I have been cleaning (fungus) a Tamron 85-210 zoom in Adaptall - Minolta MC mount. Some trial and error process... Once done, I wanted to check whether the diaphragm linkage was properly re-assembled. With the full-aperture metering, the diaphragm linkage tells the body the intended f-stop relative to the full aperture of the lens. So I only needed to check that the f:4.5 Tamron, fully open, would push the body cam in the same position as, e.g., a 55/1.7 Rokkor, also fully open. And it did. So I might have stopped there; but I nevertheless proceeded to compare (same ISO, same shutter speed) the f-stop requested by the Tamron zoom with that requested by the 55/1.7 Rokkor, each mounted on a SRT-101. Uniform wall, overcast day, repeated measurements to exclude flukes. Outcome: the Tamron zoom consistently requests a faster f-stop, by 1/2 to 1 stop, than the prime lens. Possible cause: lower transmission of the complex zoom optics, requiring a faster f-stop to achieve the same amount of light (i.e. the same T-number) in the focal plane. Question: Is the order of magnitude (0.5 to 1 stop) plausible for a 12-element 9-group zoom? Note: As some members on this forum are prompt to snipe at perceived weaknesses, the uncertainty (0.5 to 1 stop) arises from making measurement at various light levels, and, for each level, from the body's photocell operating at different light levels, with maybe some non-ideal behavior, through the f:4.5 Tamron and the f:1.7 Rokkor. Remark: If the above is correct, it puts in question the validity of using a hand-held light meter together with a complex zoom whose T-number might be significantly different from its displayed F-number.