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Excerpt from Optical Design Fundamentals for Infrared Systems, Second Edition
A lens placed at or near the focal plane of the objective lens can also serve to reduce the needed detector size for a given field coverage. The power of the lens is chosen to image the aperture stop of the objective into the detector plane. This arrangement has the same integrating effect as the cone already discussed. The lens used for the reduction of the detector is called the field lens. If placed exactly at the focus of the objective, it has no effect on the power of the objective. For a high optical gain factor, the field lens consists of several elements to correct for aberrations. From the figure below, it can be concluded that the optical gain is the ratio between the areas of the field lens and the detector.
Field lens images aperture stop onto detector.