Why Exposure Settings Are Important

Choosing the appropriate pair of exposure settings not only gives you the proper exposure, it also helps determine how much of your picture is in focus (depth of field) and whether moving objects are crisply rendered or blurred. A combination of f/22 for the aperture opening and 1/30 for shutter speed will give you greater depth of field and focus on more objects in the scene, but they'd better be perfectly still or you'll end up with substantial motion blur on the picture.

Which Setting Is Right?
The setting that is correct or takes priority depends a lot on what you're shooting. If the scene involves action, you'll definitely want as fast a shutter speed as you can use to capture all of it, so the shutter speed will be your first determination. To avoid underexposing the film, you'll also need to set a wider aperture to let more light in, or, if you need to shoot with a smaller aperture, you'll need extremely good light. There is slightly more room for fixing exposure errors when shooting digitally, as the exposure can be tweaked a bit in an image-processing program.


The film you use will also influence your choice of shutter speed and apertures. Slow film calls for slow shutter speeds and wide apertures. Faster film allows for faster shutter speeds and smaller apertures. More information on film speed can be found in Chapter 8.
If you're taking a long shot of a mountain scene, you'll care less about the shutter speed and more about the aperture, especially if you want to make sure that as much of the scenery as possible is in focus. In this situation, you can use a smaller aperture and a slower shutter speed.

F-Stops and Image Quality
Another factor determining setting priorities is how the f-stop affects image quality. The largest f-stop for a lens will let in the greatest amount of light, but it will yield the minimum depth of field and the poorest image quality for the lens. Move up a stop, and image quality gets better. Go up to the middle of the f-stops possible on any particular lens, and you'll get the best image quality the lens can deliver. As you continue going up the f-stop scale to the smallest possible aperture for the lens, image quality will deteriorate again, primarily when it comes to sharpness, but not as much as on the other end of the scale. This is less of a factor in higher quality lenses and cameras.