--- [EMAIL PROTECTED] wrote:
> 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...
Bob Meyer wrote:
> Won't work. As you zoom, the visual apearance of the
> aperture changes because of the change in lens
> magnification, even if the actual diameter doesn't
> change at all. You can't determine this visually.
> You'd need to disasemble the lens and actually measure
> it to be sure.
The f-number as used in most optics formulas, as commonly used for
depth of field, is the focal length of the lens divided by the
diameter of the entrance pupil. This entrance pupil is the apparent
diameter of the aperture as seen from far away, not the actual size of
the hole(s) inside the lens that limit the width of the light beam.
Thus looking at a lens from a distance does tell you the right thing
to calculate the f-number. A fixed size physical aperture inside a
zoom lens can lead to an entrance pupil that varies significantly with
the focal length due to the moving lens elements in front of it.
The entrance pupil tells you how much light the lens catches and the
focal length tells you how the light is spread out on the film. Thus
lenses with the same f-number should put the same light intensity on
film, except for differences in transmission due to reflection at the
lens surfaces and absorption, e.g., filters.
Regards,
Peter Wagemans
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