So you got this batch of reference photos and just into doing some image - based modeling. Awesome! Except that you vaguely remember that there was a zoom lens used on the photo camera. So you’d say “not possible” right?
Nothing further from the truth! Canons actually have these smart lenses that report their current focal length to the camera, and this is something you can use to your advantage. Just like these super-expensive films do with their Cooke/i lenses but you never will since your production couldn’t afford them ;-) So, the magic.
Step one, load the pics into some software with decent EXIF support (like Lightroom or iView). Note the lens values and the camera model:
Step two, snoop the internets for sensor sizes:
Step three, enter film back data into your matchmoving software so that it knows how to properly compute FOV from the focal length
Step four punch in the lens millimeters.
Step five - profit!