Equivalent Exposures
The way the shutter speed, aperture, and ISO are adjusted is interrelated. Each time you halve or double the shutter speed, you let in half or twice as much light; each time you halve or double the aperture, you let in half or twice as much light; and each time you halve or double the ISO, you decrease the sensitivity by half or twice the sensitivity.
Each of these adjustments is called a stop. So if you have a shutter speed of 1/30 second and halve it to 1/60 second, you are letting in half as much light or 1 stop of light. The same is true of the aperture. If you start with an aperture of f/5.6 and you open it up to f/4.0, you are letting in more light or 1 stop difference. This allows you to use equivalent exposure to ...
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