Common photography myths (12)

Myth #11: Digital cameras have a deeper DOF than film cameras

September 5th, 2009 - 08:45:51 PM:

When digital cameras with sensors smaller than the usual 35mm format were released, all kinds of confusion was created. First, there was the infamous “focal length multiplication factor”, which is really just a crop. Then came the myth that the same lens, used on a digital camera instead of a film camera, produces a deeper DOF. The myth lives on with full-frame digital cameras.

Technically, this is wrong. The depth of field depends only on magnification (on film or sensor) and aperture. “Magnification” is the relative size of the object in front of the lens compared to its image projected onto the film or sensor. This relationship does not change, no matter what kind of sensor you hold behind the lens or what size it is. With the same magnification and the same aperture, you will always get the same DOF.

For practical purposes, there is some truth to this myth. That's because you usually don't compose your shot for a certain magnification but for a certain framing. For example, you try to fill the frame with some object. With film or sensors of different sizes, this results in different magnifications, and hence in different DOF. With smaller sensors, you shoot at a smaller magnification when you fill the frame with a given object. A smaller magnification leads to a deeper DOF when you use the same aperture. With a larger sensor, e.g. medium format film, the same framing results in a larger magnification, reducing DOF. So DOF indeed appears deeper for smaller sensors and shallower for larger sensors. However, this is not because of the size or nature of the sensor, but because you typically use these formats differently.

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