Framed and Exposed: Death, Taxes, and Focal Lengths


While we can all count on the inevitability of death and taxes, photographers have had a few other certainties to depend on: black cameras look more professional than silver ones; the best shots present themselves when you don't have a camera with you; and digital zoom is always a bad idea. And for the last century and a half, photographers have also been taught that the focal length choice they make has an impact on the depth of field in their scene. In this article, I'll show you why that long-held maxim is actually not true.

Before you start dreading the need to re-learn a bunch of old habits, relax. While the old theory may have been technically incorrect, the practical upshot has been completely valid. This article is not so much about changing your hands-on technique; rather, it gives you a more accurate explanation of what happens to depth of field when you choose one focal length over another. Your everyday practice -- use longer lenses to get apparently shallower depth of field -- will still apply, but after reading this article, you might have a different understanding of why the depth of field appears different with different focal lengths.

Figure 1 shows two images that are framed the same way but shot from different positions, using different focal lengths. The image on the left was shot using a 75mm lens at f/5.6. The image on the right was shot from farther away using a 235mm lens, also at f/5.6. In both images, the goal was to keep the top of the chimney roughly the same size.

Figure 1. I shot the wide-angle image on the left at 75mm, and the more telephoto image on the right at 235mm. Click on the image for a larger version.

Because of the changes in shooting position and field of view, the backgrounds in the images look very different. What's more, there appears to be less depth of field in the right-hand image.

Traditionally, you'd say that the right-hand image has less depth of field because it was shot with a longer focal length, and longer focal lengths produce shallower depth of field.

However, if you zoom in to each image and look at the background detail, you'll see that the amount of softness and defocusing is not as different as it appears when viewing the image normally. That tall brown building in the background is the Bank of America building. Next to it is the Transamerica pyramid. In Figure 2, I enlarged both images so that the Bank of America building is roughly the same size.

Figure 2: I enlarged a particular background detail until it was about the same size in both images.

On the left, you can see the enlarged version of the wide-angle image; on the right is the enlarged telephoto image. Because the wide-angle image had to be enlarged more than the telephoto image, there are some slight differences in visible detail and contrast. However, even with these differences, it's obvious that both images are equally defocused. The telephoto image is not blurrier than the wide-angle image, as you would expect if it were true that longer focal lengths yield shallower depth of field. So why does it appear as if the telephoto image has shorter depth of field?

When you use a longer focal length, the background elements in your image always appear larger than when you use a shorter focal length. Because they're larger, it's easier to see exactly how much they've been defocused by your aperture setting. When you shoot with a shorter focal length, background elements are usually rendered small enough that you can't see how much they've been defocused by your shallow depth of field.

There are really only two factors that impact depth of field: aperture choice and sensor size. A wider aperture yields a more shallow depth of field. At any given aperture, a smaller sensor will yield deeper depth of field than a larger sensor, just as a piece of 35mm film yields deeper depth of field than medium format. You can't do anything to change your sensor size, so aperture choice is the only factor you have to control the actual depth of field in your image.

However, you can improve apparent depth of field by paying attention to how much background is visible in your scene. To achieve the look of a shallow depth of field, frame your shot so that large background elements are visible. Since it will be easy to see that these background elements are defocused, your image will appear to have very shallow depth of field. Of course, one of the easiest ways to do this is to use a long focal length. But even when using wide-angle lenses, if you can place a background element closer to the lens, you might be able to achieve a shallower look. It's worth remembering this lesson when thinking about your final framing.

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