368 W.S. Foulds and C.D. Luu

is as abnormal as a totally blurred image for a normal image contains an admixture of in-focus and out of focus content in varying proportions, according to the degree of accommodation exercised and the number and size of near and distant objects within the field of view.

It has been shown,59 that even in adults, prolonged microscopy using infinity focus binocular microscopes is associated with an increased prevalence of myopia, some 39% of initially emmetropic adult microscopists becoming significantly myopic after two years of intensive microscopy, and a larger proportion (48%) of initially myopic adult microscopists becoming significantly more myopic over the same time span. The myopia in both instances was axial myopia with a significant increase in axial and vitreous chamber lengths in those affected.

As a focused image viewed through a microscope is totally in-focus across the whole visible field of view, there is at least a possibility that this very abnormal situation might contribute to the reported development of myopia in microscopists undertaking intensive microscopy. In a study in chicks undergoing optical defocus and either preclusion of sharp vision or limited sharp vision, it was found that limited sharp vision was required to compensate for induced myopia.60

During reading with the eyes accommodated at reading distance, a large area of the visual field will be occupied by a uniformly in-focus image, so reproducing to an extent the visual situation experienced by microscopists where the visual field through a microscope is also completely in-focus.

It has been shown in chicks with lens induced myopia the viewing of a near target confocal with the retina but on a transparent background so allowing more distant visual information to contribute to the retinal image, that the myopia was reduced or eliminated, but only if accommodation were intact.34 In this situation, with the eye accommodated to the distance of the near target, additional distance visual information would add a proportion of myopic blur to the retinal image. As myopic blur is known to be protective against myopia, the presence of a proportion of myopic blur in the image appears to have been sufficient to overcome the degree of myopia that had previously been induced in these chicks by negative lens wear.

Outdoor activity and retinal image blur

In general, during outdoor activity, objects on the horizon are of less interest than objects nearer at hand and if visual activity is largely restricted to