obtaining a focus position of {TAIL ~ the object} with respect to {HEAD ~ the optical system} based on the evaluation function calculation values 
obtaining a focus position of { TAIL ~ the object } with respect to { HEAD ~ the optical system } based on the evaluation function calculation values
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH