Objectives To study the effects of photorefractive keratectomy on ocular optical aberrations and to establish correlations with glare vision and low-contrast vision. Methods Preoperative ocular aberroscopy of 15 eyes undergoing photorefractive keratectomy was compared with aberroscopy at 3 months postoperatively by means of a newly developed automated aberroscope of the Tscherning type. The correlation of the wavefront errors with best spectacle-corrected visual acuity, low-contrast visual acuity, and visual acuity under glare conditions was analyzed. Results In any individual treated, the total wavefront error increased. On average, the total wavefront error increased by a factor of 17.65; this increase was highly statistically significant ( P = .001). Also, the correlation with best-corrected visual acuity, low-contrast visual acuity, and glare visual acuity was statistically significant ( P = .02, P = .001, and P = .03, respectively). The increase in ocular aberrations was significantly related with the virtual pupil size. Conclusions Photorefractive keratectomy increases the ocular aberrations, impairing the visual performance of the eyes treated. In detail, scotopic visual measures such as low-contrast visual acuity and glare visual acuity suffer most from the myopia correction. Aberroscopy-guided photorefractive keratectomy may avoid such effects.

https://jamanetwork.com/journals/jamaophthalmology/articlepdf/412752/ecs90164.pdf

Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism.

Wave aberrations were measured with a Shack-Hartmann wavefront sensor (SHWS) in the right eye of a large young adult population when accommodative demands of 0, 3, and 6 D were presented to the tested eye through a Badal system. Three SHWS images were recorded at each accommodative demand and wave aberrations were computed over a 5-mm pupil (through 6th order Zernike polynomials). The accommodative response was calculated from the Zernike defocus over the central 3-mm diameter zone. Among all individual Zernike terms, spherical aberration showed the greatest change with accommodation. The change of spherical aberration was always negative, and was proportional to the change in accommodative response. Coma and astigmatism also changed with accommodation, but the direction of the change was variable. Despite the large inter-subject variability, the population average of the root mean square for all aberrations (excluding defocus) remained constant for accommodative levels up to 3.0 D. Even though aberrations change with accommodation, the magnitude of the aberration change remains less than the magnitude of the uncorrected aberrations, even at high accommodative levels. Therefore, a typical eye will benefit over the entire accommodative range (0-6 D) if aberrations are corrected for distance viewing.

A population study on changes in wave aberrations with accomodation

http://www.vision.csic.es/Publications/Articles/Monochromatic%20aberrations%20in%20the%20accommodated%20human%20eye.pdf

Monochromatic aberrations in the accommodated human eye.

Current understanding of the anatomy, function and performance of the accommodative system of the young, adult human eye is outlined. Most major current models of the accommodative mechanism are based on Helmholtz's original ideas but, despite of a growing volume of related research, uncertainty continues over the relative contributions made to the overall mechanism by different ocular structures. The changes with age are then discussed. Although the amplitude of accommodation decreases steadily from later childhood, the speed and accuracy of the system within the available amplitude are little impaired until the age of about 40, when the amplitude falls below that needed for normal near work. A review of the available evidence on age-related change in the lens, capsule, ciliary body and other relevant ocular structures confirms that geometric and viscoelastic lenticular changes play major roles in the progressive loss of accommodation. Other factors may also contribute in an, as yet, unquantified way and a full understanding of the origins of presbyopic change remains elusive.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1444-0938.2008.00256.x

The eye in focus: accommodation and presbyopia

Of the commonly used chromatic dispersion equations, only the Sellmeier and the Cauchy equations seem to be theoretically based. Cauchy's equation is derived from the Sellmeier equation, is simpler to implement, and was found to give an excellent fit to published refractive-index data of the human eye. We used Cauchy's equation to model the chromatic difference in refraction of the Gullstrand number 1 schematic eye with a gradient-index lens. To estimate the dispersion at different refractive-index levels within the lens, a single dispersion equation at one nominal refractive index was linearly scaled. This scaling was justified after exploring the effect of mean refractive index on dispersion by using Sellmeier's equation and finding that a dispersion equation for one wavelength is just a linearly scaled version of the dispersion equation at any other wavlength. Because Cauchy's equation is theoretically based and gives excellent fit to data in the visible spectrum, it can be used to extrapolate results into the near infrared with confidence.

