The eye has very high errors of refraction in the periphery of vision, in addition to any refractive errors "on-axis" and these errors usually increase with peripheral distance (Smith and Atchison, 1997) and vary greatly from person to person. Although these errors are not critical for resolution tasks, because these are limited by the neural system (Green, 1970 and Millodot et al., 1975), the errors can have consequences for detection in the periphery. It has been shown that correcting such errors can improve detection thresholds, including detection of movement, (Leibowitz et al. 1972, Johnson and Leibowitz 1974, Wang et al. 1997). Because these peripheral errors change with peripheral angle, correction of these errors can done by a number of separate single power ophthalmic lenses in turn whose optical axis coincides with the chief ray from the peripheral object. However, it would be convenient to correct the peripheral errors with a single lens so that it is not necessary to use separate lenses for each object position. This would be particularly valuable in automated perimetry. The lens will have to be a strip lens, similar to a vari-focal lens, with the radial or tangential curvature adjusted to give the required tangential correction and the sagittal curvature required to give the sagittal correction. Thus at any distance from the axis of the lens, it will have distinct sagittal and tangential curvatures which vary with distance and thus over a small section, the lens will be toric like.
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