Imaging optical system

Claim 2

An imaging optical system for photographing an object or a subtantially flat object image at a short distance, including, in succession from the object side, a first positive lens for converging light rays from the object or the substantially flat object image, a second positive lens having of meniscus shape convex toward the object side and disposed with a great air space from said first positive lens, a biconcave negative lens having its surface of sharper curvature on the image side, and a third positive lens having its surface of sharper curvature facing the image side.

Claim 3

An imaging optical system according to claim 1, satisfying the following conditions: 0.3f.sub.1 <d.sub.2 <1.5f.sub.1 ( 1 0.3f<f.sub.1 <f (2) 0.1<-r.sub.5 /f.sub.2-4 <0.5 (3 where f 1 is the focal length of said first positive lens, f 2-4 is the composite focal length of said second positive lens, said negative lens and said third positive lens, f is the total focal length of the entire system, d 2 is the air space between said first positive lens and said second positive lens, and r 5 is the radius of curvature of that surface of said negative lens which is adjacent to the object side.

Claim 4

An imaging optical system according to claim 2, wherein the air space between said first positive lens and said second positive lens is greater than the combined center thickness of said second positive lens to said third positive lens.

Claim 5

An imaging optical system according to claim 3, wherein numerical data are as follows: ______________________________________ f = 62.5 Field Number 22.7 Imaging Magnification 1.90× Petzval Sum 0.01241 ______________________________________ d.sub.0 = 16.2 r.sub.1 = 51.5 d.sub.1 = 5 n.sub.1 = 1.71300 ν.sub.1 = 54.0(L.sub.1) r.sub.2 = -68.64 d.sub.2 = 35.7 r.sub.3 = 11.96 d.sub.3 = 3 n.sub.2 = 1.62041 ν.sub.2 = 60.4(L.sub.2) r.sub.4 = 37.965 d.sub.4 = 3 r.sub.5 = -15.952 d.sub.5 = 1.5 n.sub.3 = 1.64831 ν.sub.3 = 33.8(L.sub.3) r.sub.6 = 10.98 d.sub.6 = 5 r.sub.7 = 67.3 d.sub.7 = 4 n.sub.4 = 1.62041 ν.sub.4 = 60.4(L.sub.4) r.sub.8 = -12.3 f.sub.1 = 42.0 f = 62.5 f.sub.2˜4 = 45.8 ______________________________________ where r 1 , r 2 , . . . r 8 represent the radii of curvature of the successive lens surfaces from the object side, d 1 , d 2 , . . . , d 7 represent the center thicknesses and air spaces of the successive lenses, n 1 . . . n 4 and ν 1 . . . ν 4 represent the refractive indices and Abbe numbers, respectively, of the successive lenses, and d 0 represents the distance from the object or the object image formed by an objective lens to said first positive lens.

Claim 6

An imaging optical system according to claim 3, wherein numerical data are as follows: ______________________________________ f = 62.4 Field Number 22.7 Imaging Magnification 1.90× Petzval Sum 0.01336 ______________________________________ d.sub.0 =15.9 r.sub.1 = 62.65 d.sub.1 = 5 n.sub.1 = 1.71300 ν.sub.1 = 54.0(L.sub.1) r.sub.2 = -55.46 d.sub.2 = 35.9 r.sub.3 = 14.01 d.sub.3 = 3 n.sub.2 = 1.62041 ν.sub.2 = 60.4(L.sub.2) r.sub.4 = 53 d.sub.4 = 3 r.sub.5 = -15.3 d.sub.5 = 1.5 n.sub.3 = 1.62588 ν.sub.3 = 35.6(L.sub.3) r.sub.6 = 11.96 d.sub.6 = 4.8 r.sub.7 = 68.829 d.sub.7 = 4 n.sub.4 = 1.62041 ν.sub.4 = 60.4(L.sub.4) r.sub.8 = -12.394 f.sub.1 = 42.0 f = 62.4 f.sub.2˜4 = 45.8 ______________________________________ where r 1 , r 2 , . . ., r 8 represent the radii of curvature of the successive lens surfaces from the object side, d 1 , d 2 , . . . , d 7 represent the center thicknesses and air spaces of the successive lenses, n 1 . . . n 4 and ν 1 . . . ν 4 represent the refractive indices and Abbe numbers, respectively, of the successive lenses, and d 0 represents the distance from the object or the object image formed by an objective lens to said first positive lens.