What is depth of field?

Depth of field refers to the area of an image before and beyond the actual focus point that appears acceptably sharp. We've discussed it a little as it relates to aperture, but there's a bit more to the story than that.

Deeper into depth of field

Depth of field is one of the most versatile tools in a photographer's arsenal. By adjusting this one aspect, you can completely change the look of an image.

In portraiture, for example, it's generally a good idea to use a narrow depth of field, so that only your subject is clear and sharp. This draws the viewer's attention to your subject, letting it command the entire image. In the image of the teacup and teapot, you'll notice that only the teapot is in focus — the cup in front and the background behind it are blurred. The quality of blur in an image is called bokeh.

In landscape photography, on the other hand, it's frequently important to use as wide a depth of field as possible, so that landmarks both close to the photographer and farther away will be sharp and in focus. A nice, wide depth of field allows photographic tricks such as the age-old "holding up the Leaning Tower of Pisa" photo, in which the illusion is achieved through having both the person close to the camera and the tower farther away appear in focus.

Let's do some math!

Okay, just kidding. Don't panic, I won't subject you to the math behind depth of field, but if you're really interested, Wikipedia's depth of field article has more formulas and numbers than you can shake a tripod at.

Here's the Cliff's Notes version. Depth of field is affected by three things: aperture/f-stop, lens focal length, and subject distance.

Aperture and depth of field

As we discussed last week, the aperture is the opening in your camera's lens that controls how much light gets through to the camera sensor. The aperture or f-stop number (usually written as f/number) is a ratio of the size of the aperture opening relative to the focal length of the lens.

The important thing to remember is that a larger aperture means a smaller depth of field, and a smaller aperture means a larger depth of field. Yes, that's completely backwards from what you might expect.

Lens size and depth of field

Another thing that affects depth of field is the focal length of your lens. This is really tied into aperture as well, but it's important enough to get its own paragraph. Again, this is somewhat backwards from what logic might suggest at first glance: Basically, the longer your lens, the smaller your depth of field, and vice versa.

For example, if you're using a 55mm lens at an aperture of f/8, you will have a very wide depth of field. If you're using a 200mm lens at the same aperture, your depth of field would be much smaller. The picture of the woman standing in the window was taken with an aperture of f/8 at a focal length of only 18mm, and as you can see, the entire visual depth of the image is more or less in focus. This is an example of a short focal length with a wide aperture.

Subject distance and depth of field

The final issue that affects depth of field is the distance from the camera to the subject. The general rule is that the farther away you are from the subject being photographed, the larger the possible depth of field will be, and vice versa.

It's fairly easy to illustrate this point. From where you're seated, focus your eyes on something in the middle distance, about 20" away. While focusing on this item, you'll probably see that other things some distance in front and behind it are also in focus. Now focus on an upper corner of your monitor, and notice how blurry everything behind it gets.

This is the effect of distance on depth of field. It's also why it's generally possible to get very wide depth of field in landscape photography and very narrow depth of field in macro photography.

Experiment

It's a common refrain in photography: Just experiment until you get it! Depth of field is very fun to play around with, but there are, as they say, a lot of moving parts. Figuring out exactly how all those parts fit together takes time and practice. It's well worth your time to spend an afternoon playing around with lenses of several different focal lengths to see how various settings affect the resulting images.

As always, the rules of photography are meant to be broken. Once you learn how to manipulate them to your advantage, try turning things on their head and breaking the rules a bit. Take a portrait with a huge depth of field. Photograph a landscape where only the smallest area is in focus. Experiment! ( yahoo.com )