Instrument-to-eye distance mildly influences accuracy of refraction

SAN FRANCISCO — Accounting for aberrometer-to-eye distance may improve the accuracy of refraction results, particularly in cases of strong refraction.

"If your instrument is off a little bit, you'll actually end up with a small error that is just due to the fact that it's off," Daniel R. Neal, PhD, said at the American Society of Cataract and Refractive Surgery meeting here. "It's just a refraction calculation. But if you don't know how far it's off, then it's difficult to make that correction."

A clinical trial included 56 eyes in which 113 measurements were taken. Patient age ranged from 29 to 74 years. An aberrometer was used to determine the sensitivity of refraction results to varying instrument-to-eye distance.

Analysis showed that errors in refraction had a linear relationship to instrument-to-eye distance. For example, in an eye with 10 D manifest refraction, a 2-mm variation in distance produced an error of about 0.2 D.

"These are not real strong effects, but if you get off too much, then you can be more than a quarter diopter off, and that could be a real source of error," Dr. Neal said.

An ideal target for accuracy would be a refraction result within 0.05 D of manifest refraction, he said.