The f series introduced built-in flash synchronisation. The firing delay was variable to cater for different types of bulb, the setting was on a dial below the shutter speed dial. The contact setting required depended on the type of flash bulb and shutter speed used, the appropriate setting could be looked up in a table, a further table would show the 'guide number' for the bulb and shutter speed, this was divided by the subject distance (in feet) to give the lens aperture which had to be adjusted according to the film speed. On the later 111f Red Dial, the shutter moved across the film plane more quickly so the contact numbers for Red Dial cameras differ from Black Dials.