simple eye

A simple eye is defined by having a single concave optical chamber — as opposed to compound eyes, which consist of many independent lens-detector units. Simple eyes are found across a wide range of invertebrates and vertebrates: vertebrate camera eyes (including the human eye), cephalopod eyes, and various forms found in spiders, scorpions, and many invertebrate larvae.

The term 'simple' refers to the single-chamber architecture, not visual capability. Some simple eyes achieve extraordinary acuity — cephalopod (octopus and squid) camera eyes, for example, rival vertebrate eyes in optical quality. Even among invertebrate simple eyes, jumping spiders have remarkably acute simple eyes for detecting prey movement. Conversely, some compound eyes have relatively poor resolution compared to high-performing simple eyes.

Simple eyes can be classified by their optical mechanism: pit eyes (the most primitive, lacking a lens), pinhole eyes (small aperture acts as a pinhole camera), simple lensed eyes (a single refractive lens focuses light), multiple-lens eyes (two or more lenses), and mirror eyes (use reflective surfaces rather than lenses). Each type represents a different evolutionary solution to achieving focused vision.