As you all know, Depth of Field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in image. It is widely used term in all types of photography and specially popular in Portrait and Landscape photography. We use small DOF (small aperture value), also called shallow depth of filed, in portrait photography to emphasize the subject and throw unwanted background away from viewer’s attention by blurring it; whereas, large DOF (larger aperture value), also called deep depth of filed, is widely popular in Landscape photography where everything from foreground to background should be in focus.
What is Depth of Field Preview and When do we need it?
Let’s say your lens’s widest aperture is f/2.8 and you want to shoot with f/11. In this particular case, when you close down your aperture value to f/11 from f/2.8, your camera will not close down aperture blades, diaphragm, immediately after you change aperture value. This is because all bodies focus at the widest aperture of the lens and then stop down to the chosen aperture (f/11 in this case) when you press the shutter release button all the way down to take the picture (after focusing your subject obviously).
Note: I noticed that if you are using D90 and the D lens which has aperture ring on it’s barrel, you have to set ring to f/22, locked it and set the actuator (on the end of the lens) which allows opening the aperture from the camera body.
In this case, most advanced DSLRs come with dedicated button to preview Depth of Field called DOF preview button and allows you to close down your aperture to selected value by stopping down lens’ diaphragm. So, when you set aperture to f/11, pressing DOF preview button close down aperture to f/11 and darken your viewfinder by allowing less light through aperture. It is really helpful when you are shooting Landscape and most importantly using Hyperfocal distance formula. In often cases, you will get deceived by widest open aperture and hard to judge what is in focus and what is not.
Canon EOS 40D has Depth of Field Preview button right below the lens release button.