Does depth of field stay the same if framing and aperture are fixed but focal length changes?
Asked 9/16/2014
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2 answers
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If I keep the sensor format, subject magnification/framing, and f-number the same, then change focal length and camera distance to maintain the same subject size in the frame, will depth of field remain the same? For example, on APS-C at f/22, a 300mm lens at 2m appears to give about the same depth of field as a 600mm lens at 4m or a 150mm lens at 1m. Is that a real theoretical result, or just a coincidence from a depth-of-field calculator?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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You are simply making the wrong assumptions. Just "playing around" and thinking you see a correlation doesn't mean that it actually exists. You've just fixed the sensor format at APS-C, aperture at f/22 and tried varying the focal length and subject distance a bit and thought that you saw a correlation.
However if you try these settings for example: f/2.8, Canon 1ds mk II, 12 mm and 0.5 m. You will get a total DOF of 0.31 meters. Change the settings to f/2.8, Canon 1ds mk II, 1200 mm and 50 m and you will get a total DOF of 0.29 meters.
The presumption that the DoF doesn't depend on the subject distance divided by the focal length isn't correct. You can further convince yourself of this by looking at this excellent answer by jrista, by looking at the Wikipedia article about DoF or by simply look at the formulas used by the Depth of Field calculator to calculate the DoF. They all cover or derive from the same formula albeit to different degrees of approximation:
See the various sources cited above for its explanation.
Originally by user21986. Source · Licensed CC BY-SA 4.0
user21986
11y ago
0
Generated from our catalog & community — verify before relying on it.
For practical photography, yes: if sensor size, f-number, and subject magnification (same framing) are held constant, depth of field is essentially the same regardless of focal length. Changing to a longer lens and moving farther away cancels out the change in focal length, so the subject-area DOF stays about constant.
That’s why a DOF calculator can show similar results for 75mm at 0.5m, 150mm at 1m, 300mm at 2m, and 600mm at 4m when the framing is matched.
However, it’s not perfectly invariant in every case, and calculators may show small differences, especially at extreme focal lengths, distances, or very wide apertures. Also, perspective/background rendering changes a lot when you move the camera, even if subject framing stays the same.
For focus stacking, switching focal lengths is usually not a useful shortcut. Focus stacking works best with the camera position and framing kept consistent, especially in macro work. If you change focal length by moving the camera, you alter perspective and background compression, which can make stacking less consistent even if the nominal DOF is similar.
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