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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Posted on: May 11, 2007

5 Comments on Framed and Exposed: Death, Taxes, and Focal Lengths

  1. but in 25 years of photography I don’t remember ever being taught, or even hearing, that DOF is related to focal length. It’s always been clear to me that DOF is related to aperture and film size (film now being sensor size). Who’s been teaching you?

    By the way, I would prefer white cameras if Nikon and Leica made them. Less heat in the sun. Black is “more professional” because it doesn’t reflect in the sun, calling more attention to the camera than necessary.

    And it’s definitely not true that the best shots present themselves when you don’t have a camera with you. The truth is that the best shoots are always ruined by the rare foul-ups that plague photography, such as film streaks or mechanical shutter failure and the like!

  2. In the scenario described, Depth of Field is virtually the same for both focal lengths. All other factors being equal (except for focal length), and the longer focal length has the shorter depth of field. Put both cameras at the SAME distance from the subject, enlarge the image from the shorter lens to match image size to the longer lens and the depth of field for the longer lens will be shallower compared to the shorter lens (perspective will be different as well).

  3. DOF is determined by lens aperture, lens focal length, distance to the subject and (to a significantly lesser degree) the acceptable circle of confusion. In fact, DOF is often stated as being computed from lens aperture and subject magnification (on the film/sensor). Subject magnification is determined by lens focal length and distance to the subject. Since Mr. Long changed *two* of the above factors (lens focal length as well as distance) to maintain the same subject size, he would, in fact, end up with the same depth of field. If he had taken an image of the roof vent with the longer lens from the same position as with the shorter lens, the DOF would have been significantly reduced.

    Also note that film/sensor size does *not* directly affect DOF. With a smaller capture medium you must use shorter lenses from the same distance in order to capture the same image. The shorter lenses result in greater DOF – not the smaller sensor.

    Richard Baker
    Sunnyvale, CA

  4. I just got it.. it looks as if the building is equally defocused, but just because it’s too far away, an infinite focues, this wouldn’t hold true if it was closer.

  5. Dear Mr. Long: I appreciate the experiment you have done in this article, but you forgot a 3rd element that affects depth of field: camera-to-subject distance. As you focus on objects closer and closer to you, you lose depth of field. When you took the two photos of the chimney, you changed two variables: 1- the focal length, AND 2- you changed your camera to subject distance to keep the size of the chimney the same for both photos. To scientifically prove or disprove a hypothesis, you should only change 1 variable in an experiment. If you had taken both photos from the same position, and only changed the focal length, you would see that there is in fact a change in the depth of field. I have done my own experiments, and have photos I am willing to share with you. Depth of Field calculation software also supports my theory. I myself may have overlooked something, and wouldn’t mind some constructive criticism. I have your 6th edition, and I am enjoying it immensely.
    Sincerely, Rob Rigole, Forensic Photography Instructor, Ontario, Canada. rob.rigole@ontario.ca

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