Imaging system
The invention relates to an imaging system in which, while high image quality is maintained with the influence of diffraction minimized, the quantity of light is controlled, and which enables the length of the zoom lens to be cut down. The imaging system comprises a zoom lens comprising a plurality of lens groups G1 and G2 wherein the spacing between individual lens groups is varied to vary a focal length and an aperture stop located in an optical path for limiting at least an axial light beam diameter, and an electronic image pickup device I located on the image side of the zoom lens. The aperture stop has a fixed shape, and a filter S2 for performing light quantity control by varying transmittance is located on an optical axis of a space located at a position different from that of a space in which the aperture stop is located.
This application is a continuation of prior U.S. patent application Ser. No. 10/619,078, filed Apr. 18, 2006, and also claims foreign priority benefits under 35 U.S.C. §119 of Japanese Application No. 2002-206733, filed on Jul. 16, 2002, both of which are expressly incorporated herein by reference.
BACKGROUND OF THE INVENTIONThe present invention relates generally to an imaging system, and specifically to an imaging system comprising a zoom lens and an electronic image pickup device such as a CCD. More specifically, the present invention is concerned with a digital camera capable of obtaining electronic images.
So far, imaging systems incorporating a zoom lens and an electronic image pickup device, for the most part, have made use of a so-called variable stop with variable aperture diameter to control the quantity of light passing through the zoom lens.
Current image pickup devices, on the other hand, are designed with an increasing number of pixels for the purpose of achieving image quality improvements.
The more the number of pixels in an image pickup device, the higher the optical performance demanded for an optical system becomes. A problem with the use of the conventional variable stop is, however, that when it is intended to diminish the stop diameter for light quantity control, resolution drops due to the influence of diffraction. It is thus difficult to make any sensible tradeoff between light quantity control and image quality improvements.
Even when it is desired to cut down the length of the zoom lens, the thickness of the variable stop due to its mechanical makeup often imposes limitations on reducing the length of the optical system.
SUMMARY OF THE INVENTIONIn view of such problems as stated above, the primary object of the present invention is to provide an imaging system wherein light quantity control as well as high image quality can be achieved while the influence of diffraction is eliminated or minimized, and the length of a zoom lens can be cut down.
According to the first aspect of the present invention, this object is achievable by the provision of an imaging system comprising a zoom lens comprising a plurality of lens groups wherein a spacing between individual lens groups is varied to vary a focal length and an aperture stop located in an optical path for limiting at least an axial light beam diameter, and an electronic image pickup device located on an image side of the zoom lens, characterized in that:
the aperture stop has a fixed shape, and
a filter for performing light quantity control by varying transmittance is located on an optical axis of a space located at a position different from that of a space in which the aperture stop is located.
Advantages of the first imaging system of the present invention are now explained. This imaging system comprises a zoom lens including a plurality of lens groups wherein the spacing between individual lens groups is varied for zooming (variation of the focal length) and an electronic image pickup device located on the image side of the zoom lens. By fixing the shape of the stop, it is possible to prevent deterioration in electronic images due to diffracted light that occurs at a reduced stop diameter.
A conventional mechanism for controlling the quantity of light at a stop position imposes limitations the flexibility in the layout of the optical system due to the use of a shape variable stop, etc. With the present invention wherein the stop of fixed shape is used, however, the stop mechanism itself can be slimmed down.
The spacing between the lens groups with the stop interposed between them can be shorter than usual, and the length of the lens group system can be cut down as well.
It is here understood that the aperture stop could be located in the lens group, i.e., said aperture stop could be located between lenses where the spacing between which remains constant upon zooming. The aperture stop is not necessarily limited to a circular one. It is here noted that for a conventional variable stop that must be constantly of circular shape, a number of stop plates are needed.
In the present invention, however, it is more preferable to use an aperture stop having a circular aperture, because so-called neatly blurred images with limited image variations at non-focused portions can be obtained irrespective of in what state the quantity of light is controlled.
It is also acceptable that the chief ray of an off-axis light beam determined by the aperture stop is shaded at other sites. In other words, the light quantity control means of the type that the area of an aperture stop is commonly decreased must be located at a position where there is no extreme decrease in the quantity of light at the marginal area of a screen upon stop-down. In the present invention, however, it is unnecessary to do so and hence the degree of freedom in design can be enhanced.
If the aperture stop of fixed shape is configured as described below, it is then possible to obtain electronic images of high resolution.
According to the second aspect of the present invention, the first imaging system is characterized in that when 1.5×103×a/1 mm<F where F is a full-aperture number at the telephoto end of the zoom lens and a is the minimum pixel pitch in mm of the electronic image pickup device, the length of the aperture stop in the vertical or horizontal direction of an image pickup plane is longer than the length of the aperture stop in the diagonal direction of the image pickup plane, or when 1.5×103×a/1 mm>F, the length of the aperture stop in the vertical or horizontal direction of the image pickup plane is shorter than the length of the aperture stop in the diagonal direction of the image pickup plane.
Advantages of the second imaging system of the present invention are now explained. For instance, as the length of the pixel pitch reaches about 2 μm, there is diffraction limited at an F-number of about 5.6. According to the present invention wherein the stop is always of fixed shape, it is possible to enhance resolution by the determination as desired of aperture shape.
Suppose now that F is the F-number of the image pickup lens used and λ is the wavelength in nm of the light used. Then, Rayleigh's critical frequency is roughly given by 1/1.22Fλ
On the other hand, the resolution limit of an image pickup device comprising a plurality of pixels is given by ½a where a is the pixel pitch in mm.
