Underwater, refraction at the plane port bends light rays so that the object **appears** to be 25% closer to the port than measured. Assuming that the separation of port and lens is small compared with the object distance, the effective underwater magnification is therefore F/(0.75*O - F). With the object at a distance O **in air**, we need to know what focal length F' produces this magnification. Clearly, it must satisfy the equation

**The effective underwater focal length is** **33% greater than in air.**.