Date of Award
Master of Science in Vision Science
William E. Preston
Astigmatism is a refractive condition in which the optical system of the eye cannot form a point image of a point object. This is a consequence of varying curvatures of the cornea and or the lens of the eye and is present in a large majority of the detection of astigmatism.
This study takes a look at three subjective tests for astigmatism. Two of these, the Jackson cross cylinder and the Pratt near cylinder, are well established tests used at Pacific University clinics. The third test was designed by Dr. William Preston. Dr. Preston’s technique utilizes a near point card with a sunburst target which presents a series of radially arranged lines every 15 degrees. The card also presents three lines of Snellen acuity letter, 20/40, 20/30, and 20/20, which are used in fogging and cylinder axis refinement.
This study concluded that there was no statistical difference between the magnitude and direction of the astigmatic refractive error found by the Preston technique and the Jackson cross cylinder. In addition, the study found that there was a statistically significant difference between the magnitude and the direction of the astigmatic refractive error found by the Preston technique and the Pratt near cylinder. Although these results were statistically significant, the clinical difference between the tests was minimal.
Graft, Leonard O. and Weaver, Jennifer C., "A clinical comparison of three astigmatic tests" (1987). College of Optometry. 776.