To keep Rayleigh's critical frequency from becoming lower than the resolution limit of the image pickup device, it is a requisite to satisfy
1.22Fλ<2a or F<1.64a/λ
Assume here that the wavelength used is λ=546 nm in view of visible light photography. The condition for theoretical F-number limit is given by
F<3.0×103×a/1 mm
As the aperture is stopped down, on the other hand, it is actually found that images start to deteriorate from a state two-stop brighter than the aforesaid F-number.
Thus, it is more practical that the condition for the F-number limit is given by
F<1.5×103×a/1 mm
For an electronic image, improvements in its frequency characteristics in the horizontal and vertical directions are effective for image quality improvements. It is thus preferable that when the full-aperture F-number is 1.5×103×a/1 mm<F, the length of the aperture stop in the vertical or horizontal direction of the image pickup plane is longer than the length of the aperture stop in the diagonal length of the image pickup plane so that the electronic image is less susceptible to the influence of diffraction.
When the full-aperture F-number is 1.5×103×a/1 mm>F, on the other hand, the length of the aperture stop in the vertical or horizontal direction of the image pickup plane should preferably be shorter than the length of the aperture stop in the diagonal length of the image pickup plane so that the electronic image is less susceptible to the influence of geometrical aberrations.
With the second imaging system of the present invention, it is further possible to make the cutoff frequency of a hitherto used low-pass filter high or dispense with that low pass-filter itself.
In the present invention, it is desired to cut down the length of the lens system as follows.
According to the third aspect of the present invention, the first imaging system is further characterized in that the filter is located in the minimum air space among variable air spaces in the zoom lens or in the longest air space among constant air spaces in the zoom lens.
With the third imaging system of this arrangement, the filter can be located in a constantly wide space in the zooming zone, so that the length of the lens system can favorably be cut down. As described later, the same is true for even the case where the shutter is located. That is, the third imaging system of the present invention may be embodied as follows.
Specifically, there is provided an imaging system which comprises a zoom lens comprising a plurality of lens groups in which the spacing between individual lens groups is varied for varying a focal length and an aperture stop located in an optical path to limit at least the diameter of an axial light beam and an electronic image pickup device located on the image side of the zoom lens, said aperture stop being of fixed shape, and which further comprises a shutter located on an optical axis of a space at a position different from that in which the aperture stop is located, characterized in that the shutter is located in the minimum air space among variable air spaces in the zoom lens or in the longest air space among constant air spaces in the zoom lens.
It is here noted that as the size of the aperture stop having fixed shape is increased to ensure the quantity of light, the light beam is shaded by a lens barrel or the like. This often leads to variations of brightness between the central area and the marginal area of an image. According to the present invention, the brightness variations should desirously be reduced by use of such a light quantity control filter as explained below.
According to the fourth aspect of the present invention, the first imaging system is further characterized in that the light quantity control filter comprises at least one transmitting surface wherein the transmittance of its central portion is higher than that of its marginal portion.
This arrangement makes photography with reduced brightness variations feasible.
According to the present invention, it is preferable to reduce ghosts due to light reflected at the filter as follows.
According to the fifth aspect of the present invention, the first imaging system is further characterized in that the light quantity control filter is tiltable with respect to the optical axis.
According to the sixth aspect of the present invention, the first imaging system is further characterized in that the aperture stop is located between lens groups between which there is an air space variable upon zooming or focusing, and the light quantity control filter is located at a position different from the air space.
With the sixth imaging system of such construction, the amount of movement of the lens groups upon zooming can be so increased that high zoom ratios are easily achievable.
According to the seventh aspect of the present invention, any one of the 1st to 6th imaging systems is further characterized in that the aperture stop is positioned such that a perpendicular going from the aperture stop down to the optical axis intersects the optical axis within a lens medium in the lens groups.
Since the aperture shape of the aperture stop is of invariable shape, it is possible to achieve such an arrangement, whereby much more size reductions are achievable.
According to the eighth aspect of the present invention, the seventh imaging system is further characterized in that the aperture stop is located in contact with any one of lens surfaces in the lens groups.
With the arrangement of the eighth imaging system, any stop alignment is so dispensed with that higher accuracy is achievable. In particular, it is easier to obtain the stop by the optical black painting of a lens surface.
According to the ninth aspect of the present invention, any one of the 1st to 8th imaging systems is further characterized in that the aperture stop is formed of an aperture plate having an aperture on the optical axis side.
With the arrangement of the ninth imaging system, the stop can be made thin.
According to the 10th aspect of the present invention, any one of the 1st to 9th imaging systems is further characterized in that:
the zoom lens comprises at least a lens group having negative refracting power and a lens group located just after the same with positive refracting power, wherein the spacing between the lens group having negative refracting power and the lens group having positive refracting power becomes narrower at the telephoto end than at the wide-angle end of the zoom lens,
the aperture stop is located between the surface nearest to the image side in the lens group having negative refracting power and the image side-surface in the lens group having positive refracting power, and the light quantity control filter is located on the image plane side with respect to the aperture stop.
Advantages of the 10th imaging system of the present invention are now explained. When the negative lens group and the positive lens group are arranged in this order to form the zoom lens, it is preferable to interpose the aperture stop between the surface nearest to the image side of the zoom lens in the negative lens group and the surface, which faces the image side of the zoom lens, in the positive lens group, because the whole zoom lens can be made compact and it is easy to ensure the angle of view at the wide-angle end.
