simple eye with multiple lenses

Multiple lenses in a simple eye serve different optical purposes. In copepods like Pontella, a parabolic outer lens corrects spherical aberration while a second lens focuses the image — together achieving sharper resolution than either lens alone. In Copilia, a pair of lenses creates a scanning mechanism where the inner lens moves across a fixed retinal area, effectively scanning the visual scene sequentially rather than imaging it all at once.

Copilia is a marine copepod crustacean with a remarkable eye: a large outer lens gathers light, and a small inner lens moves rhythmically side-to-side — scanning back and forth across the image formed by the outer lens. This scanning motion allows the single retinal sensor to sequentially sample different parts of the visual field, functioning somewhat like a flying-spot scanner rather than a conventional image-forming eye.

Simple eyes with multiple lenses are relatively rare compared to single-lens camera eyes and compound eyes. They occur in some copepod crustaceans (notably Pontella and Copilia) and a few other invertebrates. The multiple-lens solution represents an evolutionary variant that achieves specific optical improvements — aberration correction or scanning ability — within the single-chamber simple eye architecture.