EXIF keeps two focal lengths. FocalLength (0x920A) is the real optical focal length of the lens when the picture was taken, in millimeters, stored as an unsigned rational. FocalLengthIn35mmFormat (0xA405) is what that focal length would have to be on a 36 × 24 mm frame to give the same field of view. It is a SHORT, so whole millimeters only, and the EXIF spec calls it FocalLengthIn35mmFilm. A value of 0 means unknown.
The two numbers only match on full-frame cameras. Everywhere else the ratio between them is the crop factor.
Crop factor, worked through
A 35 mm lens on a Canon APS-C body (crop 1.6) frames like a 56 mm lens on full frame. On Nikon, Sony or Fujifilm APS-C (1.5) the same lens gives roughly 52 mm. Micro Four Thirds doubles it. Phones are the extreme case: their sensors are tiny, so the main camera has a real focal length of a few millimeters while reporting an equivalent around 24 to 26 mm.
Values like those of a recent phone main camera, written to a test JPEG:
$ exiftool -G1 -s -FocalLength -FocalLengthIn35mmFormat -ScaleFactor35efl -FocalLength35efl photo.jpg
[ExifIFD] FocalLength : 6.9 mm
[ExifIFD] FocalLengthIn35mmFormat : 24 mm
[Composite] ScaleFactor35efl : 3.5
[Composite] FocalLength35efl : 6.9 mm (35 mm equivalent: 24.0 mm)
$ exiftool -G1 -s -n -FocalLength -FocalLengthIn35mmFormat -ScaleFactor35efl photo.jpg
[ExifIFD] FocalLength : 6.86
[ExifIFD] FocalLengthIn35mmFormat : 24
[Composite] ScaleFactor35efl : 3.49854227405248ExifTool rounds the printed focal length to one decimal, so the stored 6.86 shows as 6.9 mm. Use -n if you are sorting or comparing.
When the equivalent is missing
Some full-frame cameras skip FocalLengthIn35mmFormat entirely; a Canon EOS RP JPEG with a 50 mm lens has only FocalLength 50.0 mm. ExifTool then estimates the crop from other clues, such as FocalPlaneXResolution and the image width, and still produces Composite:FocalLength35efl ("50.0 mm (35 mm equivalent: 50.1 mm)" for that file). Treat the computed figure as an estimate. Those clues describe the original frame, so a file that was cropped before export can come out wrong.
A zoom lens records the focal length actually used, not its range. The range is in LensInfo, covered under lens model.
Reading and changing focal length
To check these values on a phone, the Android guide covers the built-in gallery apps. In a JPEG or TIFF, the EXIF editor writes both tags, as 35 or 35 mm.