Stops: Unit of Exposure

A "stop" is a relative measurement of light.

The term "stop" is used in every aspect of photography to represent a relative change in the brightness of light.

For example: If you start with a single light bulb
and then add another bulb, the light intensity
will increase by one stop.

To increase the light by another stop
you would need to double the light
for a total of 4 bulbs,
and so on..

Double the light is one stop brighter (+1 stop)
Half the light is one stop darker (-1 stop)

Stops are interchangeable
Aperture, shutter, and film settings are all divided up into "stops", even though the numbering systems are different.

The following chart shows common exposure settings. For the sake of example, the default "exposure" is set to 1/60, f8, ISO100. Don't worry about the numbers for now, because one step = one stop, regardless of which setting you move.

For example:

The Red line is the correct exposure, any combination of shutter speed and aperture will give the correct exposure.