Lens Curvature (6.04)
Due to the curvature of the objective lens in a magnifier, the focal plane is not uniform. Good focus only occurs in the center of the magnified area. This spherical aberration is corrected by adding a second lens of a different curvature to compensate for the curvature of the first. The third lens is added to adjust for the chromatic aberration (dispersion), which causes the light waves of different colors to focus at different distances. These three lenses, the primary, the aplanatic, and the achromatic lenses make up the fully-corrected triplet loupe.