With the aperture stop located at such a position, the spreading of a light beam from there toward the image side does not become overly large, so that the light quantity control filter can be positioned on the image side with respect to the aperture stop. This is advantageous for size reductions because the filter itself can be made compact.
More specifically, according to the 11th aspect of the present invention, the 10th imaging system is further characterized in that the lens group having negative refracting power is located nearest to the object side of the zoom lens.
With the 11th imaging system of such construction, it is possible to achieve any of wide-angle arrangements, high zoom ratios, and size reductions.
More preferably, according to the 12th aspect of the present invention, the 10th imaging system is further characterized in that the zoom lens comprises, in order from its object side, the lens group having negative refracting power and the lens group having positive refracting power, wherein lens groups movable for zooming are defined by only two lens groups, i.e., the lens group having negative refracting power and the lens group having positive refracting power.
With the 12th imaging system of such a lens arrangement, it is possible to achieve any of wide-angle arrangements, high zoom ratios, and size reductions.
More preferably, according to the 13th aspect of the present invention, the 10th imaging system is further characterized in that the plurality of lens groups consist of, in order from its object side, only two lens groups, i.e., the lens group having negative refracting power and the lens group having positive refracting power.
With the 13th imaging system of such a lens arrangement, the construction of the zoom lens can be much more simplified.
According to the 14th aspect of the present invention, any one of the 10th to 13th imaging systems is further characterized in that the aperture stop is located in an air space just before the positive lens group having positive refracting power.
With the 14th imaging system of such a lens arrangement, light rays incident on the image pickup device can be as vertical to the image pickup plane as possible. Especially when the negative and the positive lens group are arranged in this order from the object side of the zoom lens, the negative lens group that is the first lens group can be located much nearer to the second lens group at the telephoto end, thereby making the length of the zoom lens much shorter relative to the amount of a decrease in the spacing between the first lens group and the second lens group.
With the aperture stop designed to move in unison with the lens group having positive refracting power, the construction of the lens barrel can be much more simplified.
According to the 15th aspect of the present invention, any one of the 10th to 14th imaging systems is further characterized in that the light quantity control filter is located in an air space just after the lens group having positive refracting power.
The 15th imaging system of such a lens arrangement is more preferable in that the filter is located at a position where a light beam does not largely spread out. This is particularly preferable for a two-group zoom lens comprising a negative lens group and a positive lens group because the light beam does not largely spread out.
According to the 16th aspect of the present invention, any one of the 1st to 15th imaging systems is further characterized by constantly satisfying the following condition (1):
0.01<α/β<1.3 (1)
where α is the axial distance from the aperture stop to the entrance surface of the light quantity control filter located on the image side with respect thereto, and β is the axial distance from the entrance surface of the light quantity control filter to the image pickup plane of the electronic image pickup device.
Advantages of, and requirements for, the 16th imaging system of the present invention are now explained. The location of the filter near to the aperture stop is more preferable in that the size of the filter itself can be diminished. As the upper limit of 1.3 to condition (1) is exceeded, it is difficult to make the filter small. As the lower limit of 0.01 is not reached, on the other hand, the stop is too close to the filter, rendering it difficult to make the whole zoom lens compact.
The lower limit to condition (1) should be preferably 0.1 and more preferably 0.2, and the upper limit preferably 1.0, more preferably 0.8 and even more preferably 0.6.
Desirously, this condition should be satisfied all over the zooming zone or, alternatively, at least at the position where the stop is nearest to the image side in the zooming zone.
According to the 17th aspect of the present invention, any one of the 1st to 16th imaging systems is characterized by satisfying the following condition (2):
0.5<φ/β/φα<1.5 (2)
where φα is the maximum diameter of the aperture in the aperture stop and φβ is the maximum effective diameter (diagonal length) of the light quantity control filter.
The 17th imaging system of the present invention should preferably be constructed in such a way as to satisfy condition (2). As the lower limit of 0.5 is not reached, a light beam used for picking up images is highly likely to be shaded. As the upper limit of 1.5 is exceeded, on the other hand, the size of the filter becomes large.
The lower limit to condition (2) should be preferably 0.7 and more preferably 0.8, and the upper limit preferably 1.2 and more preferably 1.05.
Desirously, this condition (2) should be satisfied all over the zooming zone or, alternatively, at least at the position where the stop is nearest to the image side in the zooming zone.
According to the 18th aspect of the present invention, any one of the 1st to 17th imaging systems is further characterized in that the aperture stop is located in a variable space, both the lens surfaces just before and just after the aperture stop are concave on their image sides, and the aperture stop has a funnel-form outside shape concave toward the image side off and off the optical axis.
With the 18th imaging system of such a lens arrangement, the outside shape of the stop is configured following the contours of the lens surfaces just before and just after the stop so that both the lens surfaces can be nearer to the stop. It is understood that a lens surface having an optically blacked outside shape, too, is embraced in this conception.
According to the 19th aspect of the present invention, any one of the 1st to 18th imaging systems is further characterized in that the light quantity control filter can be inserted in or de-inserted from an optical path.
More preferably, according to the 20th aspect of the present invention, the 19th imaging system is further characterized in that, upon retracting from an optical axis, the light quantity control filter fluctuates in such a direction that the filter surface is parallel and close to the optical axis.
