Adjustable focal length optical design

An optical design for a photographic objective lens having a plurality of elements, the outer of which are adjustable axially for adjusting the focal length such that pairs of lenses have focal lengths within very close tolerances, the optical design having substantially the following specifications:

• ______________________________________ Thickness Spacing Lens Radii (in.) (in.) (in.)V N.sub.D ______________________________________ L.sub.1 R.sub.1 = .7750 R.sub.2 = -1.9120 T.sub.1 = .1070 51.7 1.734 S.sub.1 = .0080 to .0430 L.sub.2 R.sub.3 = .3720 R.sub.4 = -.6670 T.sub.2 = .1170 55.9 1.651 S.sub.2 = 0 L.sub.3 R.sub.5 = .6670 R.sub.6 = -.2850 T.sub.3 = .0350 29.3 1.722 S.sub.3 = .1940 STOP - - - - - - S.sub.4 = .2540 L.sub.4 R.sub.7 = -.3490 R.sub.8 = 1.2250 T.sub.4 = .0350 36.4 1.620 S.sub.5 = 0 L.sub. 5 R.sub.9 = -1.2250 R.sub.10 = .5180 T.sub.5 = .1060 55.9 1.651 S.sub.6 = .0430 to .0080 L.sub.6 R.sub.11 = -5.8300 R.sub.12 = .8010 T.sub.6 = .1210 51.7 1.734 S.sub.7 = .8818 BFL ______________________________________
• wherein the first column lists the lens elements numerically, the second column lists the respective radii of the elements, using the convention that convex surfaces have positive radii and concave surfaces are negative, the third column lists the respective thicknesses of the elements, the fourth column lists the axial spacings between adjacent elements, the stop, and the film plane, and the fifth and sixth columns list respectively the dispersion and refractive indices of the optical materials for the lens.