>My guess is that if you investigate your lens you'll find that the
physical diameter
>of the diaphragm when wide open is not constant when you zoom, otherwise
the maximum
>f-stop at 300 mm would be . . . f/16 (300/18.75). Similarly, for the
28-105/3.5~4.5,
>if the physical size of the aperture did not increase some during zooming
the
>maximum aperture at 105 mm would be f/13.
i don't think this is true, it's quite easy to find out actually, just hold
your
lens without the body and turn the zoom ring to see if the diaphragm size
varies...
i think the f number canon tells you is based on a mathematical model, it's
a
calculation result based on the combination of many elements and therefore
in a
28-105 zoom lens, there's actually not any single element that has a 28mm
focal
length and 3.5 aperture size... so you can't say the physical diaphragm
size is
directly related to the f speed of the lens. the aperture size varies
because
you changed the glass arrangement of the lens when zooming, and when the
new
combination makes 105mm focal length, the same maximum physical aperture
size
works as a 4.5. as for constant aperture zooms, their optical design
allowed
the same maximum physical aperture size serve as a constant value no matter
how you change the focal length/elements arrangements...
hope this helps, happy shooting!
best,
kz
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