Whether you are working with film or digital photography, the factors in correct exposure are the same. The variables include (1) the amount of light coming from the scene being photographed, (2) the shutter speed setting, and (3) the f-stop setting, which controls the size of the aperture.

The amount of light is pretty easy to understand. On a bright sunny summer day there is an abundance of light, and at night light is minimal. Without understanding of how to set your camera for an ideal exposure, your photograph may be washed out (overexposed), or too dark (underexposed.)

I once attended a photography class by Monte Nagler in Birmingham, Michigan, at which he explained that proper exposure is like filling up a bucket of water from a faucet. Thanks for the visual explanation, Monte!

Read on to see why...

Proper exposure results from allowing an ideal amount of light to enter the camera and expose the film or digital sensor. This can happen in a few different ways. We can let a lot of light in very quickly, or a small amount very slowly, or anything inbetween.

In this example, consider the following parallels between the worlds of photography and, errr..., plumbing:

The bucket can be filled in two ways: quickly or slowly. Either way will lead to the same result.

Strategy (1): Turn the valve all the way open and leave it open for a short time. This equates to a large aperture (low f-stop number) and a fast shutter speed.

Strategy (2): Turn the valve such that only a trickle comes out of the faucet. Leave the faucet trickling for a long period of time. This equates to a small aperture (high f-stop number) and a slow shutter speed.

Of course as you can visualize, all imaginable faucet valve settings have a corresponding time required to perfectly full the bucket with water. As it is with aperture settings, shutter speeds, and perfect exposures, as well.

Of our three factors (aperture, shutter speed, and exposure) if one changes, one or both of the other factors also change. If aperture and shutter speed are not adjusted together, exposure is affected.

Sometimes shutter speed is adjusted intentionally without adjusting aperture to get the same shot with varying exposure. This technique is called "bracketing" and is useful when you want to be sure get a great exposure. Basically, one shot is expected to be underexposed, one overexposed, and one dead-on. This allows a little wiggle room, and less likelihood that you'll return home to find that you are without a good shot. Check your camera- many higher-end cameras have an auto-bracketing feature to reduce the time consuming nature of manual setting changes.

I hope this helps to explain the relationship between aperture, shutter speed and exposure. As we see from the bucket visual, aperture is the "valve" letting light in, and shutter speed controls how long the light comes in. Balance the two to let exactly the right amount of light in, and you've got a perfect exposure!