ARE LETTERS THE BEST REFRACTION TARGETS?

Russell Maringer

Abstract

PURPOSE. Letter optotypes are the most commonly used targets in subjective refraction. There are reasons, however, to ask whether they are the optimum targets. Letters may introduce biases if some orientations are more prevalent than others. This is a study of subjective refraction, comparing repeatability with the use of letters to those with some alternative targets. METHOD. Printed and projected letters, an array of dots, and several oriented targets similar to a fan-dial chart, were used as refraction targets. The distribution of refractive endpoints was used as an indicator of repeatability. Measurements were made on 10 subjects, seven target types, and with or without fogging blur. A modified phoroptor was used, in which the powers of the horizontal/vertical meridians can be altered independently of the powers of the diagonal meridians. Oriented targets (e.g. "+" and "x"-like symbols) offer alternative refractive techniques when used with the modified phoroptor used in this study. When viewing oriented targets, subjects judged the relative clarity of contours at different orientations, rather than the absolute level of clarity of the target.

RESULTS. Letter and dot targets result in approximately the same levels of repeatability. Special oriented targets, which present a different psychophysical task, result in better repeatability levels. With no fogging blur, the median repeatability level of oriented targets was approximately 0.18D: that of letters 0.27D. With +0.50D of fogging blur, the median repeatability with oriented targets was 0.24D, and with letters, 0.36D. Subjects were less variable in focusing diagonal contours than horizontal/vertical contours.

CONCLUSIONS. Commonly used refraction targets, such as letters, yield more variable results than oriented targets. The results also suggest that subjects have a greater ability to focus diagonal contours than horizontal/vertical contours. The procedures employed here require the use of a modified phoroptor, in which the power of the horizontal/vertical meridians can be altered independently of the power along the diagonals.

Details

Year: 2001

Program Number: Poster 106

Author Affiliation: The Ohio State University

Co-Authors: Thomas Raasch

Co-Author Affiliation: The Ohio State University

Room: Exhibit Hall C