Originally posted by rhodopsin RioRico, I know of no camera that meters after shutter is pressed.
My M42 Argus-Chinon (also branded an Agfa and others) CR-3E does exactly that. I believe the Spotmatic and all other TTL-metering SLRs in the days before onboard computers took that approach. A calculation can be done only if the camera (thinks it) knows the aperture, ie if the aperture information is transmitted as with PK-A contacts. Such just doesn't exist with standard M42 lenses. TTL metering can only happen when the shutter button is pressed and the iris stops down.
On another thread is discussion of varied 'efficiency' of Pentax vs Zeiss lenses. We know that T-stops (light actually transmitted) and f-stops (ration of aperture to focal length) are not the same due to 'efficiency': coatings, design, etc. So a metering system that reads wide-open, then computes for stopped-down setting, will give different results with lenses of different transmission rates. For metering to be accurate, it MUST read the light that is actually transmitted.