How good can vision be?

Abstract

Is 20/15 or 20/10 the best vision we can expect? We know that some patients have better corrected visual acuity than others, even with the best refractive correction in each case. Why is this the case? It may be that there is a difference in retinal receptor spacing, neurologic connections, scatter in the ocular media, some irregularity of the interfaces and/or optical aberrations. In recent years, there have been great strides made in the measurement of the optics of the eye. Instrumentation, such as the Hartmann-Shack Wave Front Analyzer, which essentially traces many rays through the pupil of the in vivo human eye, has been developed. This procedure presents a collimated beam of laser light through the pupil and then analyzes the pattern of the wave front reflected back from the retina. From the pattern of the reflected rays, the quality of the optical system is determined and the aberrations quantified. Such instrumentation allows one to analyze the optics of an individual patient’s eye. This opens the possibility to devise techniques to correct higher-order aberrations such as spherical aberration and coma. To experimentally correct the aberrations of the eye, adaptive optics has been used. Adaptive optics is used in sophisticated astronomical telescopes to correct for aberrations induced by turbulence in the atmosphere. The aberrations are corrected with either a deformable mirror (the shape of the mirror is changed in local regions to correct the aberrations) or an array of lenslets (very small lens units) where the power of each small lens unit can be changed. With such systems, higher-resolution photographs of the fundus have been obtained, including photos of individual rods.1 As this technology is advanced to the point where routine clinical measurements of ocular aberrations are possible, combined with the micronlevel lathing that now is available, it may become feasible to correct many aberrations. This could mean that 20/5 to 20/10 acuity may become common. Contact lenses are the best candidate for such corrections, as they can be maintained over the pupil. With spectacles, once you look away from the optical center of the lens the aberrations will change and one will lose the positive effect. There are a number of obstacles to be overcome. One being developing easily used clinical measuring instrumentation. Another is individual lens production to the accuracy required, although this is now possible. However, lathe-cut lenses are rotationally symmetrical which may correct some of the aberrations of the eye but many are not symmetric around the central axis. Lens movement also may be a problem. It does appear that better vision is possible. The question will be if the improvement will enhance vision enough that patients will appreciate the difference. Will the cost of the instrumentation and manufacturing make it practical? Only time and experimentation will tell. In the mean time, we need to just do a better job correcting residual astigmatism, and preventing surface drying and surface deposits.