It is well accepted that the accommodation system is characterized by steady-state errors in focus. The purpose of this study was to correlate these errors with changes in ocular wavefront aberration and corresponding image quality when accommodating. A wavefront analyzing system, the Complete Ophthalmic Analysis System (COAS), was used in conjunction with a Badal optometer to allow continuous recording of the aberration structure of the eye for a range of accommodative demands (up to 8 D). Fifty consecutive recordings from seven subjects were taken. Monocular accommodative response was calculated as (i) the equivalent refraction minimizing wavefront error and (ii) the defocus needed to optimize the modulation transfer function at high spatial frequencies. Previously reported changes in ocular aberrations with accommodation (e.g., the shift of spherical aberration to negative values) were confirmed. Increased accommodation errors for near targets (lags) were evident for all subjects, although their magnitude showed a significant intersubject variability. It is concluded that the one-to-one stimulus/response slope in accommodation function should not always be considered as ideal, because higher order aberrations, especially changes of spherical aberration, may influence the actual accommodative demand. Fluctuations may serve to preserve image quality when errors of accommodation are moderate, by temporarily searching for the best focus.

^{2}. However, retinal illuminance was not constant for each subject and accommodation level because of differences in pupil sizes.

*A*is the accommodation demand,

*L*the target vergence,

*a*the vertex distance (13 mm), and

*K*the refractive power of the correcting lens.

- by calculating the equivalent quadratic of a wavefront aberration map (given in Equation 2) using “paraxial curvature matching” (i.e., the second-order paraxial focus [
*c*_{2}^{0}] and the fourth-order spherical aberration [*c*_{4}^{0}] Zernike coefficients). This forms an approximation of spherical equivalent used in common ophthalmic calculations, and was found to be the most accurate method in predicting subjective refraction (Thibos et al., 2004), - by using a computational method that calculates the power of a focusing lens needed to optimize retinal image quality of the accommodating eye. This was calculated using a “weighted” sum of the modulation transfer function (MTF) image metric, the “optimized” MTF with a weighting function (WF) peaking at a spatial frequency of 18 c/deg (see Figure 1b). This was chosen because it has been suggested that while low spatial frequency components of a target provide a “coarse” accommodation guidance, it is the high spatial frequencies or edges of the target that refine in accuracy the final response (Charman & Tucker, 1977).

*r*, being higher than 0.85 in all cases). Note also that this linear relationship holds up to a degree of accommodative response, and that pupil constriction does not cease when the accommodative response reaches its limit.

*c*

_{4}

^{0}), vertical (

*c*

_{3}

^{1}), and horizontal coma (

*c*

_{3}

^{−1}). The most systematic change occurs for the spherical aberration,

*c*

_{4}

^{0}, which always moves in the negative direction. The magnitude of the change in

*c*

_{4}

^{0}is linearly related to the accommodative response for all the subjects (on average 0.048μm/D, the correlation coefficient,

*r*, being higher than 0.93 in all cases). Regarding the coma modes, although there is a tendency (more pronounced for

*c*

_{3}

^{−1}) for a change to more positive values with accommodation, there is a significant intersubject variability. Note that, when averaged across all subjects, both mean spherical-like and coma-like aberrations approximate to zero when the response equals −1.5 D, close to the mean tonic accommodation level (the intersection in the response/stimulus curves), although this is not the case for all individual subjects. This is further discussed in Conclusions. The other third- and fourth-order wavefront terms underwent small nonsystematic changes. It has to be stressed that the aberration data correspond to different pupil sizes (because of the accommodation-induced pupillary miosis). This analysis was purposely chosen to evaluate retinal image quality under real conditions.

*c*

_{4}

^{0}produces a lead (over-accommodation) for far targets, whereas negative

*c*

_{4}

^{0}results in a lag (under-accommodation) for near targets. For example, for subject AT (seeFigure 6), an accommodative lag equal to 1.05 D (compared to the overall 2.65 D) can be attributed to the high negative

*c*

_{4}

^{0}when accommodating, with the resulting optimal image response occurring at 7.00 D and not at 8.05 D, which equals the target vergence. Similarly, for subject TD, an accommodative lead equal to 0.18 D (compared to the overall 0.61 D) can be attributed to the positive

*c*

_{4}

^{0}with the resulting optimal image response occurring at 0.33 D instead of the 0.15-D target vergence.

*c*

_{4}

^{0}). These errors are calculated for each subject by the difference between the ideal 1:1 response and the optimized MTF. They take a positive value when corresponding to a “lag” and a negative value when resulting in a “lead.” The linear trend proves that spherical aberration is the main higher order aberration that contributes to image quality changes during accommodation.

*r*being higher than 0.89 in all cases). It is also evident that predicted focusing errors computed by the optimized MTF response are lower in magnitude, and thus can partly explain the observed accommodative “lag” and “lead.”

*SD*, 0.17), ranging between 3.23 and 4.02 D. Mean pupil diameter is 5.54 mm.

*c*

_{3}

^{−1},

*c*

_{3}

^{1}) aberrations to change to the positive direction, the most prominent transition occurs for symmetric spherical aberration (

*c*

_{4}

^{0}), which consistently moves into the negative direction with increasing accommodation demand (Cheng et al., 2004; He et al., 2000; Vilupuru et al., 2004). These are attributed to changes in the shape, refractive index distribution, and position of the crystalline lens during accommodation (Drexler, Baumgartner, Findl, Hitzenberger, & Fercher, 1997; Roorda & Glasser, 2004).

*c*

_{4}

^{0}is evident for all subjects, there are three characteristic patterns: (1) subjects with positive

*c*

_{4}

^{0}in the unaccommodated state, which shifts to negative values with accommodation (most common); (2) those with positive

*c*

_{4}

^{0}, which changes to less positive values without crossing through zero; and (3) those with negative

*c*

_{4}

^{0}, which changes progressively to more negative values with accommodation.

*c*

_{4}

^{0}is present) and a “lead” for far targets (when positive

*c*

_{4}

^{0}is present).