12/04/2014

Author: Matt Jaskulski, University of Murcia

1. Introduction

It is well documented that while reducing the retinal image quality, the chromatic aberration (CA)[1], may also aid the eye in the accommodation response[2][3]. The deficiencies of accommodation (i.e. accommodative lag) have been recently explored in terms of interplay between spherical aberration and defocus[4]. We created a computational model of the eye and performed a series of polychromatic through-focus simulations of image quality to analyze the effects of chromatic aberration on accommodation and depth of field.

2. Methodology

We performed a set of wavefront aberration measurements during accommodation in a group of young subjects under natural viewing conditions using the irx3 aberrometer (Imagine Eyes, France). The stimulus vergence was changed automatically in steps of about 0.5D to cover the entire AA span. We mext proceeded to compute polychromatic retinal image simulations. We followed the well established methodologies[6] for calculating first the PSF's (point- spread functions) and finally retinal images from monochromatic aberrations. We applied the process to each of the three channels of the target image that corresponded to the RGB primary colours – 700nm (R), 546.1nm (G) and 435.8nm (B). The three channels were then recombined into a single, polychromatic image. For each stimulus vergence we performed through-focus simulations of image quality spanning the whole AA of each subject. Every time an image simulating the effects of given eye's wavefront aberrations was generated from the target and various image quality metrics (such as the Visual Strehl Ratio) were calculated.

Figure 1. Test target (left), the well- (middle) and poorly (right) accommodated retinal image simulations. The R,G and B point-spread functions are shown below.

3. Results and discussion

Independently of viewing distance, the eye usually accommodates the minimum amount, enough place the target within its depth of field in order to see it relatively clearly[10]. Therefore, when one focuses at a nearby object, the eye accommodates a little further away, what constitutes a lag of accommodation. Our simulations have shown that in polychromatic light (which means in normal viewing conditions) the objective image quality is lower than in monochromatic light, however the effects of chromatic aberration extend the depth of field, which lowers the required accommodative effort on the part of the eye.

References

[1] N. López-Gil, A. Bradley, The potential for and challenges of spherical and chromatic aberration correction with new IOL designs. Br. J. Ophthalmol. 2012; 0:1–2.
[2] F.W. Campbell, The Depth of Field of the Human Eye, Optica Acta: International journal of Optics. 1957; 4:157-164.
[3] W.R. Bobier, M.C.W. Campbell, M. Hinch, The influence of chromatic aberration on the static accommodative response. Vision Res. 1992; 32:823-32.
[4] P. Bernal-Molina, R. Montés-Micó, R. Legras, N. López-Gil, Depth-of-field of the accommodating eye. Optometry and Vision Science. 2014; In-press.
[5] D. R. Iskander, M. J. Collins, B. Davis, L. G. Carney, Monochromatic aberrations and characteristics of retinal image quality. Clinical and experimental optometry. 2000; 1:83-6.
[6] B. Wang, K. J. Ciuffreda, Depth-of-Focus of the Human Eye: theory and Clinical Implications. Survey of Ophthalmology. 2006; 1:76-85.