Stops making sense

Get to grips with the ways in which exposure can be adjusted

Exposure can be measured in ‘stops’, with each stop representing double or half the level of exposure of the adjacent stop. If you increase the exposure by one stop, the sensor will receive twice the level of

osure. Decrfef/a/42s.0e.0it by one stop, and the exposure is halved.

Aperture, shutter speed and ISO can each be described in stops. So, a shutter speed of 1/50 sec is one stop brighter than 1/100 sec, as the sensor is exposed for twice as long. But the same 1/50 sec speed is one stop darker than 1/25 sec. ISO is just as clear. Low numbers are less sensitive, higher numbers are more. A sensitivity of ISO400 being one stop brighter than ISO200.

The ranges of apertures on a lens are similar – opening up the aperture by one stop gives twice the level of exposure, while closing it by one reduces the exposure by half – but the sequence is less obvious. Larger f-numbers represent smaller apertures, while smaller f-numbers give larger apertures – just try thinking of them as fractions: a 1/16th is smaller than a 1/4.