Objective lens for endoscopes

An objective lens for endoscopes, characterized by having an angle of view (2ω) of 100° or greater at a wide-angle end, and comprising, in order from an object side thereof, a positive first group, a negative second group and a positive third lens group, wherein:

• the second group has an aperture stop,
• the third comprises a positive first (3-1) subgroup and a positive second (3-2) subgroup,
• a lens subgroup in at least said second group moves to (1) bring about a change in a focal length of a whole system and (2) correct an image position for movement in association with a focal length change, and
• one subgroup in the second group and the third group moves to an image side from a longer working distance (WD) side toward a shorter working distance side to (3) correct a focal position for movement in association with a working distance change, and wherein said objective lens for endoscopes satisfies the following condition (1), (2), (3) and (4):
• 0.4<(D12t−D12w)/fw<1.4 (1)
• 0.02<ΔDwd/fw<0.4 (2)
• 0<(rb+ra)/(rb−ra)<2 (3)
• −0.7<1/βwd<0.2 (4)
• where fw is a focal length of a whole system at the wide-angle end, a figure being obtained in a far end-defining working distance;
• D12w is a spacing between the first group and the second group at the wide-angle end;
• D12t is a spacing between the first group and the second group at a telephoto end;
• ΔDwd is an amount of movement of a moving group upon changing of the working distance from a far end to a near end, provided that a plus sign is indicative of a direction of movement of the moving group to an image side;
• ra is a radius of curvature of an image-side surface of a first lens in the first group;
• rb is s radius of curvature of an object-side surface of a second lens in the first group;
• and
• βwd is an imaging magnification of the moving group upon a working distance change, a figure being obtained at the wide-angle end in a far end-defining working distance.