We will start with a single lens to illustrate the physics of imaging and will find the equivalence of material and geometry parameters for the image quality.
Such a lens alters the amplitude and the phase. We observe the phase change in Fig. 2.1, where a plane wave is transformed into a spherical wave. The spherical wave converges to an intensity spot at a distance f, called the focal length of the lens. Although an amplitude change results in a light intensity loss, this is ignorable here, and the phase term is much more important. It determines the optical quality, that is, the sharpness and contrast of an image. We see in Fig. 2.1 that a wavefront error of the perfect spherical output wave would result in an undesirable spot broadening.
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