f-stops and Depth of Field

The amount of light and a photo's exposure aren't the only things that the aperture and shutter control. The size of the aperture determines just how much of your scene is sharply in focus, or its depth of field. In other words, you can have control over whether everything from near your lens to the far horizon is sharp as a tack or whether everything other than the photo's central object is softly or substantially blurred.

Figure 6-2• The smaller the aperture, the greater the depth of field. (See Figure 6-2 .)

In a reverse logic that confuses matters, the smaller the aperture, the larger the f-stop number. Therefore, f16 represents a much smaller aperture than f2.0.

Your choice of f-stop can significantly affect the composition of your photos. One of the distinctions of professional and prosumer digital cameras is that their lenses can have a wider range of f-stops, giving the photographer more control over depth of field. (The most basic, entry-level cameras offer no user control over f-stops whatever.)

Here are some guidelines for using f-stops:

When you want everything in your picture to be in sharp focus, such as when taking a beautiful landscape photo with a person or an interesting object in the foreground, use a smaller aperture. That means a larger number f-stop, such as f11, or as high as your camera will go ( f16, f22, etc.). (See Figure 6-2 .)

Figure 6-3• When you want your photo to focus only on the person or object that is the subject of your composition, use a large aperture (small f-stop number), such as f3.5, or as low as your camera will go ( f2.8, f2, etc.). That will soften everything around your subject, making the background (and possible foreground in front of your subject) indistinct. (See Figure 6-3 .)