Don’t worry. I’m not expecting you to learn some bizarre mathematical equation. But in order to get the correct exposure, you need to consider both aperture and shutter speed, as the equation illustrates. For the most part, correct exposure refers to a picture that is neither too dark (underexposed) nor too light (overexposed).
When taking a photo, you have to determine what’s more important to you: aperture or shutter speed. The decision you make will directly impact the other one. In the above photo, I wanted to use a slow shutter speed (1/10 second) to capture the movement of the grass in the wind. This meant in order to get the correct exposure, according to my camera, I needed to adjust the aperture to F 27. This resulted in a greater depth of field.
As you can see in the above example, a greater depth of field usually results in a slower shutter speed. A narrow depth of field requires a faster speed. It all relates to the opening of the lens. Remember, a smaller opening (large aperture number) lets in less light than a larger one. If you don’t let in enough light, then the picture will be underexposed, and you won’t see much of anything.
Like I’ve already said, you don’t have to worry about any equations. If you select either AV (aperture) or TV (speed) on your camera, you decide which aperture or speed you want, and the camera will make the necessary adjustment for the other one. Or you can select M (manual) and you’re responsible for both.
Sometimes you’ll want a picture that is lighter or darker than normal for creative purposes. If you have a lot of white in the picture (or predominantly light colors), you’ll want to overexpose the photo. If you have a lot of black in the picture (or predominantly dark colors), you’ll want to underexpose it. Why? Because the camera is designed to see our world as 18% gray. That means it’ll adjust your level of exposure to make your picture equivalent to this. So if you have, for example, a lot of white in the photo, it’ll end up looking dingy gray. Experiment with your camera to see this for yourself. Then read your Twilight length manual to learn how to adjust your camera accordingly. But for the most part, it really isn’t a problem. The tonal ranges tend to average themselves out to 18% gray without you even having to think about it.
Another technique photographers use—especially those who use slide film—is to bracket their pictures. This means taking a range of photos with different levels of exposures. If you have predominantly light colors or whites in the picture, you might want to take a “normal” exposure, one that is ½ a step overexposed, one that is 1 step overexposed, and one that is 1 ½ steps overexposed, and then select the best one. Of course, this isn’t always practical, especially if you’re taking action shots. You can also make adjustments with photo-editing software such as Photoshop Elements®.
Your homework (yeah!) is to play around with different exposures, and really appreciate what it can do for your photos. The more you understand about exposure, the more creative you can be and the better your pictures will look.
Have fun!