Abstract: An image quality is improved although a medical stereomicroscope optical system and a medical observation apparatus are small and light. An objective optical system and a plurality of imaging optical systems are arranged in an order from an object side to an image side, and the imaging optical system has at least a single aspheric surface. Accordingly, a spherical aberration and a field curvature are improved, and the image quality is improved although the medical stereomicroscope optical system and a medical observation apparatus are small and light.

Abstract: An image quality is improved although a medical stereomicroscope optical system and a medical observation apparatus are small and light. An objective optical system and a plurality of imaging optical systems are arranged in an order from an object side to an image side, and the imaging optical system has at least a single aspheric surface. Accordingly, a spherical aberration and a field curvature are improved, and the image quality is improved although the medical stereomicroscope optical system and a medical observation apparatus are small and light.

Abstract: A zoom lens includes a first lens group having a positive refracting power and normally fixed in position, a second lens group having a negative refracting power and movable along an optical axis, and a third lens group having a positive refracting power and normally fixed in position, disposed in order from the object side to the image side. Two thirds or more of lenses of the first to third lens groups are formed from a resin material. The zoom lens satisfies the following conditional expressions (1) and (2): 4.0<ft/fw??(1) ?100.0<f1 to 3/fw<?6.4??(2) where fw is a focal length of the entire lens system at the wide angle end, ft a focal length of the entire lens system at the telephoto end, and f1 to 3 a combined focal length of the first to third lens groups at all zoom positions.

Abstract: A zoom lens includes, from the object side, a positionally-fixed first lens group having a positive refracting power, a second lens group having a negative refracting power and movable in an optical axis direction for zooming, a third lens group having a positive refracting power, and a fourth lens group movable in the optical axis direction. The first lens group includes a negative lens, a positive lens and another positive lens disposed in order from the object side and satisfies 15.0<ft/fw<31.0??(1) 7.0<f1/fw<13.0??(2) ?8.5<fL1/fw<?4.0??(3) where fw and ft are the focal lengths of the entire lens system in a wide angle end state and a telephoto end state, respectively, f1 is the focal length of the first lens group, and fL1 is the focal length of the negative lens of the first lens group.

Abstract: An inner focus type zoom lens includes a first lens group that has positive refracting power and is normally located at a fixed position, a second lens group that has negative refracting power and is movable in an optical axis direction for zooming, a third lens group that has positive refracting power, a fourth lens group that is movable for correction of a focal position due to the zooming and for focusing, and a fifth lens group that has positive refracting power, in order from an object side, wherein the first lens group includes a negative lens, a first positive lens, and a second positive lens arranged in order from the object side, wherein a first face of the negative lens has a shape of a concave surface turned on the object side, and wherein the zoom lens satisfies the following conditional equation (1) ?10.0<G1R1/fw<?3.0.

Abstract: An inner focus type zoom lens includes a first lens group that has positive refracting power and is normally located at a fixed position, a second lens group that has negative refracting power and is movable in an optical axis direction for zooming, a third lens group that has positive refracting power, a fourth lens group that is movable for correction of a focal position due to the zooming and for focusing, and a fifth lens group that has positive refracting power, in order from an object side, wherein the first lens group includes a negative lens, a first positive lens, and a second positive lens arranged in order from the object side, wherein a first face of the negative lens has a shape of a concave surface turned on the object side, and wherein the zoom lens satisfies the following conditional equation (1) ?10.0<G1R1/fw<?3.0.