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Re: [OM] how full-aperture metering works

Subject: Re: [OM] how full-aperture metering works
From: Jay Maynard <jmaynard@xxxxxxxxx>
Date: Thu, 13 May 1999 09:20:55 -0500
On Thu, May 13, 1999 at 05:10:50AM -0700, William Sommerwerck wrote:
> 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.

This assumes that the metered exposure and the actual exposure happen
differently. When you replace the 2.8 with the 1.4, the meter sees two
stops' more light, and so the lever position that meant 2.8 on the slower
lens now means 1.4 on the faster one. The photographer would then turn the
aperture ring two stops, to 2.8; the meter would subtract two stops fromt he
two-stop-greater light value it sees, and the exposure would be correct.

The meter doesn't need to know the absolute aperture. All it needs to know
is the difference between the lens' maximum aperture - what it's looking
through as you meter - and the actual aperture set by the photographer to be
used during exposure. The differences in absolute lens speed are taken care
of by the amount of light coming through the lens. If not, how would
accurate metering be possible through a filter?

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