The aim of this study was to use a Shack-Hartmann clinical aberrometer (COAS, Wavefront Sciences Ltd) to evaluate the variability of low- and higher-order aberration measurement of the eye. Using the wave aberration polynomial determined for a full-size pupil we compared the Zernike expansion coefficients for a smaller, 3 mm pupil derived by two methods: first, by re-calculating the wave aberration coefficients to the reduced sampled points corresponding to that pupil ("direct" method), and, second, by using a matrix method to reconstruct a new set of coefficients appropriate for the reduced pupil ("scaling" method) (see figure 1).

Our results suggest that, for full-size pupil, the efficiency of the measurements varies across x and y pupil position: where the wavefront is larger, measurement variance is higher, especially near the margins of the pupil, where increased standard deviation results to higher wave-aberration error. Some of this increased variance may have been due to poorer image quality in parts of the Shack-Hartmann images[36]. Also, it may be partially attributed to the fact that saccadic movements, during the time required for data collection, lead to alignment errors that continuously change the set of sensor elements contributing to wavefront sensing. Although such displacements in respect to the optical axis of the instrument cannot justify significant fluctuations of the wavefront aberration[37], we cannot exclude the possibility that the algorithm employed in COAS software may generate the increased noise in periphery during wavefront expansion, since pupil translation magnitude (~100 μm) is comparable to lenslet array spacing (as magnified by the conjugating optics) in the particular instrument.

The increased standard deviation of wavefront aberration at the periphery has implications in calculating wavefront-guided ablation patterns. An error of 0.45 μm in the measured wavefront aberration at the periphery of the treatment zone may lead to a substantial error in the calculated shot pattern depending on the laser beam delivery (scanning) method as well as beam parameters and compensation for corneal curvature[38].

As pupil becomes smaller, the magnitude of wavefront aberrations decreases. At 3 mm pupil, the "direct" method (employed by COAS) induces considerable variance in the measurement of higher-order aberration coefficients attributed to inherent fitting error. This results from the small number of sensor elements involved in the wavefront inclination measurement. Moreover, there is a clear shift in the magnitude of each coefficient to higher values, which may lead to inaccurate determination of higher-order terms. In contrast, the "scaling" method produces coefficients of higher variability, and this is not surprising since it allows the use of the information from a larger set of sensor elements, reducing instrument noise.

On the other hand, second-order terms, and especially C_{2}
^{0}, are measured with higher variability (see figure 5), and this is an interesting feature as the coefficients C_{2}
^{0}, C_{2}
^{-2}, C_{2}
^{+2} can be used in the calculation of the conventional sphero-cylindrical correction[33]. This observation, in conjuction with the high accuracy of the refraction estimated objectively from wavefront aberration data when small (~3 mm) pupils are used[13, 39, 40], is of considerable importance, as this means that the COAS clinical aberrometer may be used as a reliable and accurate autorefractor. However, care must be taken when large pupils are tested, as the objective estimation of refractive error may lead to ambiguous results, due to the influence of higher-order aberrations on the determination of correction[40, 41].

Another finding is that the variability of C_{2}
^{0} (corresponding to defocus) improves only slightly when the "scaling" method is used. This probably occurs because the defocus term is mostly contained in the Shack-Hartmann spots at the centre of pupil. In contrast, spherical aberration, for example, depends on the 4^{th} power of pupil radius, which means that most of the information is outside the central 3 mm and what is measured with a small pupil is mostly noise. Another reason may be the fact that the variance in C_{2}
^{0} is not related to the inherent noise of the instrument itself, but to the dynamic characteristics of the human visual system, such as accommodation micro-fluctuations[42, 43], tear film changes[44], and eye movements leading to alignment errors[32, 45].

Although instruments based on the Shack – Hartmann sensor have been extensively validated for experimental work[11, 13, 31, 32, 40], our results indicate that special care should be taken when measurement of aberration is used in clinical applications, such as refractive surgery, either decision making or outcome evaluation. The diversity of the measured values of various coefficients suggest that a number of measurements should be taken and averaged for each subject in order to calculate coefficients of higher efficiency[32]. This is of major importance in customized laser surgery, where aberration data are used to correct higher-order aberrations for the potential enhancement of visual performance [46–48].