6Optical Aberrations and Lens Performance
In previous chapters lenses have been considered as ‘ideal’, forming geometrically accurate images of subjects. In practice actual lenses, especially simple lenses, only approximate to this ideal. There are three reasons for this:
- The refractive index of glass varies with wavelength.
- Lens surfaces are usually spherical in shape.
- Light behaves as if it were a wave motion.
These departures from ideal imaging are called lens errors or optical aberrations. Effects due to (1) are called chromatic aberrations, those due to (2) are spherical aberrations and those due to (3) are diffraction effects. In general, the degrading effects of aberrations increase with both aperture and angle of field. ...