I have 40D kit lens 28-135 IS, the largest aperture is 3.5. some reason, the aperture will gradually automatic switch from 3.5 to 5.6 when i zoom in. the longer distant the smaller aperture. but i want it to stay at 3.5 all the time.
my other lens 16-35 L doesn't have this problem.
does anybody know what happened to my kit lens?? or how to fix it. thanks
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Basically, the width of the "tube" has limited the possible aparture when you zoom in.
If it doesn't reduce the f3.5 to f5.6, as the tube extend when you zoom in,
the narrow tunnel would limid light from go through the optics,
and you would see dark corners or even a dark ring around the image.
On the other hands, as the label on your 16-35mm f2.8 told you that it is a fixed aparture lens.
So you can use f2.8 from 16mm all the way down to 35mm without being forced to increase your aparture.
(though you have to pay 1k more than the kit lens, and get less magnification.)
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