f is the focal length of {HEAD ~ the taking lens} of {TAIL ~ the photographic film package}, 
of
f is the focal length of { HEAD ~ the taking lens } of { TAIL ~ the photographic film package } ,
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH