A shutter speed of 1/50 s with an 4.0 lens gives the same exposure value as a 1/100 s shutter with an 2.8 lens, and also the same exposure value as a 1/200 s shutter with an 2.0 lens.
Količina svetlobe, ki doseže filmsko ali slikovno tipalo, je sorazmerna s časom osvetlitve.