Chromatic dispersions of the ocular media of human eyes.

Measurement of Monochromatic Ocular Aberrations of Human Eyes as a Function of Accommodation by the Howland Aberroscope Technique

Further development of the objective version of the Howland and Howland [(1976) Science, 193, 580-582; (1977) Journal of the Optical Society of America, 67, 1508-1518 aberroscope technique for measuring ocular aberrations is described. Compensation for refractive corrections and calibration is discussed. The technique was used to investigate the effect of accommodation upon the monochromatic aberrations of the right eyes of 15 subjects. Coma and coma-like aberrations were the dominant aberrations for most people at different accommodation levels, thus confirming previous findings.

Variations in aberrations were considerable between subjects. About half the subjects showed the classical trend towards negative spherical aberration with accommodation. Changes in spherical aberration with accommodation in this study were less than that found in previous studies where all monochromatic aberration was considered to be spherical aberration.

Simple formulas based on reduced eyes have been developed to predict the variation in longitudinal chromatic aberration with variation in ametropia or accommodation. Two formulas were developed, one for axial ametropia and one for refractive ametropia. The latter also served as a model for accommodation. The results using the formulas are in close agreement with results obtained using raytracing through more sophisticated models. Combining the results of different methods gives the following predictions of change in chromatic difference of focus, between wavelengths of 400 and 700 nm, with change in each diopter of refractive error or accommodation: axial ametropia 0.012 to 0.017 D (0.6 to 0.9%), refractive ametropia 0.05 D (2.2 to 2.4%), and accommodation 0.04 to 0.05 D (2.1 to 2.6%). The chromatic aberration effects of correcting lenses with low dispersion are intermediate in effect and opposite in sign to the effects of corresponding degrees of axial ametropia and refractive ametropia.

Theoretical effect of refractive error and accommodation on longitudinal chromatic aberration of the human eye.

Simple formulae based on reduced eyes have been developed to predict the variation in longitudinal chromatic aberration with variation in ametropia or accommodation. Two formulae were developed, one for axial ametropia and one for refractive ametropia. The latter also served as a model for accommodation. The results using the formulae are in close agreement with results obtained using raytracing through more sophisticated models. Combining the results of different methods gives the following predictions of change in chromatic difference of focus, between wavelengths of 400 and 700 nm, with change in each dioptre of refractive error or accommodation: axial ametropia 0.012 to 0.017 D (0.6 to 0.9%), refractive ametropia 0.05 D (2.2 to 2.4%), and accommodation 0.04 to 0.05 D (2.1 to 2.6%). The chromatic aberration effects of correcting lenses with low dispersion are intermediate in effect and opposite in sign to the effects of corresponding degrees of axial ametropia and refractive ametropia.

Theoretical Effect of Refractive Error and Accommodation on Longitudinal Chromatic Aberration of the Human Eye

The use of visible radiation in medical diagnostic procedures, initially attempted some 60 years ago, has only very recently become feasible, due to advances in source and detector technology, and the availability of elaborate computer-based imaging processing algorithms.

Michael D. Waterworth

Papers

Measurement of Monochromatic Ocular Aberrations of Human Eyes as a Function of Accommodation by the Howland Aberroscope Technique

Measurement of Monochromatic Ocular Aberrations of Human Eyes as a Function of Accommodation by the Howland Aberroscope Technique

Theoretical effect of refractive error and accommodation on longitudinal chromatic aberration of the human eye.

Theoretical Effect of Refractive Error and Accommodation on Longitudinal Chromatic Aberration of the Human Eye

Laser-based instrumentation for medical diagnoses at visible wavelengths