Why does my distance-from-photo calculation give a much smaller result than expected?

Question

I’m estimating how far a deer is from a trail camera using this formula:

distance to object = (focal length × real object height × image height in pixels) / (object height in pixels × sensor height)

For one image I used:
- focal length: 3.1 mm
- real deer height: 700 mm
- full image height: 1944 px
- deer height in image: 303 px
- sensor height: 20.32 mm

This gives about 685 mm, which is obviously far too small. Am I using the right formula, and does “sensor height” mean the physical vertical size of the camera sensor, not the height of the camera above the ground?

user1375 · Accepted Answer

Sensor height refers to the vertical size of the sensor in mm.

It seems like either your focal length or sensor height are wrong. Probably the sensor height.

A 20mm tall sensor is about the size used in the Canon 1D series profession DSLR, for which no 3.1mm lens has ever been released!

Assuming the stated focal length of 3.1mm is correct you can work out the sensor height if you know the 35mm equivalent focal length (which is often specified in reviews and on the manufacturer's website as if it was the real focal length, should be something like 24-105mm):

sensor height = 

real focal length * 24
-----------------------
equivalent focal length

Again all units should be mm (or should match at the very least).

If the sensor size is a factor of 10 out, then your distance will be 6.9 meters, which sounds more reasonable!

Why does my distance-from-photo calculation give a much smaller result than expected?

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