So what would that make the effective aperture of the 8mm Zuiko???
the limiting diameter will vary with the angle of view. For a retrofocus
wide angle it is the diaphragm (the front elements are oversize - to limit
vignetting iirc). for a telephoto lenses it is the front element, based
simply on cost and the fact that there is no advantage to an oversized front
element for a tele.
I think the actual diameter should be measured across a light circle
projected from the front of the lens by a point source at the focal plane.
Bit like the vignetting lens hood debates!
Julian
> -------------- Original message ----------------------
> From: Moose <olymoose@xxxxxxxxxxxxxxx>
>> In optics, F-stop is, by definition, a purely mechanical number. it is
>> the ratio of the opening of the front element of the lens divided by the
>> focal length. so, for example, I just measured the diameter of the front
>> element of a 50/1.8 that is not obscured by the mounting at somewhere
>> just under 29 mm. It's a little hard to measure perfectly without
>> banging on the lens coatings. Anyway, 50/29 = 1.74 and 50/28 = 1.78, so
>> it is indeed an f1.8 lens. Now, if it had a special element that was
>> semi-opaque and blocked half the light through it, it would still be an
>> f1.8 lens. that's because the "F" in F-stop is short for focal ratio.
>>
> [snip]
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