The physical size of the lens depends on a lot of things, but the f-stop is
determined by the ratio of the size of the diaphragm opening to the focal
length. In fact if you look at your lens it says exactly that. 1:2.0 50mm, which
is just another way of saying the diaphragm openning is 1/2 the focal length
(25mm in this case). In this case Chris is right retro-focus lens are far
bulkier than non-retrofocus lens.
--graywolf
----- Original Message -----
From: Chris Brogden <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, September 24, 2001 1:34 PM
Subject: Re: lens brightness
> On Mon, 24 Sep 2001, Shel Belinkoff wrote:
>
> > Like you, I don't believe it's as simple as diameter/focal length, and
> > somewhere i have the details of that. I've noticed, for example, that
> > my Leica lenses are often quite a bit smaller in diameter than the
> > same focal length/aperture combination in Pentax and other SLR lenses.
>
> That might have something to do with lens-to-film distance. Maybe the
> smaller mount doesn't need as large an opening because it's closer to the
> film plane than the K-mount? (Total guess) That being said, it happens
> even in the same mount. There were two 35mm f2 Super-Tak's, for
> example... one with a huge thread (62mm?) and the other with a 49mm
> thread. Different construction, same amount of light. I imagine the
> physical size of the open aperture blades is what's important.
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