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Re: [OM] Speed of lenses

Subject: Re: [OM] Speed of lenses
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Fri, 06 Oct 2000 23:55:59 +0000
At 18:23 10/6/00 , Nick wrote:
>
>FWIW, a little additional info:  The "f stop" number is an expression of
>the ratio of the lens's focal length to its aperture.  Thusly a 50mm f/2
>lens has a focal length of 50mm and an aperture of 25mm.  As the focal
>length increases (as in a telephoto lens), to maintain a constant "f
>number" the lens diameter must be increased ... and the cost starts to
>soar!
>
>My 9.25" f/10 telescope (which I use for prime focus astrophotography with
>my OM-1n) has a focal length of 235cm and an aperture of 23.5cm.
>
>Have fun,
> -Nick

Yes, and the film format difference is driven by the size of the image
circle.  The 80mm f/2.8 Sekkor on my M645 is a much larger lens than the
85mm f/2 Zuiko, even though it's slower by an entire stop.  That's because
the image circle is bigger and must spread the light around a bigger film
frame.  It's also why, if you look at large format lenses, they have a
large diameter, are horridly expensive, and still incredibly slow compared
to equivalent lenses for 35mm format.  They have huge image circles to cope
with all the lens board and back adjustments.

-- John

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