10 Practical uses for the Depth of Field Preview Button

“This button just makes everything darker, it’s useless.” I’ve heard this many times from beginning as well as experienced photographers. The first part of what they say is true but the second isn’t; they’ve yet to discover the many uses of the depth of field preview button.

Not all cameras have one and if you’re new to photography you may not even know what it is. When you’re looking through your camera, what you see isn’t necessarily what you’ll get on film; if you’re using a single lens reflex camera, as you look through the viewfinder you’re looking through the lens at its widest aperture setting, which may or may not be the aperture chosen for your picture. When the preview button is pushed, the lens is mechanically closed down to the chosen aperture. This way the photographer can view the depth of field that will actually be recorded on film. But when the lens aperture is closed down, less light is allowed through and the viewfinder can get very dark, especially if a small aperture opening is used, and details can be hard to see. This is where a lot of photographers give up on using the DOF preview button. But as you’ll see, you don’t always have to use the DOF preview button at f/22 nor do you always need to see every detail.

Beyond making “everything darker”, the DOF preview button has several practical uses.

Detecting hotspots. When doing close-up photography one of the prime concerns is the background. Bright spots are very distracting and can go unseen when viewing the image while the lens is wide open. When you push the preview button the viewfinder will get dark. But that’s OK; it makes it easier to detect any distracting hot spots that may end up in the final image.
Detecting dark spots. Just as hot spots can be distracting, dark spots, especially in the foreground of landscape images, can be just as distracting. Many landscape photos are taken at an aperture of f/16 or f/22 to obtain the maximum depth of field. By pushing your preview button while the lens is set to f/22, the viewfinder will get dark, maybe too dark to really tell what’s going on. But by previewing at f/8 or so, you’ll be able to easily see where shadow areas in your foreground might be too dark for the film to record. The rest of the image will still be relatively bright but these darker areas will become readily apparent. You can then decide if fill-flash, a reflector, or perhaps a different composition will be needed to eliminate these shadow areas.
Detecting unwanted elements (or “Intruder alert”). How many times has your film come back from the lab with some out of focus branch or blade of grass sticking part way into your frame? You can stop this from happening by using your preview button before you trip the shutter. When you preview the depth of field, run your eyes around the edges of the frame and look for intruders trying to poke their way in and ruin your image. Also look for intruders trying to sneak into your backgrounds. Remember, you may not notice these things while looking through the lens while it’s wide open because of the shallow depth of field. Use your DOF preview button and catch these intruders. Again, the viewfinder will get dark but you’re not trying to see all the detail in the image, you’re just looking for intruders.
Finding the best background. Especially in close-up and portrait photography, hold down the preview button while changing the aperture until you get the best combination/compromise for a pleasing background and the focus needed for your subject.
Placing a graduated neutral density filter. Grad ND filters are essential to successful landscape photography but they need to be placed correctly so that their use isn’t obvious. Holding down the preview button while moving the filter in front of your lens will make this much easier and more precise.
Optimize the range of focus for your main subject. Whether your doing portrait photography, wildlife photographing wildlife or wildflower photography, you can use the preview button to see if everything you need to have in focus will be in focus, from the stamens and stigmas on flowers to the nose and eye of an animal or portrait subject.
Pre-visualize underexposure. By looking at a scene through your viewfinder and setting the lens one or two stops smaller than the widest aperture on your lens, you can use the DOF preview button to get an idea of what an image might look like if you underexpose it a little. You may want to underexpose to create a different mood and this will help you see what that might look like. Conversely, you can set your lens to f/8 or so and push the button, let your eyes adjust, and then start opening the lens to get an idea of what overexposing a scene might look like.
Detecting lens flare problems. This is similar to detecting intruders. When photographing into the sun, especially when the sun is low in the sky, you run the risk of lens flare, which results from direct light bouncing around the elements of your lens. By previewing the scene, lens flare will become more apparent. You can then decide if you want to do something to mitigate it, like using a wider aperture, changing composition or shading your lens. You may find that you like the effect. You can also preview the “star burst” effect that results from using small aperture openings while including the sun in the image.
Detecting vignetting from stacking filters and lens hoods. With wide-angle lenses, there’s a greater risk of vignetting, or cutting off the corners, while using filters, especially if you stack filters or put your lens hood on the filter. By previewing and paying special attention to the corners of the frames, you’ll be able to know ahead of time if there will be a problem. (Be aware that most viewfinders only show about 90% of what will end up on film. If in doubt, be sure to run tests with all combinations of filters you may wish to use. Photograph a blank wall or the blank sky and when you get your film back you should see if you’re going to have any problems.)
Use the DOF preview button as a compositional aid. By making the image darker you also eliminate much of the detail, leaving only forms and shapes in your viewfinder; in other words, just graphics. You can then arrange these graphics in a pleasing manner. This works especially well when you’re working with light and shadow or moving water.

As far as dealing with a darkened viewfinder, letting your eye adjust is probably the first thing you should try. There are several techniques to help with this. First, don’t start off with your lens at its smallest opening; hold the preview button down and close down your lens gradually. Second, use an eyecup on your viewfinder to help cut out stray light. Third, throw a jacket or dark cloth over your head like the users of view cameras do.

Your depth of field preview button can do much more than just strain your eyes. If your camera has this valuable tool, be sure to use it to its full potential.