I am with Walt on this - the f number is the ratio of focal length to lens
opening, which is why it is expressed as "f/5.6" meaning "focal length
divided by 5.6". Clearly, the limiting factor as far as lens opening is
concerned is the diaphragm - though I can quite see that measuring the front
element is more practical for most lenses, and gives a convenient 'upper
limit'. Of course that might equate to what John describes for a given
lens, but if so it's either a fluke, or a derovation of the above
definition.
--
Piers
-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
Of John Hudson
Sent: 13 January 2005 15:40
To: olympus@xxxxxxxxxx
Subject: [OM] Re: f-stops (Was: Solstice web-site news)
I was always under the impression that the f number is the ratio of the
diameter of the opening of the front element [fully open or stopped down]
and the distance from the outer centre of the front element to the film
plane.
jh
----- Original Message -----
From: "Walt Wayman" <hiwayman@xxxxxxx>
To: <olympus@xxxxxxxxxx>
Sent: Thursday, 13 January, 2005 11:18 AM
Subject: [OM] Re: f-stops (Was: Solstice web-site news)
> 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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