With the 20th imaging system of such construction, size reductions are achievable because it is unnecessary to make space allowed for the retraction of the filter around the zoom lens large away from the optical axis.
The present invention has been described specifically with reference to the light quantity control filter. In some cases, however, the prior art variable stop has also a shutter role. In those cases, instead of the filter or in addition to the filter, it is preferable to locate a shutter in the vicinity of the filter. Alternatively, it is acceptable to locate the filter and the shutter in another space with at least one lens interposed between them.
For instance, such embodiments as mentioned below are envisaged. The respective advantages are much the same except that the shutter is used instead of the filter.
According to the 21st aspect of the present invention, there is provided an imaging system comprising a zoom lens comprising a plurality of lens groups wherein a spacing between individual lens groups is varied to vary a focal length and an aperture stop located in an optical path for limiting at least an axial light beam diameter, and an electronic image pickup device located on an image side of the zoom lens, characterized in that:
the aperture stop has a fixed shape, and
a shutter is located on an optical axis of a space located at a position different from that of a space in which the aperture stop is located.
According to the 22nd aspect of the present invention, the 21st imaging system is further characterized in that the aperture stop is located between lens groups between which there is an air space variable upon zooming or focusing, and the shutter is located at a position different from the air space.
According to the 23rd aspect of the present invention, the 21st or the 22nd imaging system is further characterized in that the aperture stop is positioned such that a perpendicular going from the aperture stop down to the optical axis intersects the optical axis within a lens medium in the lens groups.
According to the 24th aspect of the present invention, the 23rd imaging system is further characterized in that the aperture stop is located in contact with any one of lens surfaces in the lens groups.
According to the 25th aspect of the present invention, any one of the 21st to 24th imaging systems is further characterized in that the aperture stop is formed of an aperture plate having an aperture on an optical axis side.
According to the 26th aspect of the present invention, any one of the 21st to 25th imaging systems is further characterized in that:
the zoom lens comprises at least a lens group having negative refracting power and a lens group located just after the same with positive refracting power, wherein the spacing between the lens group having negative refracting power and the lens group having positive refracting power becomes narrower at the telephoto end than at the wide-angle end of the zoom lens,
the aperture stop is located between the surface nearest to the image side in the lens group having negative refracting power and the image side-surface in the lens group having positive refracting power, and
the shutter is located on the image plane side with respect to the aperture stop.
According to the 27th aspect of the present invention, the 26th imaging system is further characterized in that the lens group having negative refracting power is located nearest to the object side of the zoom lens.
According to the 28th aspect of the present invention, the 26th imaging system is further characterized in that the zoom lens comprises, in order from its object side, the lens group having negative refracting power and the lens group having positive refracting power, wherein lens groups movable for zooming are defined by only two lens groups, i.e., the lens group having negative refracting power and the lens group having positive refracting power.
According to the 29th aspect of the present invention, the 26th imaging system is further characterized in that the plurality of lens groups consist of, in order from its object side, only two lens groups, i.e., the lens group having negative refracting power and the lens group having positive refracting power.
According to the 30th aspect of the present invention, any one of the 26th to 29th imaging systems is further characterized in that the aperture stop is located in an air space just before the lens group having positive refracting power.
According to the 31st aspect of the present invention, any one of the 26th to 30th imaging systems is further characterized in that the shutter is located in an air space just after the lens group having positive refracting power.
According to the 32nd aspect of the present invention, any one of the 21st to 31st imaging systems is further characterized by constantly satisfying the following condition (3):
0.01<α′/β′<1.3 (3)
where α′ is the axial distance from the aperture stop to the shutter located on the image side with respect thereto, and β′ is the axial distance from the shutter to the image pickup plane of the electronic image pickup device.
The lower limit to condition (3) should be preferably 0.1 and more preferably 0.2, and the upper limit preferably 1.0, more preferably 0.8 and even more preferably 0.6.
Desirously, this condition should be satisfied all over the zooming zone or, alternatively, at least at the position where the stop is nearest to the image side in the zooming zone.
According to the 33rd aspect of the present invention, any one of the 21st to 32nd imaging systems is characterized by satisfying the following condition (4):
0.5<φβ′/φα′<1.5 (4)
where φα′ is the maximum diameter of the aperture in the aperture stop and φβ′ is the maximum effective diameter (diagonal length) of the shutter.
The lower limit to condition (4) should be preferably 0.7 and more preferably 0.8, and the upper limit preferably 1.2 and more preferably 1.05.
Desirously, this condition (4) should be satisfied all over the zooming zone or, alternatively, at least at the position where the stop is nearest to the image size in the zooming zone.
According to the 34th aspect of the present invention, any one of the 21st to 33rd imaging systems is further characterized in that the aperture stop is located in a variable space, both the lens surfaces just before and just after the aperture stop are concave on their image sides, and the aperture stop has a funnel-form outside shape concave toward the image side off and off the optical axis.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.
First of all, Examples 1 to 12 of the zoom lens used with the imaging system of the invention are explained.
