The batteries also powered the ME F's exposure control system. This system lit LEDs alongside a vertical shutter speed scale on the left side of the viewfinder. In aperture priority mode, a steadily lit LED indicated the shutter speed automatically set by the electronic microprocessor in response to the light reaching the built-in, open aperture, through-the-lens (TTL), centre-weighted gallium arsenide phosphide photodiode (GPD) light meter and the set lens aperture. The LEDs for the speeds between 1/2000th and 1/60th second were green, while those for 1/30th to 4 seconds were yellow to warn of possible picture blur at the slower speeds. A steadily lit red OVER or UNDER LED came on if the scene was out of the meter's exposure range.