Refractive astigmatism acts predominantly as a source of high spatial frequency image distortion: the associated lineal distortions can be overcome by using a low pass spatial filter!
OBJECTIVE
Surprisingly, an important characteristic of astigmatism has been overlooked by ophthalmic and clinical scientists. Apparently, refractive astigmatism is due largely to a form of high spatial frequency image distortion.


METHODS
Characteristic astigmatic image distortion can be minimized or eliminated by using a low-pass spatial filter (here, a ground glass plate was employed for this purpose). The ground glass is placed a short distance in front of a visual acuity chart, or it may be used with other tests, such as vernier acuity.


RESULTS
This test has been performed by us on large numbers of patients and test subjects. A clinician can try this test for himself/herself. Place a +2.00 D.C. (any axis) lens in front of the eye; the usual distortions will be observed. Locate the ground glass plate as described. The usual distortions associated with the conoid of Stürm will not be visible or will be virtually eliminated, although some image blur will remain.


CONCLUSIONS
This technique has significance, e.g., in visual screening programs in developing nations, or in assessing patients with media disorders prior to ophthalmic surgery.
Enoch
J M
JM
School of Optometry, University of California at Berkeley, 94720-2020, USA. jmenoch@socrates.berkeley.edu
Lakshminarayanan
V
V
Kono
M
M
Shih
P
P
Strata
E
E
D001251:Astigmatism / Q000503:physiopathology* / Q000628:therapy; D005139:Eyeglasses; D005260:Female; D005403:Fixation, Ocular; D006801:Humans; D008297:Male; D055095:Optics and Photonics; D012029:Refraction, Ocular; D014792:Visual Acuity