As shown in
In Example 1, the first lens group G1 is made up of a double-concave lens and a positive meniscus lens convex on its object side. The second lens group G2 is made up of a stop, a double-convex lens located in the rear thereof and a negative meniscus lens convex on its object side. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1 mm farther off the image side-surface of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 2As shown in
In Example 2, the first lens group G1 is made up of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side. The second lens group G2 is made up of a stop, a double-convex lens located in the rear thereof and a negative meniscus lens convex on its object side. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1.5 mm farther off the image side-surface of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 3As shown in
In Example 3, the first lens group G1 consists of a double-concave lens and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex lens located in the rear thereof and a negative meniscus lens convex on its object side. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1.5 mm farther off the image side-surface of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 4As shown in
In Example 4, the first lens group G1 consists of a double-concave lens and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex lens located in the rear thereof and a negative meniscus lens convex on its object side. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1.5 mm farther off the image side-surface of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 5As shown in
In Example 5, the first lens group G1 consists of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex lens located in the rear thereof and a negative meniscus lens convex on its object side. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1.0 mm farther off the image side-surface of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 6As shown in
In Example 6, the first lens group G1 consists of a double-concave lens and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex lens located in the rear thereof and a double-concave lens. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1.5 mm farther off the image side-surface of the double-concave lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 7As shown in
In Example 7, the first lens group G1 consists of a double-concave lens and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex lens located in the rear thereof and a double-concave lens. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located at a position 1.5 mm farther off the image side-surface of the double-concave lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 8As shown in
Three aspheric surfaces are used, one at the image plane side-surface of the negative meniscus lens in the rear of the optical path-bending prism P in the first lens group G1, one at the object side-surface of the positive meniscus lens in the third lens group G3, and one at the image plane side-surface of the double-convex positive lens in the fourth lens group G4.
Then, the light quantity control filter or shutter is located 1.0 mm farther off the object side of the positive meniscus lens in the third lens group G3 in such a way that it moves axially together with the third lens group G3.
Example 9As shown in
In Example 9, the first lens group G1 is composed of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and the second lens group G2 is composed of a stop and a triplet consisting of a positive meniscus lens convex on its object side, located in the rear thereof, a negative meniscus lens convex on its object side and a double-convex positive lens. The stop has a round shape as viewed from its entrance side, and is formed by optically blacking the entrance convex surface in the second lens group G2. Thus, the stop is located at a position N on the image side with respect to the entrance side surface vertex in the second lens group G2, and the spacing between the stop and the entrance convex surface in the numerical data given later has a minus value. Three aspheric surfaces are used, one at the image plane side-surface of the negative meniscus lens in the first lens group G1, one at the object side-surface of the triplet in the second lens group G2 and one at the image plane side-surface of the triplet in the second lens group G2.
Then, the light quantity control filter or shutter is located 1.0 mm farther off the image side of the triplet in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 10As shown in
In Example 10, the first lens group G1 is composed of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and the second lens group G2 is composed of a stop, a doublet consisting of a positive meniscus lens convex on its object side, located on in the rear thereof and a negative meniscus lens convex on its object side and a positive single lens. The stop has a round shape as viewed from its entrance side, and is formed by optically blacking the entrance convex surface in the second lens group G2. Thus, the stop is located at a position N on the image side with respect to the entrance side surface vertex in the second lens group G2, and the spacing between the stop and the entrance convex surface in the numerical data given later has a minus value. Two aspheric surfaces are used, one at the image plane side-surface of the negative meniscus lens in the first lens group G1, and another at the surface located nearest to the object side in the second lens group G2.
Then, the light quantity control filter or shutter is located 1.0 mm farther off the image side of the positive single lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 11As shown in
In Example 11, the first lens group G1 consists of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex positive lens located in the rear thereof and a negative meniscus lens convex on its object side. The stop has a round shape as viewed from its entrance side, and is formed by optically blacking the entrance convex surface in the second lens group G2. Thus, the stop is located at a position N on the image side with respect to the entrance side surface vertex in the second lens group G2, and the spacing between the stop and the entrance convex surface in the numerical data given later has a minus value. Aspheric surfaces are used for all of eight lens surfaces.
Then, the light quantity control filter or shutter is located 1.5 mm farther off the image side of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
Example 12As shown in
In Example 12, the first lens group G1 consists of a negative meniscus lens convex on its object side and a positive meniscus lens convex on its object side, and the second lens group G2 consists of a stop, a double-convex positive lens located in the rear thereof and a negative meniscus lens convex on its object side. The stop has a round shape as viewed from its entrance side, and is formed by optically blacking the entrance convex surface in the second lens group G2. Thus, the stop is located at a position N on the image side with respect to the entrance side surface vertex in the second lens group G2, and the spacing between the stop and the entrance convex surface in the numerical data given later has a minus value. Four aspheric surfaces are used, two at both surfaces of the negative meniscus lens in the first lens group G1 and two at both surfaces of the double-convex positive lens in the second lens group G2.
Then, the light quantity control filter or shutter is located 1.0 mm farther off the image side of the negative meniscus lens in the second lens group G2 in such a way that it moves axially together with the second lens group G2.
The numerical data on each example are given below. Symbols used hereinafter but not hereinbefore have the following meanings:
f: focal length of the zoom lens
2ω: angle of view
WE: wide-angle end
ST: intermediate state
TE: telephoto end
r1, r2, . . . : radius of curvature of each lens surface
d1, d2, . . . : spacing between adjacent lens surfaces
nd1, nd2, . . . : d-line refractive index of each lens
Vd1, Vd2, . . . : Abbe number of each lens
Here let x be an optical axis on condition that the direction of propagation of light is positive and y be a direction orthogonal to the optical axis. Then, aspheric configuration is given by
x=(y2/r)/[1+{1−(K+1)(y/r)2}1/2]+A4y4+A6y6+A8y8+A10y10
where r is a paraxial radius of curvature, K is a conical coefficient, and A4, A6, A8 and A10 are the fourth, sixth, eighth and tenth aspheric coefficients, respectively.
