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