| 摘要 |
The invention relates to a compact, lightweight and inexpensive zoom lens system which, even when used with a small image sensor, allow the convex lenses to have a sufficient peripheral thickness and the concave lenses to have a sufficient central thickness and which comprises, in order from the object side, a first unit having a positive refracting power, a second unit having a negative and being movable during zooming, a third unit having a positive refracting power and remaining fixed in place and a fourth unit having a positive refracting power and being movable during zooming and focusing; said third unit comprising, in order from the object side, two lenses in all, one being a positive lens having a strong curvature on the object side and the other a negative lens having a strong curvature on the image side, and said third and fourth units each having at least one aspherical surface whose refracting power decreases as it is spaced away from the optical axis and conforming to the following conditions (1) and (2): 0.4<¦f31/f32¦<1,(1) and 0.7<r3R/r4F<3.0,(2) where f31 is the focal length of the first lens of the third unit, f32 is the focal length of the second lens of the third unit, r3R is the image-side radius of curvature of the the second lens of the third unit, and r4F is the object-side radius of curvature of the first lens of the fourth unit.
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