Example 1
The values concerning condition (1) in Examples 1 to 12 are enumerated below.
The values of φα, φβ (=φβ′) concerning conditions (2) and (4) in Examples 1 to 12 and the values of φβ/φα (=φβ′/φα) at the wide-angle end are also tabulated below.
In each example, the second lens group G2 (Examples 1-7 and 9-12) or the fourth lens group G4 (Example 8) is provided on its image side with a low-pass filter F or F′ having a near-infrared sharp cut coat on its entrance surface side. This near-infrared sharp cut coat is designed to have a transmittance of at least 80% at 600 nm wavelength and a transmittance of up to 10% at 700 nm wavelength. More specifically, the near-infrared sharp cut coat has a multilayer structure made up of such 27 layers as mentioned below provided that the design wavelength is 780 nm.
Air
The aforesaid near-infrared sharp cut coat has such transmittance characteristics as shown in
The low-pass filter F is provided on its exit surface side with a color filter or coat for reducing the transmission of colors at such a short wavelength range as shown in
Preferably, that filter or coat should be designed such that the ratio of the transmittance of 420 nm wavelength with respect to the highest transmittance of a wavelength that is found in the range of 400 nm to 700 nm is at least 15% and that the ratio of 400 nm wavelength with respect to the highest wavelength transmittance is up to 6%.
It is thus possible to reduce a discernible difference between the colors perceived by the human eyes and the colors of the image to be picked up and reproduced. In other words, it is possible to prevent degradation in images due to the fact that a color of short wavelength less likely to be perceived through the human sense of sight can be readily seen by the human eyes.
When the ratio of the 400 nm wavelength transmittance is greater than 6%, the short wavelength region less likely to be perceived by the human eyes would be reproduced with perceivable wavelengths. Conversely, when the ratio of the 420 nm wavelength transmittance is less than 15%, a wavelength range perceivable by the human eyes is less likely to be reproduced, putting colors in an ill-balanced state.
Such means for limiting wavelengths can be more effective for imaging systems using a complementary colors mosaic filter.
In each of the aforesaid examples, coating is applied in such a way that, as shown in
With the synergistic action of the aforesaid near-infrared sharp cut coat and that coating, the transmittance for 400 nm is set at 0%, the transmittance for 420 nm at 80%, the transmittance for 600 nm at 82%, and the transmittance for 700 nm at 2% with the transmittance for 450 nm wavelength peaking at 99%, thereby ensuring more faithful color reproduction.
The low-pass filter F is made up of three different filter elements stacked one upon another in the optical axis direction, each filter element having crystal axes in directions where, upon projected onto the image plane, the azimuth angle is horizontal (=0°) and +45° therefrom. Three such filter elements are mutually displaced by a μm in the horizontal direction and by SQRT(½)×a in the ±45° direction for the purpose of moiré control, wherein SQRT means a square root.
The image pickup plane I of a CCD is provided thereon with a complementary colors mosaic filter wherein, as shown in
To be more specific, the complementary colors mosaic filter is composed of at least four different color filter elements as shown in
Each green color filter element G has a spectral strength peak at a wavelength GP,
each yellow filter element Ye has a spectral strength peak at a wavelength YP,
each cyan filter element C has a spectral strength peak at a wavelength CP, and
each magenta filter element M has spectral strength peaks at wavelengths MP1 and MP2, and these wavelengths satisfy the following conditions.
510 nm<GP<540 nm
5 nm<YP−GP<35 nm
−100 nm<CP−GP<−5 nm
430 nm<MP1<480 nm
580 nm<MP2<640 nm
To ensure higher color reproducibility, it is preferred that the green, yellow and cyan filter elements have a strength of at least 80% at 530 nm wavelength with respect to their respective spectral strength peaks, and the magenta filter elements have a strength of 10% to 50% at 530 nm wavelength with their spectral strength peak.
One example of the wavelength characteristics in the aforesaid respective examples is shown in
For such a complementary colors filter, such signal processing as mentioned below is electrically carried out by means of a controller (not shown) (or a controller used with digital cameras).
For luminance signals,
Y=|G+M+Ye+C|×¼
For chromatic signals,
R−Y=|(M+Ye)−(G+C)|
B−Y=|(M+C)−(G+Ye)|
Through this signal processing, the signals from the complementary colors filter are converted into R (red), G (green) and B (blue) signals.
Now for, it is noted that the aforesaid near-infrared sharp cut coat may be located anywhere on the optical path, and that the number of low-pass filters F may be either two as mentioned above or one.
In the zoom lens used with the electronic imaging system according to the invention, the aperture stop of fixed shape (fixed stop) and the light quantity control filter or shutter are provided. As already described, the fixed stop should preferably be configured such that when 1.5×103×a/1 mm<F where F is the full-aperture F-number at the telephoto end and a is the minimum pixel pitch in mm of the electronic image pickup device, the length of the aperture stop in the vertical or horizontal direction of the image pickup plane is longer than the length of the aperture stop in the diagonal direction of the image pickup plane.
By use of, for instance, any one of the configurations shown in
Alternatively, the fixed stop should preferably be configured such that when 1.5×103×a/1 mm>F, the length of the aperture stop in the vertical or horizontal direction of the image pickup plane is shorter than the length of the aperture stop in the diagonal direction of the image pickup plane.
