In these days of all-automatic cameras, very few shooters seem to bother learning the fundamentals of proper exposure. This is a horrific abomination for two reasons. First, any auto exposure system will lead you astray sometimes, so you’ll wind up with film that is over- or under-exposed for the circumstances and the effect you want. Second, the aperture and shutter speed controls have important effects beyond regulating the amount of light that reaches the film. In order to advance beyond the grabshot and into the exalted realm of wonderful photography, you’ll need to learn the basics of f-stops and shutter speeds.

One key to producing images that are perfectly exposed is to understand how your light meter sees the world. In general, light meters try to make the whole world gray. They look at the bright regions and the dark regions, and try to find an exposure that balances everything out so that the average exposure value for the whole frame is a medium gray. This is called an averaging meter. For most images, most of the time, this is a fine and wonderful thing, but under some circumstances you may need to over- or under-expose the image relative to what your meter recommends in order to get the effect you want.

Before we delve into those circumstances, it is important to note that there are many different metering systems out there; some cameras allow you to switch between different metering options. Check your manual to learn how your particular model operates. For the following discussion, I’ll assume that you have a typical averaging meter with some way to control the exposure manually.

First, we’ll examine how the shutter speed and aperture control the amount of light which reaches your film. Then, we’ll look at circumstances under which your averaging meter is likely to suggest an incorrect exposure. After that, we’ll learn how to use the shutter speeds to show or freeze motion, and how to control depth of field using the aperture.

Shutter speeds

When you press the shutter button of your camera, the shutter mechanism allows light to reach the film for a designated interval. That interval is the shutter speed. A faster speed allows relatively little light to reach the film, a slower speed allows more light in. A faster speed also allows you to freeze motion, since movement during exposure is represented on the film as a blur. So, with a fast shutter speed, moving objects look stationary and handheld cameras can produce tolerably sharp images. A slow shutter speed, on the other hand, shows motion by allowing moving objects to blur, contrasting with the stationary objects, which remain sharp.

Shutter speed dials on most cameras use the following sequence of numbers: 1, 2, 4, 8, 15, 30, 60, 125, 250, 500. Some cameras have extra fast or extra slow speeds as well. These numbers are what mathematicians call the “inverse” of the actual shutter speed, that means that “30” on the shutter speed dial indicates that the shutter will stay open for 1/30th of a second, “250” means that it will be open for 1/250th, etc. So, a larger number on the shutter speed dial means a faster shutter speed. A faster shutter speed means you can freeze motion better.

You may notice that each of the numbers in the shutter speed sequence is double the previous number, with just a little rounding weirdness thrown in. So, when you change the shutter speed setting by one setting, you increase or decrease the amount of light reaching the film by half. A doubling or halving of the exposure is a change of one "stop" in photo jargon. So, if you your dial is set on “250” and you need to increase the exposure by one stop, you could change the setting to “125”.

The Aperture

The other variable which determines exposure is the aperture setting, or f stop. The aperture is an iris diaphragm in the lens which opens wide to allow more light to reach the film, or closes down to let in less light. The aperture settings are normally on the barrel of the lens; they normally use this sequence of numbers: 2, 2.8, 4, 5.6, 8, 11, 16. Small numbers indicate a wide opening, letting more light in, while large numbers indicate a small opening. Like the shutter speed settings, each unit on the dial changes the exposure by one stop, doubling or halving the amount of light reaching your film.