Perceived segment misalignment in anisometropic spectacle corrections

In anisometropic bifocals or trifocals, the magnification difference between left and right lenses will often distort the perceived alignment of the segments. The effect is most noticeable with the segment tops in bifocals or the dividing line between near and intermediate fields in trifocals. The perceived misalignment occurs because off-axis image points are displaced unequal amounts in right and left fields, which forces the lines of sight to diverge to achieve fusion. Hence, the left and right segments will appear at different levels even though they may be perfectly aligned in the spectacle lenses. This article explains why the perceived misalignment occurs, how it can be determined and quantified, and how it can be reduced or eliminated.