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Physical explanation for camera DOF (depth of field)

  1. Nov 11, 2006 #1
    Depth of field stands for the range that is sharp on a photograph.

    The bigger the diafragma (camera opening) the smaller the range that will be sharp on the picture. A bigger diafragma also means a shorter shutter opening (so the picture doesn't become too bright).

    I'm interested in the physical explanation of this effect. The explanation on wikipedia is much too technical for me to follow.

    I am not a physicist I just enjoy reading about it, so please explain as if to someone who has no idea about the field, thanks.
  2. jcsd
  3. Nov 11, 2006 #2
  4. Nov 11, 2006 #3


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    Staff: Mentor

    See the attached diagrams.

    For perfect focus (top diagram), the image has to be located exactly at the sensor plane. Only one object distance can produce this, for a given focal length and lens-sensor distance.

    When we move the object to the left (middle diagram), the image also moves to the left, in front of the sensor, and we get a circular spot on the sensor, which is often called the blur circle. In practice, we can tolerate a blur circle up to a certain maximum size, which corresponds to a certain maximum object distance.

    When we move the object to the right (bottom diagram), the image also moves to the right, behind the sensor, and again we get a blur circle. The maximum tolerable blur circle size corresponds to a certain minimum object distance.

    The distance between these maximum and minimum object distances is the depth of field.

    If we make the lens effectively smaller by putting a diaphragm in front of it, but keep the object and image distances the same, we get a smaller blur circle. To get the same maximum blur circle size as before, we need to increase the maximum object distance and decrease the minimum object distance, which increases the depth of field.

    Attached Files:

    • dof.gif
      File size:
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  5. Nov 11, 2006 #4
    In theory, every lens has zero depth of field (focusing light into an image from only one distance exactly) and a pinhole-camera has infinite depth of field (rays from any distance produce an image). In practice, there is always a continuum (ie. you can close the aperture slightly to get a slightly more pinhole-camera-ish result).

    Incidentally, this is also why big heavy expensive cameras tend to produce photos with aesthetically blurred backdrops, whereas in a small digicam's photo your eye is distracted by the detail in every item of litter in the background.
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