Why does depth of field often extend farther behind the focus point than in front?
Asked 2/26/2017
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I often see depth of field diagrams showing less in-focus area in front of the focus distance and more behind it, sometimes described as a 1/3 front and 2/3 rear split. Why does this happen, and is that ratio always true?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Why is the area in focus in front of the focus distance narrower than behind it?
It isn't. Not always. Just usually for landscape shooters using wider lenses aiming somewhere not that close. :) The 1/3-2/3 proportion rule is basically a rule of thumb that does not apply in all cases.
The larger your aperture, longer your lens, or closer your shooting distance (i.e., thinner the depth of field), the more that proportion will actually range closer to 50/50 (think: hyperfocal distance).
See also:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm (in particular, the table showing how focal length varies the proportion of front/back in the DoF.
http://www.photopills.com/articles/ultimate-guide-depth-field#step8
Originally by user27440. Source · Licensed CC BY-SA 4.0
user27440
9y ago
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Generated from our catalog & community — verify before relying on it.
It’s not always true. The common “1/3 in front, 2/3 behind” rule is only a rough guideline, not a law.
The front/rear depth-of-field balance changes with focus distance, focal length, and aperture. At close distances, or with longer lenses and wider apertures, depth of field gets very thin and the split moves closer to 50/50. At longer focus distances—especially with wider lenses used for landscapes—the far side often appears to extend more.
Why? Depth of field is based on how quickly blur changes with distance. As subjects get farther away, a given change in distance produces a smaller change in image geometry, so acceptable sharpness tends to stretch farther behind the focus plane than in front.
There is no fixed ratio that works in all cases. For any given lens and aperture, the near/far depth-of-field ratio varies with the chosen focus distance. The exact result also depends on assumptions behind depth-of-field calculations, such as viewing size and the acceptable circle of confusion.
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