ABO 1 Credit (Techincal Level 3) Florida 0 Credit Under ideal circumstances a spectacle lens would reproduce a perfect image on the retina. The ideal, however, is not the rule and as a consequence the lens designer must deal with a variety of lens aberrations Aberrations are what we call the errors that lenses may have, compared to reality. There are two basic types of lower order (ABBE value) aberrations; monochromatic aberrations and chromatic aberrations. Chromatic aberrations are a result of the refractive index of the lens and are actually a function of the wavelength of light. The most obvious example of this is a prism. Light entering a prism is split into its individual wavelengths and is called dispersion. This is because blue light experiences a higher refractive index than red light. Hence the blue light is refracted at a greater angle.