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3.1 Introduction In the previous chapters, comments were made relating to aberrations. Now it is time to discuss and explain them and find out how they affect the performance of an optical system. In general, aberrations are deviations of an image point from its ideal position These aberrations come in various forms. For this fundamental tutorial text we limit ourselves to the so-called primary aberrations: • Spherical aberration • Coma • Astigmatism • Field curvature • Distortion • Axial chromatic aberration • Lateral chromatic aberration. These seven aberrations will first be described and then expressions will be presented to be used for determining the quantitative impact of each on image quality. 3.2 Primary Aberrations One can separate these aberrations by general categories. The first five types listed deal with monochromatic radiation; the last two address polychromatic effects. Sometimes the categories are split between on-axis and off-axis aberrations. By this definition, spherical and axial chromatic aberrations are on-axis aberrations because they refer to object points located on the optical axis (the system's axis of symmetry). The rest are off-axis aberrations. 3.2.1 Spherical aberration Applying Snell's law across a spherical surface of a lens or a mirror shows that rays closer to the edge of such an element are bent more strongly than they should to meet with rays closer to the center of the element at the optical axis crossover point of these rays. However, because spherical surfaces are much easier to manufacture than aspheric ones, they are by far the more standard surface to be found in optical systems. Figure 3.1 indicates that the marginal axial ray, after passing through the lens, intercepts the optical axis at point M. The ray offset from the optical axis by an infinitesimal amount (a ray in the paraxial region) intersects the axis at P.
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