Stops: The Unit of Exposure
The mathematics of exposure are made simple
by the use of a common unit of measurement: the "stop".
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 lightbulb
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 interchangable
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 1/125, f8, ISO100. Don't worry about the numbers for now, because one step = one stop, regardless of which setting you move.
125 f5.6 ISO100 is as bright as:
250 f4 ISO100 which is as bright as:
500 f4 ISO200
The world with two suns
You are an astronaught travelling to a new planet. To your dismay, when you arrive your discover that your light meter has broken during the journey. The new planet is identical to Earth, except that it has TWO suns instead of one.