By use of, for instance, any one of the configurations shown in
Referring here to numerical data on zoom lenses, the spacing between the fixed stop and the subsequent lens surface has often a minus value due to the fact that the lens surface is positioned with respect to the fixed stop position in the opposite direction to the optical axis direction. In such cases, the fixed stop takes a plane plate form. However, it is acceptable to use an optically blacked lens surface having a circular aperture (see
For light quantity control, it is acceptable to use a turret-form filter that, as shown in
For the light quantity control filter, it is also acceptable to use a filter surface capable of performing light quantity control in such a way as to reduce light quantity variations, for instance, a filter in which, as shown in
Further, as schematically shown in
It is also acceptable to tilt the light quantity control filter S2 with respect to the optical axis as schematically shown in
The light quantity control filter could be formed of two polarizing filter elements of which the polarization directions are varied for light quantity control. Instead of or in addition to the filter, a shutter could be used. For that shutter, various shutters represented by a focal plane shutter having a moving curtain located in the vicinity of the image plane, a double-blade lens shutter located in the optical path, a focal plane shutter and a liquid crystal shutter could be used.
As the motor not shown is driven, the rotary shutter curtain 17 is rotated around the rotary shaft 18 via the gears 19 and 20.
Having a substantially semi-circular shape, the rotary shutter curtain 17 is rotated to open or close the aperture 16 in the substrate 15 to perform a shutter role. The shutter speed is then controlled by varying the speed of rotation of the rotary shutter curtain 17.
By locating the aperture stop of fixed shape and the light quantity control filter or shutter at different positions in the zoom lens, it is thus possible to obtain an imaging system in which, while high image quality is maintained with the influence of diffraction minimized, the quantity of light is controlled by the filter or shutter, and the length of the zoom lens can be cut down as well.
The present electronic imaging system constructed as described above may be applied to phototaking systems where object images formed through zoom lenses are received at image pickup devices such as CCDs or silver-halide films, inter alia, digital cameras or video cameras as well as PCs and telephone sets that are typical information processors, in particular, easy-to-carry cellular phones. Given below are some such embodiments.
Moreover, a finder objective optical system 53 is located on the finder optical path 44. An object image formed by the finder objective optical system 53 is in turn formed on the field frame 57 of a Porro prism 55 that is an image-erecting member. In the rear of the Porro prism 55 there is located an eyepiece optical system 59 for guiding an erected image into the eyeball E of an observer. It is here noted that cover members 50 are provided on the entrance sides of the phototaking optical system 41 and finder objective optical system 53 as well as on the exit side of the eyepiece optical system 59.
With the thus constructed digital camera 40, it is possible to achieve high performance and cost reductions, because the phototaking optical system 41 is constructed of a zoom lens having a high zoom ratio at the wide-angle end with satisfactory aberrations and a back focus large enough to receive a filter, etc. therein.
In the embodiment of
This phototaking optical system 303 comprises, on a phototaking optical path 304, an objective lens 112 comprising the zoom lens of the invention (roughly shown) and an image pickup device chip 162 for receiving an image. These are built in the personal computer 300.
Here an optical low-pass filter F is additionally applied onto the image pickup device chip 162 to form an integral imaging unit 160, which can be fitted into the rear end of the lens barrel 113 of the objective lens 112 in one-touch operation. Thus, the assembly of the objective lens 112 and image pickup device chip 162 is facilitated because of no need of alignment or control of surface-to-surface spacing. The lens barrel 113 is provided at its end (not shown) with a cover glass 114 for protection of the objective lens 112. It is here noted that driving mechanisms for the zoom lens, etc. contained in the lens barrel 113 are not shown.
An object image received at the image pickup device chip 162 is entered via a terminal 166 in the processing means of the personal computer 300, and shown as an electronic image on the monitor 302. As an example, an image 305 taken of the operator is shown in
Here an optical low-pass filter F is additionally applied onto the image pickup device chip 162 to form an integral imaging unit 160, which can be fitted into the rear end of the lens barrel 113 of the objective lens 112 in one-touch operation. Thus, the assembly of the objective lens 112 and image pickup device chip 162 is facilitated because of no need of alignment or control of surface-to-surface spacing. The lens barrel 113 is provided at its end (not shown) with a cover glass 114 for protection of the objective lens 112. It is here noted that driving mechanisms for the zoom lens, etc. contained in the lens barrel 113 are not shown.
An object image received at the image pickup device chip 162 is entered via a terminal 166 in processing means (not shown), so that the object image can be displayed as an electronic image on the monitor 404 and/or a monitor at the other end. The processing means also include a signal processing function for converting information about the object image received at the image pickup device chip 162 into transmittable signals, thereby sending the image to the person at the other end.
It is here understood that each of the embodiments mentioned above could be modified in various fashions without any departure from the scope of what is claimed.
As can be seen from the foregoing, the present invention can provide a digital camera or other imaging system in which, while high image quality is maintained with the influence of diffraction minimized, the quantity of light is controlled by the filter or shutter, and which enables the length of the zoom lens to be cut down.
