Field flattener lens
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Field flattener lenses in binoculars improve edge sharpness and lower the distortion.
Field flattener lenses counteract the Petzval field curvature of an optical system. In other words, the function of a field flattener lens is to counter the field-angle dependence of the focal length of a system.
The object in designing a field flattening lens is to create a lens that shifts the focal points of the Petzval surface to lie in the same plane. Consider inserting a pane of glass in a focusing beam. Due to refraction, the focal point of the beam is shifted by dependent on the thickness of the glass. Thus we have a thickness as a function of focal shift:
is given by the radius of curvature of the Petzval surface, . It can be shown, then, that the radius of curvature for the lens that would flatten out the field is given by
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