Depth of Field of Optical Systems-Knowledge Encyclopedia-Shenzhen Kai Mo Rui Electronic Technology Co. LTD

Depth of Field of Optical Systems

Source：Shenzhen Kai Mo Rui Electronic Technology Co. LTD2026-07-13

Thinking Question:

Why do many photographs appear sharp both in the foreground and background?

Theoretically, only the object plane conjugate to the image plane can form perfectly sharp images. Images of all other object points turn into blur circles. However, if the angular size of such a blur circle as seen by the human eye is smaller than the eye’s minimum resolution angle of 1 arcminute, the human eye perceives it as a single sharp point. Under this condition, the blur circle can be regarded as the planar image of a spatial point.

As shown in the diagram, when blur circles Z₁’ and Z₂’ are small enough to be recognized as single points, all objects within the spatial depth range from B₁ to B₂ form clear images on the image plane. This spatial depth is defined as the depth of field (DoF).

Key conclusions: Shorter focal length yields larger depth of field; greater distance to the focused plane expands depth of field; smaller entrance pupil diameter increases depth of field. In addition, the far depth of field is always larger than the near depth of field.

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Further Thinking Questions

Does a farther focused plane always produce a larger depth of field?
Are the above formulas applicable to all types of optical systems?
How to control depth of field during photography?

Answer to Question 1

Suppose the focal length is 50 mm, entrance pupil diameter is 12.5 mm, and the allowable blur circle Z' ≤ 0.05.

The second configuration delivers a larger depth of field. Therefore, to maximize depth of field, set the far plane at infinity.

Answer to Question 2

Only the first two sets of formulas are universally applicable; the subsequent formulas apply exclusively to photographic systems.

Answer to Question 3

The f-number of a lens is the reciprocal of its relative aperture.

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To capture images with shallow depth of field (e.g., portrait shots): Use a long focal length, large relative aperture (small f-number), and focus at a close distance.

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To capture images with deep depth of field (e.g., landscape shots): Use a short focal length, small relative aperture (large f-number), and focus at a far distance.

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