|The dioptric power of a lens is always dependent on the ray path for which it was determined. In bifocal, trifocal and progressive lenses in particular, the ray|
path during use of the lenses for near vision differs considerably from the measuring ray path. It is for this reason that a different dioptric power is measured in the focimeter from that effective in front of the patient’s eye.
In the focimeter the dioptric power F’v is obtained using a parallel bundle of light. This means the focimeter measures the back vertex focal length for an infinitely distant object. The lens is arranged on the
focimeter mount in such a way that the principal ray of the measuring ray bundle leaves the lens at an angle of 90°.
During use of the lenses for near vision,
however, a different ray path is present:
Ray Path during Use