Common photography myths (15)
Myth #14: Depth of field around the focus distance is 1/3 towards the camera and 2/3 away from the camera
September 5th, 2009 - 08:53:53 PM:
That's only a very rough rule of thumb, valid for medium magnifications. Depth of field is very variable. At larger magnifications, e. g. larger than 1:15 (!), DOF is roughly symmetric, i. e. front DOF and rear DOF are almost the same. As magnification becomes smaller (typically due to increased distance), DOF becomes more and more asymmetric. At some point there is indeed a 1:2 relationship between front DOF and rear DOF. But if you decrease magnification more, rear DOF will become larger and larger. At some distance, rear DOF will even reach infinity. We call this point “hyperfocal distance”. Also see the explanation on the optical formulas page.
As an example, here's a short table of front and rear DOF for 50 mm focal length and f/4 for various distances:
| Distance [m] | Front DOF [m] | Rear DOF [m] | Front:rear ratio |
|---|---|---|---|
| 0.5 | 0.012 | 0.012 | 1:1 |
| 1 | 0.046 | 0.05 | 1:1.09 |
| 5 | 0.968 | 1.579 | 1:1.63 |
| 10 | 3.243 | 9.231 | 1:2.85 |
| 15 | 6.279 | 38.571 | 1:6.14 |
| 20 | 9.796 | ∞ | 1:∞ |
Morale: If you want to know depth of field, use a DOF calculator or the
DOF preview function of your camera.
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