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I have a DJI drone image. When I use gdalinfo -mdd all myphoto.jpg at the command line I can read the EXIF and XMP tags for the image. I see there are two values for focal length.

EXIF_FocalLength=(8.8)

and an XMP tag that reads

drone-dji:CalibratedFocalLength="3666.666504

What is this calibrated focal length and how does it affect the EXIF focal length value? Should the calibration be added or subtracted from 8.8mm value to get the true focal length? If so, what is the unit of measure of the calibrated focal length value?

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  • \$\begingroup\$ I would guess one of two things (or both) - 1) one of them takes into account the crop factor, so it's an "effective" focal length and the other is the actual focal length, and 2) one is in different units than the other (which, mathematically is really just a variation on #1)... \$\endgroup\$
    – twalberg
    Aug 31, 2023 at 18:14

1 Answer 1

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The focal length is 8.8mm. Often this value is expressed in terms of the equivalent focal length lens that delivers the same angle of view when mounted on a 35mm full frame camera format 24mm height by 36mm length. This factor is 2.73 which translates to a 24mm equivalent lens.

All lenses have residual aberrations. When mapping, distortion aberration spoils. 3666.666504 is a correction factor in pixel units used to improve mapping accuracy.

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