a focus detection unit configured to performing focus detection on {HEAD ~ the basis} of {TAIL ~ the phase difference detection signal} 
of
a focus detection unit configured to performing focus detection on { HEAD ~ the basis } of { TAIL ~ the phase difference detection signal }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL