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I'm trying to basically find the width and height of someone's face

I know the following:

  • distance from face to camera in mm
  • the sensor height in mm
  • the focal distance in mm
  • distance in pixels distance between my points
  • Height/Width of image in pixels

How can I accurately get the width and hight measurements in MMs. I took a look at this. Could I use it for width measurements as well? Is there a more accurate way to go about doing it?

enter image description here

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    \$\begingroup\$ This question is basically a geometry question, not a photography question. Having said that, one will be severely disappointed when trying to use a consumer grade camera (this includes even very expensive "pro" models and lenses) designed to produce photographs rather than lab grade equipment designed to produce accurate measurements. \$\endgroup\$
    – Michael C
    Commented Dec 17, 2020 at 4:18
  • \$\begingroup\$ With cameras designed to take creative or documentary photographs, too many things are approximated to make them accurate measuring instruments: focal lengths when focused to infinity are rounded to the nearest marketing size, focal lengths change as the same lens is focused closer than infinity, focus shift with changing apertures, geometric distortion that makes a lens' focal length slightly different at different points in the image field, etc. all conspire to make scientific measurements inaccurate using such cameras and lenses. That's all just with prime lenses. \$\endgroup\$
    – Michael C
    Commented Dec 17, 2020 at 4:19
  • \$\begingroup\$ For other scientific pursuits, the same is true of exposure time/shutter speed that is not as precise nor as consistent from one shot to the next as needed for scientific observation. Apertures aren't accurate enough or consistent enough from shot to shot, either. \$\endgroup\$
    – Michael C
    Commented Dec 17, 2020 at 4:19
  • \$\begingroup\$ Even if this question is not off topic, it is certainly a duplicate of dozens of other more or less identical existing questions here. They're not that hard to find, even with SE's poor search engine. \$\endgroup\$
    – Michael C
    Commented Dec 17, 2020 at 4:19
  • \$\begingroup\$ Related: Why isn't my computation of the actual size of an object in a photograph matching the expected value? and Calculate object size when I have physical pixel size \$\endgroup\$
    – Michael C
    Commented Dec 17, 2020 at 4:42

1 Answer 1

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You draw an imaginary triangle, call it the image triangle. The base of the triangle is that portion of the image that correlates to your subject’s physique. In other what is the length in millimeters of the image of the face. You can count pixels, and if the size and spacing (from spec. sheet) is known you can derive this value. This distance is the length of the base of image triangle.

The height of this triangle is the focal length of the lens. You can now draw this triangle image triangle with pencil and paper. You can use a protractor or trigonometry to find the size of the angles in degrees.

Find the ratio of height to base. Say the focal length was 80mm and the face height covered 8mm. Find the ratio 8 ÷ 80 = 0.10.

Now we compute the object triangle. It will be a larger triangle however all angles will be identical to the image triangle. In other words they are similar triangles. The base of the object triangle is an imaginary line, center of camera lens to subject. Say this subject distance is 10 feet = 120 inches. The face of the subject measures 120 x 0.10 = 12 inches. (a ratio is dimensionless).

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  • \$\begingroup\$ What if the face being measured isn't perfectly flat nor perfectly perpendicular to the camera's image plane? Have you ever seen a perfectly flat face? \$\endgroup\$
    – Michael C
    Commented Dec 19, 2020 at 1:19
  • \$\begingroup\$ Seems this would reduce subject measurement accuracy. A South African Game Preserve ask me to help measure horn length of Cape Buffalo from image taken with known subject distance and focal length. This method accurate to fraction of inch with correction factor based on test with archery target. \$\endgroup\$
    – Alan Marcus
    Commented Dec 19, 2020 at 5:17
  • \$\begingroup\$ Thats some good accuracy, I will try that method. Im using the front facing camera on the new iPhone with its 3d dot mapping tech to get the distance from the face to the camera accurately \$\endgroup\$
    – user3015221
    Commented Dec 21, 2020 at 5:05

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