There is more to accommodation of the eye than simply minimizing retinal blur.

Eyes of children and young adults change their optical power to focus nearby objects at the retina. But does accommodation function by trial and error to minimize blur and maximize contrast as is generally accepted? Three experiments in monocular and monochromatic vision were performed under two conditions while aberrations were being corrected. In the first condition, feedback was available to the eye from both optical vergence and optical blur. In the second, feedback was only available from target blur. Accommodation was less precise for the second condition, suggesting that it is more than a trial-and-error function. Optical vergence itself seems to be an important cue for accommodation.

Drifts in real-time partial wavefront correction and how to avoid them.

In visual experiments that require real-time partial correction of wavefront aberrations, small errors occur that accumulate over time and lead to drifts in Zernike coefficients of the uncorrected aberrations. A simple algorithm that does not require the inclusion of an additional optical path to obtain independent measurements of the eye's aberrations is described here, and its effectiveness in preventing these drifts is demonstrated.

Accommodation and the Stiles-Crawford effect: theory and a case study.

PURPOSE: Fincham (The accommodation reflex and its stimulus. Br. J. Ophthalmol. 35, 381-393) was the first to suggest that the Stiles-Crawford effect (Type I) might provide a stimulus for accommodation, but the possibility has not been investigated experimentally. The present paper outlines a theoretical basis for such a mechanism, and includes a case study on a subject with a nasally decentred Stiles-Crawford (S-C) function. METHODS: Accommodation to a monochromatic sine grating was monitored continuously with the natural S-C function intact, or with apodising filters imaged in the subject's pupil to neutralise, reverse or double the natural S-C function. RESULTS: Mean accommodative gain was not reduced significantly when the normal S-C function was either neutralised or reversed. CONCLUSIONS: For the present subject, the average S-C effect does not mediate the accommodation response to defocus, but more subjects should be examined. Other methods by which directionally sensitive cone receptors could detect light vergence are discussed.

Measurement of the eye's near infrared wave-front aberration using the objective crossed-cylinder aberroscope technique.

We used the crossed-cylinder aberroscope technique to obtain the near infrared (784 nm) wave-front aberration of the human eye. We compared the results with those obtained under the same conditions using red light (633 nm). Other than the greater retinal scattering of the near infrared light, third- and fourth-order wave-front aberrations are similar in both wavelengths. Values of the calculated near infrared point spread function show a typical half-height width of around 2 arcmin, which is in good agreement with previous work.

Retinal image quality in the human eye as a function of the accommodation.

The changes in the retinal image quality with accommodation in the human eye were studied by using a near-infrared double-pass apparatus. A slightly better modulation transfer function (MTF) in the unaccommodated eye with respect to the accommodated eye was found when using an artificial pupil with a fixed diameter. The technique allows the estimation of the MTF of the accommodated eye discounting the effect of the accommodative defocus error. Most of the reduction found in the MTF with accommodation could be explained in terms of the accommodative defocusing error. However, the shape of the retinal images clearly changes with accommodation, indicating that other aberrations are also altered with accommodation. In general, the double-pass image for the accommodated eye tends to be more symmetric than that of the unaccommodated eye. This is probably due to either a decrease in the amount of coma-like aberrations with accommodation or to an increase of other symmetric aberrations, such as defocus or spherical aberration, that hide the asymmetries present in the retinal image of the unaccommodated eye.

Generation of third-order spherical and coma aberrations by use of radically symmetrical fourth-order lenses.

We have extended the method of Alvarez [J. Am. Optom. Assoc. 49, 24 (1978)] to generate a variable magnitude of third-order spherical and/or coma aberration by using a combination of fourth-order plates with a magnification system. The technique, based on the crossed-cylinder aberroscope, is used to measure the wave-front aberration generated by the plates. The method has been applied to correct the third-order spherical aberration generated by an artificial eye as well as the coma produced by a progressive addition ophthalmic lens. The simplicity of the method and its relatively low cost make it attractive for partial correction of the aberrations of the eye.

Reconstruction of the point-spread function of the human eye from two double-pass retinal images by phase-retrieval algorithms.

In the double-pass technique used to measure the optical performance of the eye, the double-pass image is the cross correlation of the input spread function with the output spread function [J. Opt. Soc. Am. A 12, 195 (1995)]. When entrance and exit pupil sizes are equal, the information on the point-spread function is lost from the double-pass image, although the modulation transfer function of the eye is obtained. A modification of the double-pass technique that uses unequal-sized entrance and exit pupils allows a low-resolution version of the ocular point-spread function to be recorded [J. Opt. Soc. Am. A 12, 2358 (1995)]. We propose the combined use of these two double-pass measurements as input in a phase-retrieval procedure to reconstruct the ocular point-spread function. We use an adapted version of the iterative Fourier-transform algorithm consisting of two steps. In the first step, error-reduction iterations with expanding weighting functions in the Fourier domain yield an estimation of the phase that serves as an initial guess for the second step, which consists of cycles of hybrid input-output iterations. We tested the robustness and limitations of the retrieval algorithm by using simulated data with and without noise. We then applied the procedure to reconstruct the point-spread function from actual measurements of double-pass retinal images in the living eye.

Comparison of double-pass estimates of the retinal-image quality obtained with green and near-infrared light.

We compared retinal point-spread functions obtained by the double-pass method with two different wave-lengths, green (543 nm) and near-infrared (780 nm), in both cases under the best conditions of focus. The best refractive state at each wavelength was determined with two procedures: subjective refraction and analysis of the recorded double-pass images as a function of focus. Since the refraction results agree quite well, we assume that in both cases, green and near-infrared light, most of the light of the central core in the double-pass images comes from a layer close to that of the photoreceptors. The central spread of the double-pass images was also quite similar for the two wavelengths: a width of approximately 2-3 arcmin at half-intensity relative to the peak. However, larger differences were found in the tails of the images, with the infrared images presenting a larger scattering halo, probably as a result of a more important contribution of retinal and choroidal scattering for that wavelength. By using the central core in the double-pass images and ignoring the tails, we can use the near-infrared data to predict the modulation transfer function measured with the use of green light. These results raise the possibility of using near-infrared illumination the double-pass method to estimate the optical performance of the human eye.

Double-pass measurements of the retinal-image quality with unequal entrance and exit pupil sizes and the reversibility of the eye's optical system.

We have used a modified double-pass apparatus with unequal entrance and exit pupil sizes to measure the optical transfer function in the human eye and have applied the technique to three different problems. First, we confirm that in the eye the double-pass spread function is the cross correlation of the input spread function with the output spread function [J. Opt. Soc. Am. A 12, 195 (1995)]. Consequently, when entrance and exit pupil sizes are equal, phase information is lost from the double-pass images. Second, we show that in double-pass measurements the eye behaves like a reversible optical system. That is, when entrance and exit pupils are equal, the double-pass image results from two passes through an optical system having a transfer function that is the same in both directions. To test for reversibility in the living eye we have used a double-pass apparatus with different exit and entrance pupil sizes (one of them small enough to consider the eye diffraction limited), so that the ingoing and the outgoing transfer functions are different. The measured image quality was unchanged when the pupils were interchanged, i.e., when the first-pass entrance pupil size becomes the second-pass exit pupil size, and vice versa. Third, the technique provides a means for inferring the complete optical transfer function of the eye, including the phase transfer function, and the shape of the point-spread function.