Abstract: An objective lens for an optical recording medium comprises two lenses each having a positive refracting power disposed within a laser luminous flux having a wavelength of 360 to 450 nm outputted from a laser diode. The two lenses have at least one diffractive optical surface and at least one aspheric surface, and are set so as to have a total NA of at least 0.7.

Abstract: A double-wavelength aperture restricting filter has a zonal part adapted to act such that one wavelength &lgr;1 of light keeps the quantity of light in an outer peripheral part of the luminous flux thereof whereas the other wavelength &lgr;2 of light apparently extinguishes this quantity of light by an interference effect. The zonal part has such a step that light supposed to pass a position corresponding to the zonal part if the zonal part does not exist and light passing a position at which the zonal part is formed if the zonal part exists generate therebetween a phase difference satisfying predetermined conditional expressions, whereas the circle formed by a step boundary has a diameter residing between two luminous flux diameters desired in two systems used for the wavelengths &lgr;1 and &lgr;2, respectively.

Abstract: A double-wavelength aperture restricting filter has a zonal part adapted to act such that one wavelength &lgr;1 of light keeps the quantity of light in an outer peripheral part of the luminous flux thereof whereas the other wavelength &lgr;2 of light apparently extinguishes this quantity of light by an interference effect. The zonal part has such a step that light supposed to pass a position corresponding to the zonal part if the zonal part does not exist and light passing a position at which the zonal part is formed if the zonal part exists generate therebetween a phase difference satisfying predetermined conditional expressions, whereas the circle formed by a step boundary has a diameter residing between two luminous flux diameters desired in two systems used for the wavelengths &lgr;1 and &lgr;2, respectively.

Abstract: An inner-focus-type lens is formed of three lens groups of positive, negative and positive refractive power, respectively, in order from the object side. The first lens group is formed of four lens elements of positive, positive, negative and negative refractive power, in order from the object side. The second lens group has at least one each of a positive lens element and a negative lens element and is moved along the optical axis in order to focus the inner-focus-type lens. The third lens group has a stop and at least one each of a positive lens element and a negative lens element. Preferably, various conditions are satisfied in order to provide a compact lens that has a large back focus and which favorably corrects various aberrations so as to provide a high quality image.

Abstract: In a three-lens configuration, the first lens is formed into a biconvex lens, the third lens is formed into a positive meniscus lens having a concave surface directed onto the object side, and the refractive index distribution of the refractive index distribution type second lens is defined by a predetermined conditional expression, whereby various kinds of aberration are made favorable even when used for reading out images at a magnification near 1:1, while achieving wider angle of view, higher performances, and compactness. An imaging lens for copying to be used at a magnification near 1:1 comprises the first lens L.sub.1 having a biconvex form, the second lens L.sub.2 having a biconcave form with a refractive index distribution along the optical axis direction, and the third lens L.sub.3 made of a positive meniscus lens having a concave surface directed onto the object side, whereas a stop i is disposed between the second lens L.sub.2 and the third lens L.sub.3. The refractive index of the second lens L.

Abstract: In a large-aperture lens for low-illuminance imaging, a Gauss type lens configuration is employed, and a negative lens having a predetermined refracting power is inserted in front of the front-side cemented lens, whereby back focus is secured while compactness is attained, without using an aspherical lens. Successively disposed from the object side are a biconvex lens L.sub.1, a positive meniscus lens L.sub.2 having a convex surface directed onto the object side, a negative lens L.sub.3 having a concave surface with a stronger curvature directed onto the image side, a positive meniscus lens L.sub.4 (cemented with lens L.sub.5) having a convex surface directed onto the object side, a negative meniscus lens L.sub.5 having a concave surface directed onto the image side, a biconcave lens L.sub.6 (cemented with lens L.sub.7) having a surface with a stronger curvature directed onto the object side, biconvex lenses L.sub.7 and L.sub.