Keeping the world in focus: how might this be achieved?

To see clearly, the eye needs to have a sharply focused image on its retina. For distance vision, the eye should ideally develop toward emmetropia, and for near vision, the accommodation system should produce an appropriate increase in ocular power. The characteristics of the emmetropization and accommodation mechanisms are compared. Following Howland (paper presented at the 13th International Myopia Conference, July 26-29, 2010, Tubingen, Germany), it is suggested that emmetropization may be guided by imagery in the peripheral retina, perhaps making use of oblique astigmatism; however, such a mechanism can only be effective if dioptric stimuli approximate to 0 across the visual field, as is the case in many outdoor environments. In contrast, the accommodation system must be driven mainly by the axial image, because during most close work, dioptric stimuli vary widely across the peripheral field. Thus, the differences between the emmetropization and accommodation mechanisms may reflect the nature of the spatial distribution of object vergence in the outdoor and near environments.