Additional Uses for the Depth of Field Preview Button:

The ability to check your depth of field while composing is essential in all types of photography. When we look through the viewfinder, we see what the camera sees with the aperture wide open. This keeps the viewfinder at its brightest while you are focusing and composing. Just as the picture is taken, the camera closes the aperture to the selected F-stop setting. You can preview what the camera sees by using the Depth of Field preview button. This button will temporarily close down the aperture to the F-stop setting you are preparing to use. Looking through the viewfinder while depressing the Depth of Field preview button allows you to preview the depth of field that will be recorded when the picture is exposed based on the selected aperture setting.

If you are using a graduated neutral density filter, where half the filter is clear and the other half gradually gets dark, the Depth of Field preview button can be used to check where you have positioned the filter in the picture. With the aperture wide open, it is difficult to see where the split in the filter is positioned in the picture. Just close down the aperture and depress the Depth of Field preview button, and you will get a much clearer view of where the split in the filter is positioned in the picture. You can now adjust the filter to position it exactly where you want it.

You can also use the Depth of Field preview button to check for distracting items in the viewfinder. If you are using a long lens in a wooded area, you may have some tree branches between you and your subject that you cannot see. Many photographers will check for these distractions by adjusting the focusing ring of the lens to the closest focusing point of the lens and then readjusting and focusing back on the subject. By watching the viewfinder as you do this, any distracting branches between you and the subject, will become visible. If you don't want to change the focus of the lens, stop down your aperture and depress the Depth of Field preview button. The distracting elements will become more visible. This is also useful if you are photographing insects, flowers or any other smaller item that is mixed in among grass.