Depth-of-field is defined as that area within a photograph which is in sharp focus. By understanding and implementing a few basic concepts, you can take complete control of your image-making. When you determine depth-of-field manually, you are able to render any scene in a myriad of ways–from isolating a subject against a soft, blurred background to maintaining sharp focus front to back. This lesson on depth-of-field is the second half of understanding exposure (see Photo Tip #1: Determining Exposure).
Let’s review some basics. There is an inverse relationship between aperture (lens opening) and shutter speed (how long light strikes the sensor or film). Once proper exposure has been determined, and you change one setting, you have to adjust the other to compensate for the increase or decrease in light reaching the camera’s sensor. Aperture controls depth-of-field. Large lens openings (f/1.8 or f/2) produce very shallow depth-of-field, that is, only a very limited area is in sharp focus and the rest of the photograph is a soft blur. A small lens opening (f/16 of f/22) will produce sharp focus throughout most or all of the photograph. At different times, one or the other may yield stronger or more pleasing results. You should familiarize yourself with your camera’s depth-of-field preview, which allows you to see what is in focus at a particular aperture before you take the photograph. If your subject is static or posing, you can always take the photograph, view it, and adjust accordingly. But if you’re photographing a fast-paced event, say, a sprinter dashing across the finish line for a gold medal, you may only have a split second and one chance to get your shot. Your depth-of-field preview allows you to pre-determine exactly which areas in the photograph are sharp and which are not–so when the subject moves into position, you are ready to depress the shutter.
Also, you will find a depth-of-field scale on your camera’s lens barrel. It tells you, in feet and meters, what area is in sharp focus at all the various f-stops. Using my 50 mm f/1.8 lens, if I focus on a subject 10 feet away with an aperture setting of f/4, the area in sharp focus, according to my scale, is between about 9 and 12 feet. If I stop down to f/16, that area in sharp focus increases to between approximately 6.5 and 28 feet, for a much greater depth-of-field. Keep in mind that by stopping down to f/16, you have decreased your exposure by 4 stops, and so you need to reduce (slow) your shutter speed, and/or increase your ISO setting a total of 4 stops. That is, if you are shooting with an ISO of 100 at 1/1000 second at f/4, you need to set your shutter to 1/60 second–or increase the ISO setting (from ISO 100 to 1600, etc.) to compensate for a much smaller lens opening
A lens’ focal length also affects depth-of-field. The shorter the focal length, the greater the depth-of-field. The longer the focal length, the shallower the depth-of-field. I have a 24 mm f/2.8 super-wide angle lens (89 degree field of view) that I use with my film camera. I love the lens for its sharpness throughout the image area, even wide open. I also love it for its distortion, which becomes more exaggerated as you move closer to the subject.
In the above photo, Fly on a Leaf, I used a 200 mm f/4 Canon lens with a small (12 mm) extension tube to increase magnification. Telephoto lenses have an inherently shallow depth-of-field and are wonderful for isolating a subject. Using the extension tube allowed me to magnify the subject (in this case, the fly) and still maintain enough distance to preserve a soft, pleasing background.
The final factor that determines depth-of-field is the distance between camera and subject. Using the same example as above, if I am shooting a subject 10 feet away at f/4, the area in sharp focus lies between approximately 9 and 12 feet. If my subject is 30 feet away at f/4, that area in sharp focus is between approximately 23 and 45 feet. The basic premise is: when you focus on a subject, 1/3 of the area that is sharp falls in front of the subject and 2/3 falls behind it. So needless to say, when you start getting very close to a subject–using extension tubes or macro lenses–depth-of-field is sometimes a fraction of an inch. This can provide for all kinds of interesting challenges. At such high magnification, or with long telephoto lenses, you want to use a tripod and cable release. Exposures are often long, and it’s easier to focus and compose the photograph, and it assures that critical areas will be sharp.
The top photo, Pebble Creek, Yellowstone, was taken using a 35-105 mm f/3.5 zoom on my (tripod-mounted) Canon film camera. I zoomed wide, stopped way down for increased depth-of-field, and used a slow shutter speed to soften the flowing water. Fly… was shot hand-held. The fly wasn’t going to stick around long enough for me to set up a tripod, so I seized the moment. Notice the telephoto’s ability to isolate the subject using shallow depth-of-field. This works well with both portraiture and photographing wildflowers.
A note on tripods and image-stabilization: Turn off the image-stabilizer when mounting the camera on a tripod. When shooting long exposures (1/15 second or slower), I recommend turning off your camera’s autofocus and focusing manually, and then locking up your mirror. Remember to bracket.
I welcome your comments, questions, and suggestions.
Until next time, happy image-making…