Wherever possible, spectacle lenses should be designed to minimal amounts of off-axis aberration. In simpler terms, as the eye rotates behind the lens surfaces, the image presented to the eye by this zone of the lens should be the same as or similar to the image presented by the paraxial zone as possible. There are five monochromatic aberrations (the Seidel aberrations) that affect the optical performance of lenses:

1. spherical aberration, 
2. coma, 
3. oblique astigmatism 
4. curvature of field 
5. distortion.

Both spherical aberration and coma are reduced by using a limiting aperture within an optical system. The eye contains the most beautifully designed adjustable aperture, the pupil, and hence these aberrations are of lesser concern to the spectacle lens designer. Of more concern are oblique astigmatism, curvature of field and distortion.