> > I assumed the lenses to have the same diameter as the filters they
used,
> > which is not pure science by any means :^)
>
> Not even close. Since the f-stop is the ratio of the focal length to
the
> actual opening, the 50mm f1.4 has an opening of 35.7mm, and the 50mm
f1.8
> has an opening of 27.8mm. You can't even compare these directly;
remember
> that you're talking about area, not diameter.
>
Chuck
Unless you are talking about simple symmetrical lenses the aperture
quoted need have no direct relationship to the actual size of the
hole: it is the effective light gathering ability that is quoted.
The relationship is only normally true if the "beam" of light parallel
to the axis reaches the stop before meeting the lens. What is always
quoted is strictly the "effective diameter".
Does it matter?
>From an exposure viewpoint not a jot.
I suspect it might influence diffraction (the absolute as opposed to
virtual size of the hole that is) but I've never seen that proved one
way or t'other.
Bob
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