Most DSLRs have a small sign showing the precise position of the sensor plane.

In what practical photographic applications would one need to use this information?

I know that focus distances are measured from the sensor plane (e.g. the minimum focus distance specification of lenses).

  • The answer is in a comment on the accepted answer to this question, as well as several macro-related questions. – user35658 Jan 27 '15 at 18:29
  • @user35658 Thanks, that's interesting. So is that how people used to work with macro before TTL metering? Why don't you post an answer? – Szabolcs Jan 27 '15 at 20:35

This is key to measure the exact distance to the objects in a shot.

Especially for tasks in visual effects like:

  • reconstructing real world cameras of an image or a video via match moving techniques to check the calculation of the solver or make sure the distances are correct
  • shooting panoramas or simply extend images to make sure nodal point is correct so that objects are always captured with the same size

Further scenarios:

  • building a camera rig with more than one camera to make sure you can easily stitch the images
  • building a camera microscope to calculate the required lens to get the desired magnification
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    For the first two applications mentioned (match moving, panoramas) it's the distance to the entrance pupil of the lens that is critical, not the distance to the sensor. – Matt Grum Jan 29 '15 at 8:27
  • Thanks @MattGrum! Interesting - what is an entrance pupil? Never heard of it. Can you please explain it a bit more? New question? :) – p2or Jan 29 '15 at 9:50
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    The entrance pupil is where the aperture stop appears to be in a lens, the location coincides with the optical "centre" of the lens, the vertex of the camera's field of view. This point is referred to as the "centre of projection" or "centre of perspective" in certain fields (such as computer graphics), and more commonly the "no parallax point" in photography. – Matt Grum Jan 29 '15 at 10:05

In addition to the excellent answer by Poor, I can add from my own experience in astrophotography. When you want to attach your camera to a telescope to capture photos of deep sky objects, you'll need to know the exact focal point of the telescope objective. In a newtonian reflector, for example, the focal point/plane of the primary mirror falls somewhere in the focusing tube, and to get the image on the camera sensor, you'll attach the camera to the focuser using T-adapter and T-rings, and adjust it so that the sensor plane lies at the focal point of the primary mirror. Knowing the exact position of the sensor plane makes life much easier when you're trying to focus your telescope.


For macro photography it is also very useful to know the exact distance from the sensor to the object. That allows you to measure sizes from the photos taken in a very precise way.

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    Can you elaborate on how? I can see how it could be done for a thin lens, assuming we know the precise focal length. But with a complex optical system made of many (not so thin) lens elements, it's not clear to me how this would be done. – Szabolcs Feb 4 '15 at 3:44

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