668 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



AR 



X 



FIG. 26. Chromatic aberration data of Wald & Griffcn 

 (89), cited by Fry (37). AR is the distance from the cornea to 

 the conjugate focus of the retina. X represents wave length. 

 [From Fry (37).] 



-6 -5 -4 -3 -2 -I 



BEST SPECTACLE CORRECTION FOR ZONE. DIOPTERS 



FIG. 27. Spherical aberration of an eye with various amounts 

 of accommodation in play. [From Koomen et al. (52).] 



light is assumed to be uniformly distributed over the 

 blur circle or blur ellipse. This approach to the prob- 

 lem of throwing the eye out of focus is usually quite 

 adequate. 



Geometrical ray tracing is also adequate for 

 describing and defining aberrations, but in order to 

 evaluate the effect of aberrations upon the distribu- 

 tion of illuminance in the retinal image it is necessary 

 to deal with the physical image. 



The a.xial chromatic aberration of the eye can i^e 

 measured by locating the conjugate focus of the retina 

 for different wavelengths of light. Figure 26 gives the 

 average data for the seven subjects of Wald & Griffin 

 (89) expressed in terms of an eye focused on a yellow 

 (589 mfx) point at infinit\-. 



The spherical aberration of the eye can be expressed 

 in terms of the conjugate foci of the retina for different 

 zones of the pupil. Figure 27 shows the data of 



Koomen et al. (52) for a typical eye for several dif- 

 ferent amounts of accommodation. Many arrange- 

 ments (9, 34, 43, 51, 52) have been used for measuring 

 the spherical aberration of the eye. 



Chromatic dispersion (37, p. 89) of the eye is 

 dependent upon the axial chromatic aberration of the 

 eye and the lateral displacement of the pupil from the 

 incident ray directed through the primary nodal 

 point of the eye (see fig. 28). Blur produced by chro- 

 matic dispersion is akin to astigmatism in being 

 radially asymmetrical. Other aberrations in the 

 human eye, such as coma and radial and irregular 

 astigmatism, have not been extensively studied. 



Figure 29 shows the effect of diffraction (37, p. 57) 

 upon the image of a monochromatic point source in 

 an eye free from aberrations and astigmatism and in 

 perfect focus. The geoinetrical image would be a point. 

 Reducing the size of the pupil increases the blur due 

 to diffraction and minimizes the effect of being out of 

 focus and the effects of chromatic and spherical aber- 

 ration. A pupil size of about 4 mm yields maximum 

 sharpness of \ision in an eve which is well-focused 



(17)- 



Once the distriljution of illuminance for a single 

 point is known, the distribution of illuminance on the 



CHROMATIC 

 DISPERSION 



OPTIC 

 AXIS 



BLUE 



AXIAL 

 CHROMATIC ABERRATION 



FIG. 28. Dependence of chromatic dispersion on axial chro- 

 matic aberration and lateral displacement of the pupil. 



FIG. 29. Disli iliution iit ilhiniinance across the center of the 

 phvsical image of a monochromatic point source in an eye free 

 from spherical aberration and astigmatism and focused for 

 the sharpest possible image. 



