Abstract
Abstract: :
Purpose: It is well accepted that the accommodation system is characterised by steady–state errors in focus, which are known as "lags" (under–accommodation for near targets) and "leads" (over–accommodation for far targets) of accommodation. It is believed that these errors may be the consequence of the change in pupil size, and/or the ocular aberrations with accommodation, both of which are known to affect retinal image quality. The purpose of the study was to record these errors of accommodation and evaluate their effect on image quality. Methods: The COAS wavefront sensor was used in conjunction with a purposely–modified Badal optometer to allow continuous recording of the pupil size and the monochromatic aberration function of the eye when presented to accommodative stimuli from 0 to –8D. Fifty consecutive recordings from the right eyes of 8 subjects were taken. Full–size pupil were used. Accommodative response for each vergence was calculated (i) by the defocus term of the Zernike coefficients and (ii) by optimising three different image quality metrics (RMS, PSF, MTF). Analysis was performed using Focus Zemax–EE and MATLAB mathematical software. Results: The previously reported changes in ocular aberrations with accommodation (e.g. the shift of spherical aberration from positive to negative values) were confirmed. Moreover, fluctuations in accommodation increased at higher accommodation demands. Although, the increased errors in focus for near targets were evident for all the subjects, the exact value of the accommodative lag, as well as the resting state, showed a significant inter–subject variability. Moreover, the calculated response varied between the image quality metrics used, with the response based on optimising the MTF, resulting to lower errors in focus. Conclusions: It is concluded that the accommodative response is less "accurate" for high dioptric stimulus levels, with the observed errors in focus showing a significant variability not only between subjects, but also between different techniques used to evaluate the response. This happens because there are numerous factors contributing to the accommodative response (ie pupil diameter, spherical aberration, fixation axis) which have to be taken into consideration, before accurate comparisons can be made.