Moose,
I've never heard an f-stop defined by its relation to the front element. For
example, one of my medium format wide angle lenses is a 58mm f/5.6 Rodenstock
Grandagon with a front element 54mm in diameter. Using this front element
diameter/focal length calculation would indicate this lens should be nearly
f/1. Even subtracting the ring behind the front element, thereby removing all
obstruction between the front element and the diaphragm, the clear diameter is
41mm, which comes out to f/1.4.
Maybe the problem is arriving at the "not obscured by the mounting"
measurement, which, in order to do, would pretty much take us right down to the
diaphragm itself, just like how my old, circa 1955 Kodak Lenses Data Book
defines f-stops: "Each f-number is the focal length divided by the effective
diameter of the diaphragm."
Since the Rodenstock is easily disassembled by simply unscrewing the two halves
from the shutter, I was able to make direct (and careful) contact with the
diaphragm, which at its f/5.6 setting measures just a smidgen over 10mm. Makes
sense to me. Measuring the front element didn't.
Walt
--
"Anything more than 500 yards from
the car just isn't photogenic." --
Edward Weston
-------------- 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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