Why is depth of field greater behind the focus point than in front?
Asked 12/10/2018
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I understand that for a given focus setting there is a tolerance on either side of the exact focus distance where blur remains acceptable. But depth of field is usually not symmetrical: the region behind the subject is often larger than the region in front.
If the acceptable blur circle at the image plane is the same on both sides of the focus point, why doesn’t that translate into equal distances in front of and behind the subject? What causes the far depth of field to extend farther?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Elaborating on Aganju's comment:
An object of size S at the far end (D+E) appears smaller on the film plane than an equal-sized one at the near end (D-E).
So the film-plane blur-blob of the far object is smaller than the near-object one, making it appear sharper, and meaning that (based on the film-plane blob size) you can go further out until you reach the same level of blur.
With short focus distances, it's close to symmetric. The further you go to infinity, the more asymmetric the range becomes.
Originally by user79539. Source · Licensed CC BY-SA 4.0
user79539
7y ago
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The key is that equal blur on the image plane does not correspond to equal distance changes in object space.
An object the same physical size placed slightly farther away appears smaller on the sensor/film than an identical object placed the same amount closer. So although defocus may produce a similar blur circle at the image plane on both sides of the focus distance, that blur is spread over a smaller image of the farther object. It therefore looks acceptably sharp over a larger distance range behind the focus point.
For nearer objects, the image is larger, so the same amount of defocus is more noticeable. That makes the near depth of field shorter.
At close focusing distances, depth of field is closer to symmetrical. As focus distance increases, especially toward infinity, the asymmetry becomes stronger, with much more depth of field extending behind the focus point than in front.
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AI7y ago
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