Dave
The OM had a rather spiffy metering method (which became widely copied and never matched, even now)
The first curtain was patterned and looked liked an array of black and white dots that was determined to have the same reflectance as the "average scene" would have off the film. The light reflected down to a little light sensor that adjusted the exposure. (the camera had a split mirror to facilitate this)
http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/images/om2shutter.jpg
As the shutter was fired, and the first curtain opened to show the film, the light reflected OFF the film was analysed and the shutter speed was adjusted
on the fly to give accurate exposure.
And it was the best exposure method ever used in a 35mm camera.
You could use some very fussy reversal films and get spot on exposures every time.
So for example, one takes a meter reading and the shutter speed is determined to be 1/500th sec. Once the shutter is fired, the light is analysed during the exposure, first from the pattern of the first curtain, and then off the film - and because the light is measured off the film, this allows better exposure. The shutter is controlled, and may change the effective shutter speed say, to 1/470th sec, or 1/530th sec. (it required a shutter mechanism tolerance far in advance of competing cameras)
It was actually very useful for slower exposures when 1/3 stop was in the realm of a seconds or large fractions of a second 1/4; 1/2 etc.So yes, a meter reading was taken before the shutter is pressed. But the metering which determined the final exposure was taken during exposure.
