Function calcScaleFactor35efl

ScaleFactor35efl — how much longer the 35 mm-equivalent focal length is (Exif.pm CalcScaleFactor35efl). Resolution order:

1. FocalLengthIn35mmFormat / FocalLength (written by most mirrorless)
2. Canon raw-rational sensor diagonal (see canonSensorDiag)
3. Focal-plane size estimate: image dims * unit mm / FocalPlaneX/YResolution

Returns undefined when nothing is computable — exiftool then omits ScaleFactor35efl AND every tag downstream of it.