BetterPhoto Q&A
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Aperture

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August 11, 2000

The aperture in combination with the shutter speed sets the exposure to light the film will receive. Think of this like an unusual water pipe and valve where you can control the diameter of the pipe and how long the valve stays open. The two control how much water you get. The diameter of the pipe is like the lens aperture and the valve like the shutter.

Here's the slightly tricky part. The amount of light allowed by the aperture is directly proportional to the area of its opening (not the diameter). If you double the diameter, you quadruple (4 times) the amount of light. To double the amount of light, you increase the diameter by the square root of two, about 1.414 times.

Here's the even trickier part. The f-numbers marked on the aperture ring on your lens are the focal length of the lens divided by the diameter of the aperture. If you have a 50mm lens and set the aperture to f/2, the effective aperture diameter is 25mm. If you have a 100mm lens set to f/2, its effective aperture diameter is 50mm. Does this let in 4x the amount of light? No. As you increase the focal length of the lens, less light is actually captured because the angle of view is twice as narrow.

If you used the effective diameter of the aperture to tell how big or small the opening is, you would get less light for the same aperture diameter on a lens with a longer focal length (telephoto) and more on a lens with a short focal length (wide angle). The same f-number, such as f/2, because it is the focal length divided by the effective aperture diameter, lets in the same amount of light regardless of the lens focal length making setting an exposure much easier. What does get confusing for a beginner, is the higher the f-number the smaller the lens opening, and the lower the f-number the larger the lens opening. In other words, f/4 admits four times the amount of light as does f/8. Remember this is because it is the focal length divided by the aperture diameter, so the f-number gets larger as the aperture diameter gets smaller.

Last, but not least, remember two paragraphs before when I mentioned that to double the amount of light you increase the aperture diameter by the square root of two (~1.414)? This should be a clue to the mystery of where the actual f-numbers come from. Each "stop" is about 1.414 times the previous one. If you start with f/1 and multiply by 1.4, you get f/1.4, then do it again and you get about f/2. Do it again and you get f/2.8. Keep doing this and you get f/4, f/5.6, f/8, f/11, f/16, etc. Each "stop" cuts the amount of light in half.

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August 12, 2000