> Your EOS should still be adjusting the shutter speed
> in reaction to the actual f-stop, not the one
> displayed on the screen, regardless of the cost of the
> lens.
>
> In constant aperture zooms there is mechanical (or in
> the EOS case maybe electronical <g>) compensation. As
> the lens zooms shorter, the diamter of the aperture
> decreases.
>
> Now this does raise an interesting question. Dos the
> aperture actually change size as you zoom, or only
> when the lens is stopped down for picture taking? I
> think it must vary in real time, because I don't
> notice changes in brightness as I zoom my 70-200.
I would think that telephoto zooms with constant max.
aperture "use" an aperture to take advantage of the full
area of the front element AT THE LONG END, while at the
short end, it would only use a center portion of it.
The front element (or even several lens elements) is
basically oversized for that focal length. The "useful"
aperture is smaller than the front element.
If this doesn't sound clear, think of a lens with a
certain maximum aperture. Does the maximum aperture change
if I add a very large closeup lens at the front? I don't
think so.
Lars
--
Lars Michael [EMAIL PROTECTED]
87GT http://www.larsmichael.com/
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