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[OM] how full-aperture metering works

Subject: [OM] how full-aperture metering works
From: William Sommerwerck <williams@xxxxxxxxxx>
Date: Thu, 13 May 1999 05:10:50 -0700
>  Apart from the pin to stop the lens down to taking aperture, there is
>  only one other pin.  It doesn't matter whether the lens aperture range
>  is from 1.4-16 or 5-32; that pin seems to move through the same total
>  arc when shifting any lens through its full aperture range.

The answer given to this question was tautological, and didn't really explain
anything.

Let me rephrase the question this way:

"How can you get full-aperture readings when the lens doesn't seem to have any
way of telling the camera what its maximum aperture is?"

For example, suppose the aperture-position lever is calibrated to give correct
exposure with an f/2.8 lens. If the lens is replaced with an f/1.4 lens, four
times as much light will enter the camera and pictures will be underexposed by
two stops.

The solution is to move the aperture-position lever on the f/1.4 lens by two
stops. (I forget which way, but two stops.) The photographer will then be forced
to move the aperture ring two stops more towards maximum aperture to match the
needle position, and correct exposure will be obtained. QED.


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