MANY vertebrate eyes possess a depression (fovea) in the retinal tissue that overlies an area of receptor surface specialised for high acuity vision. The shape of this depression varies but two extreme forms have been characterised1. Many birds and lizards2 and some fish1,3 possess deep ‘convexiclivate’ foveas, while shallow bowl-like ‘concaviclivate’ foveas are characteristic of the eyes of some primates and the binocular field of some birds (Fig. 1). Various suggestions have been made to explain the existence and shape of these different foveal surfaces. The functions proposed have been mainly concerned with visual acuity and include reduction of chromatic aberration4, clearing of the retinal tissue to minimise image degradation (concaviclivate foveas)5 and image magnification (convexiclivate foveas)6–8. It has also been suggested that the deep convexiclivate fovea acts as a fixation device and/or movement detector at the expense of loss of visual acuity over a small area of image9. We show here how the deep convexiclivate fovea can act as a sensitive, directional focus indicator.