. . "signals=(j-k)-K.times.{(h-i)+(l-m)} RFsignals=j+k+swhere a to t are output values of the light receiving areas on the photodetector device and K is a coefficient.Referring to FIGS. 6A and 6B, a modification of the optical head according to the present invention is explained.An optical head 2, shown in FIGS. 6A and 6B, shapes the beam spot using an anamorphic prism.In an optical recording medium re" . .