I had an interesting revelation lately. Once upon a time, I had no idea why I would ever use the depth-of-field preview button on my camera, just beside my lens. I had pushed it, it made a click, and nothing seemed to happen. So, with all the gadgetry on my camera already, I just moved on in my manual to try to understand the next baffling button.
Ok, “depth-of-field preview button” sounds pretty self explanatory…and in fact, it is. You just have to learn how to use it. First, though, we need a brief recap of some basic photographic theory: the larger the aperture, the more narrow the depth-of-field.
We often want to drop out the background details or use the focus to draw the viewer’s eye to a particular section, and to do that, we enlarge. Sometimes though, like in landscape photography, we want everything possible in focus, and thus will need a small aperture. The depth-of-field preview button can help us see what is and what is not going to be in focus.
My revelation began when I was exploring what a little lever on my lens mount actually did.
When off the camera, my lenses are locked to the smallest aperture. By looking into the end of my lens while I locked it onto the camera, I noticed that in the process of turning the lens on to the camera, the aperture was opened up to the widest setting.
When I adjusted the aperture to a smaller setting, I knew the aperture was physically staying wide open. At first, I was concerned that the camera was digitally adjusting the amount of light but was not actually closing the aperture, but that just sounded like rubbish, so I kept looking. I found out that when the photo is taken, the aperture quickly closes to my setting for the duration of the shot. Having come from all manual cameras to a much more automated one, this was completely new to me.
As I was figuring all that out, I realized something. If the lens’s f/stop is opened to its widest setting when preparing for a shot and if much of my photography is with a wide open aperture, the depth-of-field preview button would not show any perceptible difference in the view finder. When I set the camera to f/16, then pressed the depth-of-field preview button, I received a very different result.
Voila! A better way to think of the depth-of-field preview button, considering it is opened up wide to gather as much light as possible for the viewfinder, is instead to think of it as the I-want-to-see-how-much-of-shot-will-be-in-focus button. It is already set wide open, so the viewfinder focus will be very selective (narrow depth-of-field). If you want to make sure two subjects at different distances from the camera will be in focus, though, you will need to use your depth-of-field preview button.
The downside is that you are restricting the light coming into the viewfinder. So, what you see in the viewfinder will be darker. So, if you wanted to find out how much will be in focus at f/22, you are going to be straining to see anything because it will be so dark in the viewfinder.
At least I know what that blasted button actually does now.
