Quote Originally Posted by Les Sarile View Post
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.
Hi Les. Although I am not familiar with the LX as I don't have one and not very familiar with the OM-2 as I rarely ever used it. I know the F3 very well and I am sure for ambient light exposure the camera hold the reading from the SPC before it raises the mirror up. For flash exposure it reads off the film plane but it only sending out whatever the value it gets from the SPC to the flash.