 | 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
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