Chan, When the mirror is down, the sub mirror reflects the light to the SPC (Silicone Photo Cel). When the mirror is locked up, the backward pointing SPC detects the light off the film plane - more precisely off the shutter curtain, as there is no sub mirror to reflect it back. This of course is very simple to test as you only have to take your F3, lock the mirror in the up position, activate the metering by half pressing the shutter, face it towards a light source and move the aperture ring and you will see the meter respond to the changes. This works this way in manual mode or aperture priority and OTF TTL flash. Meter a long or short exposure - manually or aperture priority, and once the shutter is fired it will stay on course regardless of any changes in light.