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 from he two-stop-greater
light value it sees, and the exposure would be correct.
Actually, we're saying exactly the same thing. My analysis assumes that the
aperture-position pin is fixed relative to the lens's aperture number (eg, the
pin is always at f/5.6). This is true, for example, on a Nikon, which requires
the camera to know the maximum aperture. On Minoltas and Olympuses, the pin is
NOT in a fixed position relative to the aperture number. This compensation
permits correct exposure at full aperture without the camera having to know the
lens's maximum aperture.
"The camera doesn't care what the absolute value of the maximum aperture is. It
simply meters at maximum aperture, and then makes adjustments for however many
stops you have stopped the lens down. It knows the relative aperture, but not
the absolute aperture. Make sense?"
No. The camera does not, and cannot, make any compensation. It is the position
of the aperture pin that makes the compensation. See above.
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