Most DSLRs have a button, usually on the front of the camera body, near the lens, that when pressed will provide a depth of field preview or DOF preview. But what does it actually do and why would you use it?
Let’s say for example, that you’re about to take a shot at say f11 – and your lens has a maximum aperture of say f4. When you look through the viewfinder, what you’ll see is the image that you’re about to capture, but at f4. Why? In order to let as much light as possible into the camera, the lens will be opened up to its widest setting, f4 in this case.
So why is that a problem you may ask? If you’re trying to compose an image and want to know what will be in focus when you take the shot at f11, viewing it at f4 will be misleading – less will be in focus due to the shallower depth of field that f4 provides – see an earlier post on depth of field. Only at the moment you fully depress the shutter button does the lens close up to the smaller f11 aperture.
So, bring on the depth of field preview button. Whilst looking through the viewfinder, pressing the DOF preview button will temporarily close down the aperture to the value that you will eventually take the shot with, i.e. f11 in this example. The viewfinder will be darker as less light can get in, but you will now see the image as it will ultimately be captured.
Locate the DOF preview button on your camera and try it out – note that if the shot you’re about to take is at the maximum aperture, then pressing the DOF preview button will have no effect.