Photoreceptor optics I: introduction to formalism and excitation in a lens-photoreceptor system.

An analysis is presented of the optical factors influencing photoreceptor excitation. Small receptors are considered and the incident light is focussed onto them by a lens. The theoretical principles involved are outlined and explained in simple terms and by using analogies, so that they are also accessible to the nontheoretician. System parameters relevant to the fly are used as a basis for study. The theory is used to discuss the choice of lens parameters, the effects of defocussing and chromatic aberration. The emphasis is on results and in all cases graphs are presented so that magnitudes and trends are readily appreciated.