ABSTRACT
Fixed-focus visual systems are adequate for some purposes. For example, there seems to be no good evidence for accommodation in rodents (Hughes, 1977). Presumably animals such as rats and rabbits have no need to vary accommodation because their visual acuity is low, and even a large defocus of the retinal image does not materially affect their spatial discrimination. Humans and other foveate animals, on the other hand, have high acuity and a behavioural repertoire that includes the need to make fine spatial discriminations at a range of distances, and hence accommodation is necessary.
