Optical Aberration

One of the major challenges one could encounter while setting up an optical system is Optical Aberration. Optical aberration could be defined as the departure of an optical system from the predictions of paraxial optics [49]. This effect is not caused because of the flaws in an optical system but because of the limitations of the simple paraxial optics in completely defining the effect on light by an optical system [50]. In paraxial optics the marginal and chief rays completely define an optical system. The marginal ray starts at the center of the object and heads to the top of the pupil. The marginal ray determines the location of image planes, effective focal length, pupil size, F number. While the chief ray starts from the top of the object

The effect of such an aberration is less when the incident light scatters at the retina and comes out because it suffers those set of aberrations again while coming out. But still, the effect of aberrations caused by the eye is large on the images obtained. Ocular components mainly lens and cornea contributes to the aberration of the eye. Studies on the magnitude of the corneal aberration has been done by Jenkins, 1963 and Sivak and Kreuzer, 1983 [51, 52]. But one of the main challenge in those studies were the inability in determining the actual shape of the cornea and lens. Because of their variation in shape when taken in vitro. A suitable way to study the contribution of these ocular components is to determine the total aberration individual contribution of the eye and then to determine the aberration is of either the cornea or the lens. Anterior cornea is the easiest component to measure. The shape of it could be easily determined with the help of corneal topographical instruments. The departure of the cornea from its ideal shape gives its aberration, which does not give the aberration contribution of the chosen reference axis. The asphericity Q for the frontal surface conicoid of an ideal cornea, given a distant object, independent of its radius, relates to the refractive index of the cornea ‘n’ by Q = -1/n2 ≈ -1.376 ≈ -0.53 (7)
Aberration contribution by the cornea for any ray traced through the cornea in wave aberration is given by Corneal aberration ≈ (n-1) z