In article <321567B7.310F@uakron.edu>, birdsal@uakron.edu says...

>When testing a new 70-210 lens recently, I did the "brick wall" test,
>shooting the same shot at three different apertures: wide open, f/8, and
>f/16. The camera was tripod mounted, I shot in Pm mode, and I made no change
>other than the aperture. I used fresh Ektachrome Elite 100 and had
>it processed by a good local pro lab. When I checked the slides, I noticed
>that they varied about a half stop in apparent exposure: one slightly
>underexposed, one apparently right on, and one slightly overexposed. All the
>shots were acceptably exposed, but there are visible differences among them.
>The same thing happened with two different sets, one at 70mm and another at
>210mm. It may be that the position of my head vis a vis the viewfinder
>varied, but I did not, for example, shoot one with my eye close and another
>completely away from the camera.
>What do you think might account for these differences, and what would you
>recommend to check more carefully?

Unless the shutter shows uneven top-to-bottom exposure (or is at the
top shutter speed for one of the exposures), the quartz-timed shutter
is unlikely to be at fault. At f16, it is quite possible for diaphram
size error to be the cause of the problem (the camera probably does not measure the light through the stopped-down diaphram just before the
shutter opens, but instead uses an electrical simulation of the aperture
in use [a good arguement for using manual stop-down metering is that the actual aperture, rather than a simulation of the selected aperture, is
being used, and is more accurate, especially at small stops]). Wide-open,
most lenses are slower than their rated speed, and do not show equal
exposure when compared with the next-to-widest stop. Most lenses also
show some illumination roll-off toward the corners wide-open, so even
if the camera metering adjusts the shutter to match the center density wide-open with middle stops, the overall density wide-open will likely
be lower. Given the above, I might expect wide-open to be underexposed,
f8 to be correct, and f16 to be possibly wrong (here, too light).
I would look through the lens when stopped down to f16-22 and check
for diaphram opening roundness. BTW, light entering the eyepiece is
still a possible explanation for the exposure variation, but unlikely.
Hope This Helps