In addition to being the main refractive components, the cornea and the crystalline lens are the main sources of aberrations in the human eye (Fig 1). The relative contribution of each of these components can be deduced from total ocular and corneal aberrometry data. The magnitude of the aberration is strongly dependent on individual factors such as age, the state of accommodation, refractive error or the path light travels through the ocular media. The human eye has both monochromatic aberrations and longitudinal (up to 2 diopters across the visible spectrum) and transverse chromatic aberrations; the latter become an issue when using wide bandwidth light sources for retinal imaging.
The standard representation of ocular aberrations is in terms of Zernike polynomials. Zernike polynomials are a series of mathematical expansion that are orthogonal over a unit circle. Any wavefront profile can be decomposed into a weighted sum of these polynomials. The low order terms can be translated into the common sphere and cylinder notations used in optometric fields and are easily corrected using spectacles or contact lenses. The higher order Zernike polynomials are traditionally not correctable by such methods and require advanced technologies such as AO. Figure 2 shows the first 15 Zernike terms and their corresponding far field point spread functions (PSF).

Figure 2. (a) The ocular aberrations can be represented as a weighted sum of Zernike polynomials  each representing a specific aberration. (b) By Fourier transforming and multiplying by the complex conjugate, the PSF for each mode can be calculated. Defocus and astigmatism are termed low order modes and are corrected by conventional refractive methods, whereas the higher order modes generally have lower amplitudes but require more elaborate correction technologies. 
