Optical film, retarder film, and method for manufacturing same

An optical film formed of a resin C including a copolymer P containing a polymerization unit A and a polymerization unit B, wherein

• the optical film includes a phase separation structure that expresses structural birefringence,
• the phase separation structure includes a phase (A) containing as a main component the polymerization unit A and a phase (B) containing as a main component the polymerization unit B,
• an absolute value |n(A)−n(B)| of a difference between a refractive index n (A) of a polymer (A) formed of the polymerization unit A and a refractive index n(B) of a polymer (B) formed of the polymerization unit B is 0.12 or more, and
• a value of Rth/d is 0.5×10−3 or more wherein the Rth represents a thickness-direction retardation of the optical film and the d represents a thickness of the optical film.