4. Thick lenses

In practice, lenses need to be sufficiently rigid (and therefore thick) to be feasible. Thickness is frequently small relative to focal length, and thin lens formulas may be sufficiently accurate for the purpose. Indeed, it's common to use the thin lens approximation when introducing a starting configuration into an optimization calculation, with the correct thicknesses being assigned to the various components only during optimization. In some cases, the thickness of a component may be altered to correct an aberration, but in general, the thicknesses chosen are as small as possible (while still being compatible with a suitable design) in order to achieve lightweight systems and reduce lens costs. As a general rule, the thickness of a component is taken to be around one-sixth of its diameter, but can be as low as one-tenth.