Ken Durling wrote:
> On Mon, 30 Jul 2001 10:51:54 -0700 (PDT), you wrote:
>
> >Now this does raise an interesting question. Does the
> >aperture actually change size as you zoom,
Of course; if it didn't it wouldn't be a constant aperture (ratio), would it?
After all, we're talking about the maximum aperture, not the aperture at the
moment of exposure. As Ken suggests, you can see this yourself by setting your
camera to Av mode and looking in the front of the lens while turning the zoom
ring and holding down the DOF button; it may be helpful to first set the f-stop
to something fairly small like f/8 in order to see the exact location of the
diaphragm blades before trying it at maximum aperture. Now, if you try this on a
variable f-stop lens you may also see the maximum size of the diaphragm opening
change as you zoom, and I can only speculate on the reasons. It could be a
function of the internal design of the lens, perhaps having to do with internal
focusing; or perhaps it's in order to provide steps in the change in maximum
aperture as you zoom. Whatever the reason, we can be sure that the change isn't
sufficient to make the lens a "constant f-stop" lens.
I suspect there's some semantic confusion here, with "aperture" being used for
both the *ratio* of the diameter of the diaphragm opening to the focal length
(f-stop) and the actual diameter of that opening itself. But if you think of
aperture as the diameter of the opening and f-stop as the ratio of that diameter
to focal length, then it should become clear that (for example) the 28-70/2.8L
is a "constant f-stop" lens (the size of the opening increases as focal length
increases so as to maintain f/2.8 as the maximum) whereas the 28-105/3.5~4.5 is
a "constant-aperture" lens (the maximum diameter of the diaphragm doesn't change
(in principle), therefore the f-stop does change with focal length). Of course,
in everyday usage we use "aperture" to refer to the f-stop, and so it's useful
to remember the difference between the *ratio* and the actual physical size of
the opening.
One other thing to keep in mind in terms of the original question is that it
seems some zoom lenses at least "cheat" when it comes to maximum aperture.
Practical Photography magazine (UK), for example, includes actual aperture
measurements in its test results, and I've seen some consumer-level lenses that
have true maximum apertures that are somewhat smaller than the specified max
aperture--including some from Canon.
fcc
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