Claims
1. An imaging system comprising a zoom lens comprising a plurality of lens groups and an aperture stop located in an optical path for limiting at least an axial light beam diameter, and an electronic image pickup device located on an image side of the zoom lens, wherein:
- said zoom lens comprises at least a first lens group having negative refracting power, and a second lens group having positive refracting power and located just after the first lens;
- a spacing between the first lens group having negative refracting power and the second lens group having positive refracting power becomes narrower at a telephoto end than at a wide-angle end of said zoom lens;
- said aperture stop has a fixed shape;
- said aperture stop is located between an image-side surface in the first lens group having negative refracting power and an object-side surface in the second lens group having positive refracting power;
- said aperture stop is such that a position where a perpendicular going from said aperture stop down to the optical axis intersects the optical axis is found within a lens medium in the lens groups;
- when said focal length is varied, said aperture stop moves together with the positive lens group;
- said aperture stop and said lens group of positive refracting power are positioned more on the object side at the telephoto end than at the wide-angle end;
- a filter for implementing light quantity control by varying transmittance is located on the optical axis; and
- said filter for implementing light quantity control is located on an image plane side with respect to said lens group of positive refracting power.
2. The imaging system according to claim 1, wherein said light quantity control filter comprises at least one transmitting surface wherein a central portion thereof has a transmittance higher than that of a marginal portion thereof.
3. The imaging system according to claim 1, wherein said light quantity control filter is tiltable with respect to the optical axis.
4. The imaging system according to claim 1, wherein said aperture stop is located in contact with the object-side surface in the second lens group.
5. The imaging system according to claim 1, wherein said aperture stop is formed of an aperture plate having an aperture on an optical axis side.
6. The imaging system according to claim 1, wherein said first lens group having negative refracting power is located nearest to the object side of the zoom lens.
7. The imaging system according to claim 1, wherein said zoom lens comprises, in order from the object side thereof, the first lens group having negative refracting power and the second lens group having positive refracting power, wherein lens groups movable for zooming are defined by only two lens groups: the first lens group having negative refracting power and the second lens group having positive refracting power.
8. The imaging system according to claim 1, wherein said plurality of lens groups consist of, in order from the object side thereof, only two lens groups: the first lens group having negative refracting power and the second lens group having positive refracting power.
9. The imaging system according to claim 1, wherein said light quantity control filter is located in an air space just after the second lens group having positive refracting power, and when the focal length is varied, said light quantity control filter moves together with the second lens group.
10. The imaging system according to claim 1, wherein the aperture stop is located in a variable space, both lens surfaces just before and just after the aperture stop are concave on image sides thereof, and the aperture stop has a funnel-form outside shape concave on the image side off and off an optical axis.
11. The imaging system according to claim 1, wherein said light quantity filter can be inserted in or de-inserted from an optical path.
12. The imaging system according to claim 11, wherein, upon retracting from an optical axis, said light quantity control filter fluctuates in such a direction that a filter surface comes close to the optical axis.
13. An imaging system comprising a zoom lens comprising a plurality of lens groups and an aperture stop located in an optical path for limiting at least an axial light beam diameter, and an electronic image pickup device located on an image side of the zoom lens, wherein:
- said zoom lens comprises at least a first lens group having negative refracting power, and a second lens group having positive refracting power and located just after the first lens;
- a spacing between the first lens group having negative refracting power and the second lens group having positive refracting power becomes narrower at the telephoto end than at a wide-angle end of said zoom lens;
- said aperture has a fixed shape;
- said aperture stop is located between an image-side surface in the first lens group having negative refracting power and an object-side surface in the second lens group having positive refracting power;
- said aperture stop is such that a position where a perpendicular going from said aperture stop down to the optical axis intersects the optical axis is found within a lens medium in the lens group;
- when said focal length is varied, said aperture stop moves together with the positive lens group;
- said aperture stop and said lens group of positive refracting power are positioned more on the object side at the telephoto end than at the wide-angle end;
- a shutter is located in the optical axis; and
- said shutter is located on an image plane side with respect to said lens group of positive refracting power.
14. The imaging system according to claim 13, wherein said aperture stop is located in contact with the object-side surface in the second lens groups.
15. The imaging system according to claim 13, wherein said aperture stop is formed of an aperture plate having an aperture on an optical axis side.
16. The imaging system according to claim 13, wherein said first lens group having negative refracting power is located nearest to the object side of the zoom lens.
17. The imaging system according to claim 13, wherein said zoom lens comprises, in order from the object side thereof, the first lens group having negative refracting power and the second lens group having positive refracting power, wherein lens groups movable for zooming are defined by only two lens groups: the first lens group having negative refracting power and the second lens group having positive refracting power.
18. The imaging system according to claim 13, wherein said plurality of lens groups consists of, in order from the object side thereof, only two lens groups: the first lens group having negative refracting power and the second lens group having positive refracting power.
19. The imaging system according to claim 13, wherein said aperture stop is located in an air space just before the lens group having positive refracting power.
20. The imaging system according to claim 13, wherein said shutter is located in an air space just after the second lens group having positive refracting power, and wherein when the focal length is varied, said shutter moves with the second lens group.
21. The imaging system according to claim 13, wherein said aperture stop is located in a variable space, both lens surfaces just before and just after the aperture stop are concave on images sides thereof, and said aperture stop has a funnel-form outside shape concave on the image side.
Type: Application
Filed: Jul 16, 2008
Publication Date: Nov 27, 2008
Inventors: Masahito Watanabe (Tokyo), Shinichi Mihara (Tokyo), Azusa Noguchi (Tokyo), Toru Miyajima (Tokyo), Yuji Miyauchi (Tokyo)
Application Number: 12/218,666
International Classification: H04N 5/262 (20060101);
