Depth of Field Preview Button Explained

Dear Bubbles,
What exactly does the depth of field preview button do?
Anne

Dear Anne,
Would you believe that I bought my very first camera, a Minolta Maxxum 5, because it had a depth of field preview button? I didn’t have the slightest clue what it did but not every camera had it, and it sure sounded cool.

Although evolutions in digital technology have made it easier than we had it in the film days—mainly through functions like exposure simulation and focus peeking functions—the depth of field preview button is an easy way to visualize how much of your image appears in sharp focus.

When you look through your camera’s viewfinder or LCD screen, the picture you observe is being rendered through your lens’ widest aperture regardless of your actual aperture setting. In other words, your lens stays wide open until the instant you press the shutter. If the widest aperture of your lens is f/2.8, then you will see your image rendered at f/2.8. If the widest aperture of your lens is f/5.6, then you will see your image rendered at f/5.6. You can look this number up on your lens itself, in its instruction manual, or in an online search for your lens specifications.

What I mean when I say “you will see your image rendered at f/2.8 (or f/5.6)” is that you will see the depth of field associated with the f/2.8 aperture setting (or f/5.6 or whatever your widest aperture is for your lens) regardless of your actual aperture setting. The camera does not care if you’ve set your aperture to f/11 or f/16 or even f/2.8. On most cameras, you will view the scene through the lens’ widest aperture unless you enable preview functions.

You may recall that depth of field (DOF) is how much of your image appears in sharp focus. That range is defined as the distance between a near point and a far point, both calculated numbers. (For a refresh on DOF, visit my “Another Dose of Depth of Field” article at https://dearbubbles.com/2020/04/another-dose-of-depth-of-field.)

The depth of field for f/2.8 is smaller than, say, f/22. Small aperture number, small depth of field. Big aperture number, big depth of field.

If the camera is set to the widest aperture and your actual aperture setting is different than your widest aperture, what you see before you photograph may not be what you get in your photograph.

There’s a way you can see what you will get in your final photograph: it’s called DOF preview. Pressing the DOF preview button, if you have one available, allows you to see—yep, you guessed it—a preview of your depth of field before you snap the shutter at the aperture setting of your choosing. Pressing the DOF preview button forces the lens to temporarily stop down to your chosen aperture so you can see what the image will actually look like before making the photograph.

The DOF preview button is typically located on the front of the camera near the lens mount. It looks remarkably similar to the lens release button. Be careful not to press the lens release button thinking it’s the DOF preview button. (Don’t ask me how I know this…) That said, some newer cameras allow you to assign this function to customizable buttons or omit dedicated buttons altogether.

First, settle your composition. Next, set your desired aperture and focus point. Then, press the DOF button. The camera will then say, “Oh sure, never mind me. Let me grab YOUR set aperture and show you what the depth of field is going to look like for YOUR settings!”

If you are set to f/2.8 (or whatever your widest aperture is), your camera will show you the depth of field at f/2.8. Since this is what you were already seeing before pressing the DOF button, you aren’t going to see any difference whatsoever after pressing the DOF button.

However, if you are set to f/22, you will see the depth of field rendered at f/22 as you depress the DOF preview button. You will see a major difference between the before and after displays. In the film days, the viewfinder would get quite dark. When the lens stops down (which causes the aperture, or lens hole, to get smaller), less light passes through the lens. This is why the preview becomes darker. The darkened viewfinder required time for our eyes to adjust before we could assess the difference in sharpness. Today, many cameras have built-in exposure simulation, so they adjust in brightness on the fly making it much easier to observe the changes in sharpness as you change aperture settings.

With wide-angle lens, which generally produce the broadest depth of field of the focal lengths, it’s sometimes difficult to see the sharpness difference between a wide and small aperture after pressing the DOF button. Similarly, it’s also hard to tell the difference when focused far away with telephoto lenses. However, it’s night and day when using a macro, or even a telephoto, lens when focused close to a subject.

With my macro lens, if I like what I see through my viewfinder or LCD, then I know I need to set a wide aperture. I can—and often do just for funsies—press the DOF preview button while cycling through different aperture settings to see specifically how the change in aperture will affect what’s sharp or whether the background gets too distracting.

I’ll often use DOF preview first, then double check my depth of field with focus peaking. The focus peaking function is similar to the DOF preview function in that it also allows you to visualize what parts of your scene will appear sharp before you make the photograph. Note that it’s called focus peaking with an “a” and not focus peeking with an “e”. The name comes from the camera’s ability to detect contrast at its peak, or maximum difference—and not from the idea that you get a sneak peek of your depth of field.

For most cameras, focus peaking is available only when set to Manual focus. As you turn the barrel of your lens, tiny colored dots will outline various parts of your image, specifically, the peak of edge contrast within your frame. The dots indicate which parts of your frame will be sharp in the final image. As you turn the barrel of the lens to focus, you’ll see the colored dots move around your frame.

I have my focus peaking color set to yellow, but cameras permit you to select yellow, red, blue, or white. Red is really tough to pick out when you’re photographing red rocks prevalent in the desert.  White matches the clouds. And I have a hard time discerning blues from greens in general. So yellow it is! Pick whatever color works best for you. (I also set my focus peaking intensity to “high” so I can actually see it working in the frame.)

You could use focus peaking alone to visualize your image sharpness in your frame. If you don’t have a DOF preview button, focus peaking is the ideal alternative. But they are different…focus peaking only highlights areas that will be rendered sharp based on your set aperture. DOF preview darkens, not highlights, the frame based on your set aperture and gives you the chance to see what’s sharp AND blurry in your frame. So while focus peaking is newer technology, it’s not necessarily a one-for-one replacement for the more legacy DOF preview button.

However you decided to approach it, taking a second to visualize image sharpness before you pack up is one of the easiest ways to be deliberate about your work in the field, when you have time to resolve issues. You sure don’t want to get home and realize you had the perfect composition but it’s not focused properly. Stay sharp out there—and in your frame!

Be well, be brave, be wild,
~Bubbles

 

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