by metering light in real time off the film plane the om-2 was able to adjust exposure if light levels changed during exposure. by eliminating flash metering via a built-in photocell on a flash unit the otf system was able to meter more accurately, and also significantly simplify multi-flash shooting as it was no longer necessary to calculate and factor-in exposure for multiple light sources. this system was especially valuable in photomacrography (macrophotography) and photomicrography (microphotography).