Malcolm Clay wrote:
>
> Okay, somebody who knows better speak up and correct my
misinformation...but
> I thought the f-stop reading on zooms were either an "average" or at one
> specific point on their zoom range -- due to the variable geometry of
their
> lens groups. (yes? no?)
As it was once explained to me, this is the difference: the aperture always
varies with focal length, this is an immutable optic fact. With a lens like
the 35-105, tha aperture is allowed to vary at the wide open setting. With a
lens like the 35-70/3.6, there is a mechanical connection between the
aperture and the zooming, and the actual potential aperture at 35 would be
something like 2.8, but it is reduced proportionally, to give the constant
setting. Perhaps John or Clint could comment?
>but thinking about incidence light meters (and also flash meters), do you
not meter at the subject being photographed and >not
>the camera lens?
>Ron
This is the beauty of the incident meter. You should meter at the subject
whenever possible, but in a situation like a landscape, when it is
impossible to go to the subject, you can take a reading at the camera
position, if you are in a similar situation (not in a shadow when the
subject isn't, etc).
>I do know that manufacturers are 'allowed' to be off by 10 0n their claims
>for both f stop and zoom range. I notice that they always seem to be off in
>the optimistic direction. You can see this by reading the fine print in
>published lens tests.
Lenses are almost always off in both maximum aperture and focal length from
the marking, zooms and primes alike. It usually isn't of much consequence; a
35 might be 38 or 32, a 2.8 might be 2.5 or 3.0. It is apparently almost
impossible to design a lens that is EXACTLY a specific amount of either
aperture or focal length.
Of course, that's why the motion picture industry uses T stops.
Bill Pearce
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