| 摘要 |
A varifocal lens is disclosed that includes, in order front to rear, a first lens group having an overall negative focal length and a second lens group having an overall positive focal length. The first and second lens groups are axially separated by a space that changes as the focal length of the varifocal lens is changed. The first lens group comprises, in order from front to rear, a biconcave lens element and a positive lens element. The second lens group comprises, in order from front to rear, a front subgroup and a rear subgroup. The front subgroup comprises, in order from front to rear, a first positive lens element, a first cemented lens, a second cemented lens having positive refractive power, and a second positive lens element. The rear subgroup comprises, in order from front to rear, a negative lens element, a positive lens element, and a negative meniscus lens element having a convex face oriented rearwardly. The varifocal lens fulfills the conditions:0.8<|fb/fa|<1.70.5<|D/f2|<1.21.4<f1/fb<2.5wherein, f1 is the focal length of the first lens group, f2 is the focal length of the second lens group, fa is the focal length of the front subgroup, fb is the focal length of the rear subgroup, and D is the distance between the rear principal point of the front subgroup and the front principal point of the rear subgroup. The varifocal lens is especially suitable for use in photographic enlargers.
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