Effect of small focal errors on vision.

PURPOSE: The aim of this study was to quantify the effects of small plus focal errors on the quality of vision of wearers who habitually wear either no correction, or a low powered, distance only correction.
METHODS: Patients wore spectacles with small errors deliberately included for a 2-day trial wear period; a short subjective questionnaire, and some laboratory measures of visual function were also used to assess the effects of these focus errors.
RESULTS: A significant number of wearers notice, and are dissatisfied with, focal errors in distance vision as small as +0.25 D, in both sphere and cylinder.
CONCLUSION: This finding supports clinical practice in refining the refraction to the nearest 0.25 D. It suggests that for critical applications and for critical patients, the practice of slightly undercorrecting hypermetropes should be applied, so that patients can use a little of their own accommodation for optimal distance vision.