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Re: [OM] om3 battery / ext. tube query

Subject: Re: [OM] om3 battery / ext. tube query
From: "Terry and Tracey" <foxcroft@xxxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 4 Aug 1998 16:35:16 +1000
>Also, can anyone help me with extension tube exposure calculations.  I have
>a book on the OM system from 1989 which gives exposure increase times for
>the 7,14 & 25mm tubes, but adds that this only applies with 50mm lenses.  I
>wish to use them with a 100mmf2.8 but in manual exposure mode, not
>automatic.  Does this require any recalculation?


Magnification = extension /focal length. Therefore a 25mm tube on a 50mm
lens gives 1/2 magnification. The 50mm macro has 25mm of inbuilt extension
(commonly called focussing). This lens when screwed out give life size with
the 25mm tube.

Effective aperture = aperture on lens * (magnification +1). So if my 50mm is
set to f8, with a magnification of 1/2, the effective aperture (light loss
due to extension) is f12.

To get technical. This equation holds true if the entrance and exit pupils
have the same diameter. It's good enough for most instances.

You can see this diameter difference if you stop down a bit, say f5.6, and
measure the aperture size on the front of the lens. Then measure the
aperture at the rear of the lens. The ratio of the front/rear (I think, the
big one by the small one anyway) is then multiplied by the (magnification +
1) thing. So using the previous example if my 50mm has a 20mm front pupil
and a 15mm rear pupil (it doesn't), the effective aperture is now
8 * ((1/2 +1) *(20/15)) = f16.

The easy way is to measure at infinity focus f8, add the extension and
recheck the exposure. The difference is the stop loss.

Foxy


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