Theoretical and ray-tracing calculations on an accommodative eye model based on published anatomical data, together with wave-front experimental results on 15 eyes, are computed to study the change of spherical aberration during accommodation and its influence on the accommodation response. The three methodologies show that primary spherical aberration should decrease during accommodation, while secondary spherical aberration should increase. The hyperbolic shape of the lens' surfaces is the main factor responsible for the change of those aberrations during accommodation. Assuming that the eye accommodated to optimize image quality by minimizing the RMS of the wave front, it is shown that primary spherical aberration decreases the accommodation response, while secondary spherical aberration slightly increases it. The total effect of the spherical aberration is a reduction of around 1/7 D per diopter of stimulus approximation, although that value depends on the pupil size and its reduction during accommodation. The apparent accommodation error (lead and lag), typically present in the accommodation/response curve, could then be explained as a consequence of the strategy used by the visual system, and the apparatus of measurement, to select the best image plane that can be affected by the change of the spherical aberration during accommodation.

Aberrations change with accommodation and with age. SA changes more with accommodation do than other higher-order aberrations. SA becomes more negative with accommodation, and this change is larger in older individuals. Accommodative miosis is useful for ameliorating the increase in higher-order aberrations with accommodation.

PURPOSE:We studied the accuracy and precision of 32 objective wavefront methods for finding the amplitude of accommodation obtained in 180 eyes. METHODS: Ocular accommodation was stimulated with 0.5 D steps in target vergence spanning the full range of accommodation for each subject. Subjective monocular amplitude of accommodation was measured using two clinical methods, using negative lenses and with a custom Badal optometer. RESULTS Both subjective methods gave similar results. Results obtained from the Badal optometer where used to test the accuracy of the objective methods. All objective methods showed lower amplitude of accommodation that the subjective ones by an amount that varied from 0.2 to 1.1 D depending on the method. The precision in this prediction also varied between subjects, with an average standard error of the mean of 0.1 D that decreased with age. CONCLUSIONS: Depth of field increases subjective of amplitude of accommodation overestimating the objective amplitude obtained with all the metrics used. The change in the negative direction of spherical aberration during accommodation increases the amplitude of accommodation by an amount that varies with age. KEY WORDS: image quality metrics; amplitude of accommodation; presbyopia; subjective methods; objective methods. RESUMEN OBJETIVO:Estudiamos la exactitud y la precisión de 32 métodos objetivos de calidad de imagen aplicados al frente de onda ocular y utilizados para calcular la amplitud de acomodación en 180 ojos. MÉTODOS: Se estimuló la acomodación ocular modificando la vergencia del estímulo a pasos de 0,5 D, de forma que se cubriese todo el rango de acomodación de cada sujeto. Se midió la amplitud de acomodación monocular subjetiva utilizando dos métodos habituales en la práctica clínica: uno basado en el uso de lentes negativas y otro basado en un optómetro de Badal adaptado. RESULTADOS: Con ambos métodos subjetivos se obtuvieron resultados parecidos. Los resultados obtenidos con el método basado en el optómetro de Badal se utilizaron para evaluar la exactitud de los métodos objetivos. En todos los casos, con todos los métodos objetivos se obtuvo una menor amplitud de acomodación que con los subjetivos; la diferencia entre ambos tipos osciló entre 0,2 D y 1,1 D, dependiendo de cada sujeto concreto. La precisión de esta estimación también varió entre un sujeto y otro: el error estándar de la media fue, en promedio, igual a 0,1 D, disminuyendo con la edad. CONCLUSIONES: La profundidad de campo aumenta la amplitud de acomodación subjetiva que sobrestima los valores objetivos encontrados por cualquiera de las métodos objetivos usados. Los resultados indican que la variación, hacia valores más negativos, de la aberración esférica durante el proceso de acomodación hace que aumente la amplitud de acomodación en una cantidad que varía con la edad. PALABRAS CLAVE: métricas de calidad de imagen; amplitud de acomodación; presbicia; métodos subjetivos; métodos objetivos.

We analyze theoretically, by means of both computer simulations and laboratory experiments, the limitations of correcting aberrations with ideal customized contact lenses. Four experiments are presented: In the first one, we have analyzed the limitations of a static correction on the dynamic wavefront. In the second one, we studied the rotations of a contact lens on the eye using an optical method. The third one researched the limitations of the wavefront correction, focusing on a group of normal and highly aberrated eyes, when the correction suffers from a permanent rotation or translation. The fourth one estimates, under a simple approximation, the error made when applying on the corneal plane the correction corresponding to the wavefront measured at the entrance-pupil plane. Results show that a static correction of the wavefront leaves a residual aberration of 0.15-0.25 microm for a 5 mm pupil. Rotation of the contact lens (up to +/-4 degrees) diminishes the effectiveness of the correction. Horizontal or vertical translations of 0.5mm could generate a high-order-aberration RMS that is higher than the remaining one after a standard low-order correction. In particular, the group of eyes having normal values of high-order aberrations are more sensitive to translations than the one having higher values. Most of the results could be applied to other methods of aberration correction, such as refractive surgery or correction by means of intraocular lenses.

Large values of coma and trefoil (approximately 1 mum) significantly reduced visual performance. Only patients with high 3rd-order aberrations, such as those that occur in refractive surgery or in cases of distorted optics, would benefit from this correction.