Unfocused effect on the measurement of ocular aberrations.

Objectives: To evaluate the effect of unfocused measurement in the value of ocular aberrations.

Material And Method: The present study was conducted at Siriraj Hospital, Mahidol University. Ocular aberrations of 20 eyes in 10 myopic patients (myopia less than -6.00 diopters (D) and astigmatism less than -2.00 diopters (D)) were analyzed by using Zywave aberrometer (Bausch and Lomb, USA). The measurements were done at the pupillary plane (focus) and 6 other different planes (unfocus); 5, 10 and 15 mm inside and outside pupillary plane respectively. The value of each measurement was analyzed to find the effect of unfocused measurement on ocular aberrations.

Results: The magnitude of error (root mean square, RMS) of the spherical component of refraction at 5, 10, 15 mm inside and outside the pupillary plane was 0.16, 0.44, 0.57 D and 0.21, 0.38, 1.35D respectively. The RMS of astigmatic component of refraction inside and outside the pupillary plane at the same distance was 0.19, 0.50, 0.80 and 0.18, 0.52, 1.55 D respectively. The RMS of higher order aberrations inside and outside the pupillary plane at the same distance was 0.05, 0.13, 0.15 and 0.06, 0.1, 0.1 microns respectively.

Conclusion: There was more effect of outward unfocused measurement in the value of refraction (spherical and astigmatism) than inward unfocused measurement. Higher order aberration showed a similar error in both inward and outward defocus. The magnitude of error was 10 - 12% within 5 mm unfocused planes.