focal length

Cameras record their exposure information automatically. It is called EXIF data.

EXIF data will usually include lens focal length setting at time of shooting, if the lens in question is equipped to provide it.

EXIF is stored along with the picture data, and can be accessed by viewing and editing programs, etc.
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Regards,
Baz
"Ahh... But the thing is, these guys were no ORDINARY time travellers!"