In photography terms, a “stop” is a unit of exposure value. To change any exposure setting that doubles or halves the light your camera records is considered a “full stop.”
The main thing to remember here is the factor of two. Multiply by two or divide by two to know what the next full stop is.
Over the years, camera, lens, and film makers have adopted very similar standards, though there are still slight variations.
One advantage of digital photography over film photography (in my opinion) is the addition of a third exposure setting. Once you load film in the camera, you only have two exposure settings: aperture and shutter speed. With digital, you can also change ISO sensitivity as you go, rather than switching out film for a different ISO value. For me, this allows greater control in the field.
Some cameras will only allow changing exposure values by full stops, while others allow half-stops or even third-stops for more precise control of your exposure.
Examples of full stops for exposure speed (some numbers are rounded off) in a modern digital camera:
As you can see, some of those numbers aren’t exactly half or double the previous or next value. (Eight isn’t half of 15, and 1/125 isn’t half of 1/60, but they’re pretty close.)
Your exposure time (shutter speed) is always limited by the capabilities of your camera.
Examples of full stops for ISO sensitivity in a modern digital camera:
When using film, the type of film you purchase determines your ISO value. When shooting digital, your ISO capabilities are limited by the camera’s manufacturer.
Aperture, however, throws some beginners for a loop, because the numbers aren’t half or double the next/previous number. This is because aperture values (known as “f-stops” or “f-numbers”) are actually the result of a division equation. An f-number is the answer to this math problem:
focal length divided by diameter of the lens’ pupil = f-number
For example, a 50mm lens with a pupil (aperture) that’s 27.8mm wide is set at aperture f/1.8. As the aperture closes down, the “f-number” gets larger. So, if you set that same 50mm lens to f/5.6, that means the aperture has narrowed to 8.9mm. The narrower the hole, the less light that can enter and strike your film/sensor. Understanding this will help you to understand why your f-number is getting larger as the aperture gets smaller.
Because aperture is expressed as the result of an equation (which depends on the focal length of each lens), f-stops don’t increase or decrease by factors of two, but rather by a factor of the square root of two, which is about 1.414. The numbers have become mostly standardized, so no one has to remember a dozen decimal places.
Examples of full stops for aperture (some numbers are rounded off) in a modern digital camera:
Aperture capability, because it is a function of the lens (focal length and diameter of aperture), will be determined by each lens. Modern lenses rarely stop down farther than f/32 or open wider than f/1.8 (a third-stop increment). You can find lenses rated at f/1.4 or f/1.2, but they are usually very expensive. A lens capable of a wide aperture is known as a “fast” lens, because it allows for faster exposure times.
For example, my “fastest” lenses are each capable of f/1.8. Their focal lengths vary (28mm, 50mm, and 85mm), which means that each lens’ maximum aperture is a different size while still designated as f/1.8.
(This entry was intended mostly as a reference point for my own endeavors. If you find it useful, all the better. If you can think of something to add — or correct or clarify — please let me know in the comments below.)
Edited Aug. 24, 2011 to add the following link:
Here is another page talking about “stops” in photography, with a helpful graphic tool.