Master a camera's depth of field

My daughter recently began subjecting me to an anime series in which ... oh, never mind. It doesn't really matter what it's about. The action is lethargic; the plot is incomprehensible; and watching each episode is slow-motion agony.

"If this show moved any slower," I told my wife, "it would be a photograph."

I watch it because I love my daughter.

Thankfully, the show has one redeeming value: The production values are amazing. In particular, every animated scene has an exaggerated depth of field, in which the subject is in sharp focus, but the background is hazy and indistinct. And it was this show that recently reminded me how crucial depth of field can be in crafting powerful photographs.

The three pillars of depth of field

Simply put, depth of field is the region in a picture that's in sharp focus (or, to be more precise, acceptably sharp). So how do you control the depth of field in your own photos? It's easy, once you understand the three factors that contribute to depth of field: aperture setting, focal length, and the distance to your subject.

The aperture setting is the most obvious influence on depth of field. The aperture is the size of the lens opening that determines how much light reaches your camera's imaging sensor. Aperture is measured in f/stops, where lower f/numbers represent bigger openings and higher f/numbers represent smaller openings. What matters to us is that the smaller the aperture's actual opening (or, in other words, the higher the f/number), the greater the depth of field will be.

The focal length of your lens is important too. The focal length is just a measure of your lens's ability to magnify a scene. Your camera might have a 28-105mm lens, for instance, in which the low number is its wide-angle mode and the higher number is telephoto. The more you magnify your subject, the shallower the depth of field becomes. When shooting with a normal or wide-angle lens, you have a lot of depth of field. If you zoom in to a telephoto magnification, your depth of field drops dramatically.

Likewise, macro photography (also known as close-up photography) has very little depth of field, since you are greatly magnifying a small object.

Finally, subject distance is an often overlooked variable in the equation. Your distance to the subject determines how much depth of field you can get in your scene. If you photograph a subject that is far away, the depth of field will be much greater than it is for a subject that is close to the camera. In practical terms, that means the region of sharp focus for a macro shot - where the subject is only a few inches from the camera - is extremely narrow, and you need to focus very, very precisely. If you're photographing something very far away - like a distant horizon - a vast region in front of and behind the image will be in sharp focus.

Calculating depth of field

The best way to get familiar with the effect of depth of field is to experiment. Set your camera to its aperture priority mode, for instance, and shoot a handful of pictures of the same subject with several different f/stops. Get close and move far away. Compare the pictures you take, and you'll start to get a sense of what your camera can do for you.

If you'd rather be proactive, there are tools available on the Web that let you predict depth of field very accurately. The DOFMaster site, for example, is an amazing depth of field resource: http://www.dofmaster.com.

On the site's front page, you'll find a downloadable depth of field program. It's okay, but my advice is to skip right over it and press on to the online depth of field calculator -- a powerful program that runs right in your Web browser: http://www.dofmaster.com/dofjs.html.

Specify your camera model, the lens you're using, and the f/stop, and you'll be rewarded with detailed and accurate information about the actual depth of field available under those conditions. You'll know exactly where to position your subjects, for instance, if you're arranging a scene and want certain elements in or out of focus.

But what if you want depth of field information presented differently? Suppose you want to compare different depth of field values at a range of f/stops for a given distance? "I am shooting a subject 16 feet away. How deep is my depth of field at f/4 and f/22," you wonder? There's a table on the site that gives you this as well: http://www.dofmaster.com/doftable.html.

Like the calculator, you start by entering your camera model and lens focal length. Click Calculate, and a custom depth of field table appears in your browser. Be sure to put this site in your favorites list.

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