During use of the lenses for near vision, however, a different ray path is present:
Here, a divergent ray bundle emerges from a near object. In near vision, prismatic effects in the near reference point of the lens cause a deviation of the centrally imaging principal ray, with the result that the principal ray is not perpendicular to the back surface of the lens. The dioptric power in front of the eye is not the same as that measured in the focimeter. Lenses from Carl Zeiss are produced so that the prescribed power is actually present in front of the eye. The power measured in the focimeter is specified on the lens packet.
Due to the oblique ray bundle, additional astigmatic error occurs in near vision and must be corrected. This correction value is also taken into account in all bifocal, trifocal and progressive lenses from Carl Zeiss.
Ray path during wear: the principal ray is not perpendicular to the back lens surface |  Carl Zeiss near You
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