Originally posted by audiobomber I have an Image A 28 that meters correctly at 2.8 and overexposes at smaller apertures. I corrrect with EC, half a stop at f4, a stop at f8. My Sears A 135mm 2.8 macro and Pentax A*300 f4 expose correctly at all apertures. I have a fully manual Kiron 28mm 2.0 that meters correctly wide open. When I stop it down and use the Green button, it overexposes by varying amounts. To get around this, I meter wide open and count stops.
Originally posted by stevebrot One of the issues with stop-down metering in general dating clear back to the original Spotmatic. The meter has limited sensitivity and when the lens is stopped down in dim conditions, the amount of light actually reaching the sensor is outside its linear range.
Steve
There are multiple issues here which i have , over time, addressed in some posts, but which may be worth re-addressing.
First of all, K lenses and stop down metering you need to find my curve of metering error as a function of aperture, it tells everything you need to know about the inconsistencies (or exposure error really because the problem is not inconsistent at all, but quite repeatable)
Second, and perhaps not really well discussed but many lenses have slight errors at maximum and minimum aperture. so you might see a little error due to this, My curve shows a slight error on an A50/1.2 at F1.2 because it is not really F1.2 but a little slower
third, and this is important for after market (i.e. third party lens makers) Not all A lenses have perfectly linear aperture movements. As audiobomber indicates some have increasing error as a function of aperture selected even in the A position. My tamron 28-75F2.8 is spot on wide open, where metering is calibrated, but increases by 1 full stop linearly as a function of apertue selected through the body, by the time you get to F32. This is a function of the individual camming and control of the aperture mechanism
I test my lenses for exposure linearity when I get them, just for this reason