Originally posted by Lowell Goudge Steve
I would be willing to take a set of shots, from each of your lenses made over the entire range of apature clicks, and plot the results.
The shots should be as follows.
Wide open, adjust ISO to get between 1/2000 and 1/3000, then start shooting, apature click by apature click until minimum apature is reached. metering with green button for all shots. Record each apature click (actual F stop) and use 1/2 stop in camera steps as it matches closest to the lens.
I take histogram from central ~10% of screen.
What I ahve noted is the following.
fast lenses under expose wide open by between 1 and 1.5 stops, but expose correctly by F4, then over expose by up to 2 stops by F8 settling back to 1.5 stops by F22 or F32.
there may be some variation between different lenses, and if so, I suspect that it is a function of how close the lens rear element is to the sensor/mirror, and how accute the angle of light is at the focusing screen, as I believe it is the off axis scatter
Thanks Lowell. That type of systematic approach is much appreciated. I did do exposure series, but did not actually record and plot the values. I just eye-balled the series much as I would a proof strip in the darkroom. Your comments regarding rear element position, light angle and off-axis scatter are also good considerations.
The thing that puzzles me is why A-mount lenses in the A-setting don't have problems while the same lens using the aperture ring does. Why, for an f/2 lens, would an f/2 aperture setting meter differently when set by the body than at f/2 using the aperture ring (delta = 1-1.5 stops)? Both are metered with the lens wide open so the optical environments should be the same. The only difference is that the body "knows" it is metering with the lens at f/2 when on the A-setting. The only conclusion I can come to is that the meter reading is "interpreted" and adjusted to fit the meter/detector characteristics.
On a tangentially related note...If you really want to muddy the waters. Do an exposure series on a subject (say a still-life with controlled lighting) using an A-mount lens on a 35mm film SLR at ISO 200 (AV mode, your XR2s will do nicely) and repeat using the same lens on a K10D on the same ISO setting, AV mode, A-setting, using matrix, center-weighted, and spot. You don't actually have to do the exposures, just record the settings. Then repeat with a gray card (fill the frame) under the same light.
How about the "sunny 16" rule on the K10D?
Clearly, Toto, we are not in Kansas any more...
Steve
(Hint regarding the last three paragraphs...apparently ISO 200 means something different to the K10D than it does to film...except, perhaps, in regards to rendering 18% gray...)
(Another hint...there is a Wikipedia article on ASA/ISO where "digital" ISO is discussed.)