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The primary aberration characteristics of thin-lens models of common relay systems

Medical and industrial endoscopes usually employ relay systems composed of only positive components and consequently their image quality may be limited by field curvature and astigmatism, to an extent strongly dependent on relay system type. Using these aberrations as criteria, the performance of the traditional, rod-lens and two-doublet relay systems is studied, for the thin-lens case. Formulae representing the minimum aberration per unit length of relay system (for a known Lagrange invariant and clear aperture), are derived and tabulated as an aid to selection for different applications. In addition it is shown that, when the two-doublet system is designed to have a flat tangential field and is permitted to have some vignetting, then, despite its simple construction, it can match the high level of performance achieved by the widely-used rod-lens system.