Unfocused Effect on the Measurement of Ocular Aberrations
Sabong Srivannaboon MD*, Narisara Adulbhan MD*
Affiliation : * Department of Ophthalmology, Faculty of Medicine, Siriraj Hospital, Mahidol University
Objectives : To evaluate the effect of unfocused measurement in the value of ocular aberrations.
Materials and Methods : 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 & 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, 1.35 D 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.
Keywords : Unfocused, Aberrations
