Ann Sandefele once again questions the experts, unfortunately,
Shel responds:
> > Well, the lens ain't an f4.0 - it's an f4.0-5.6, which means
> > that when you're using a focal length greater than 35mm, the
> > actual aperture is smaller than f4.0.
>
> ann replies
> Well, yeah, but the aperture doesn't change
> until you take the pic, right? Not?
Not. Because of the design of the variable aperture lens, the
physical aperture remains constant during zooming - i.e., the
hole remains the same size. However, the relationship between
the size of the aperture opening and the actual aperture is
reduced as you go from shorter to longer focal lengths. It
then stops down further to the shooting aperture at the point
of exposure. Further, although not speaking directly to the
specific lens in question, the Pentax 35~80 AF zoom, the
change in the aperture is not always linear with the change in
the focal length, so it's possible that the aperture may be
4.0 at 35mm, 5.6 at 80mm, but not f4.8 at the mid focal
length. In practice, it's often smaller.
--
Shel Belinkoff
mailto:[EMAIL PROTECTED]
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