Common photography myths (10)

Myth #9: Tele-photo lenses have a shallow DOF

September 5th, 2009 - 08:43:30 PM:

In this short form, this statement is wrong. The accurate version would be “Tele-photo lenses used at short distances and at wide apertures have a shallow DOF”. Maybe too inconvenient, but with enough omissions even a correct statement can turn into an incorrect one.

Here're the facts: DOF is a function of aperture and magnification (on film or sensor), and magnification is a function of focal length and distance. When you shoot a longer lens from a greater distance you can get the same magnification as when shooting a shorter lens from a closer distance. When you also use the same aperture, you will get the exact same DOF. So DOF with a longer lens will only be shallow when you also shoot from a short distance.

Many super-tele lenses can't focus particularly close. You often don't get magnifications greater than 1:6 or 1:8. For example, a 600/4 shot at its closest distance of 6 m will have a deeper DOF than a 100/2.8 shot at 80 cm (and at f/4), even though it's six times as long.

Often you get a smoother out-of-focus background with a longer lens. But that's not because DOF is shallower. The longer lens with its narrower angle of view just sees a smaller section of the background, and it's easier to find a smooth section of the background when it's small rather than large. For example, you have to turn a 600 mm lens by only 4° to get a completely new background. To do the same with a 100 mm lens you have to turn it by 24°. So if you're after a smooth background, using a longer lens may be a good idea. But if you're actually after a shallow DOF, using a longer lens may not be enough.

There are a number of general statements about photography passed off as “the truth”. They are repeated again and again in introductory texts about photography and on the Internet. Repetition, however, doesn't make a false statement true. Here are the most common myths I've encountered: