Aberrations of Lenses and Eyeglasses
Optical lens systems (eyeglasses or lenses) always have minor aberrations. These aberrations are not material flaws, rather they are due to the laws of physics. Expensive optical systems can reduce these aberrations by using many different lenses in a specific order.
This means that the refractive power of the lens varies according to thewavelength of the light.
Light consists of a blend of various wavelengths. Light with a shortwavelength such as blue is refracted more than light with a long wavelength such as red (Fig. 16.20). This is why monochromatic light (light of a single wavelength) produces a sharper image on the retina.
Chromatic aberration is the basis of the red-green test used for fine refraction testing.
This means that the refractive power of the lens varies according to the loca-tion at which the light ray strikes the lens.
Patients may report being able to see better when looking through a disk with a pinhole (a stenopeic aperture) than without it. This usually is a sign of an uncompensated refractive error in the eye.
The further peripherally the light ray strikes the lens, the more it will be refracted (Fig. 16.21). The iris intercepts a large share of these peripheral light rays. A narrow pupil will intercept a particularly large share of peripheral light rays, which improves the depth of field. Conversely, depth of field is signif-icantly poorer when the pupil is dilated.
Patients who have received mydriatic agents should refrain from driv-ing.
A punctiform object viewed through a spherical lens appears as a line.
If one looks through a lens obliquely to its optical axis, it will act as a prism (Fig. 16.22a). A prism refracts a light ray toward its base (Fig. 16.22b). In addi-tion to this, the light is split into its component spectral colors. Light with a short wavelength (blue) is refracted more than light with a long wavelength (red). Astigmatic aberration is an undesired side effect that is present whenever one looks through a lens at an oblique angle.
This phenomenon should be distinguished from astigmatic or toric lenses, which correct for astigmatism of the eye when the patients looks through them along the optical axis.
This means that the magnification of the image changes as one approachesthe periphery. The result is a sharp image with peripheral curvature. Convexor plus lenses produce pincushion distortion; concave or minus lenses pro-duce barrel distortion (Fig. 16.23).