The object consists of an illuminated transparency containing a field of o's and x's. Light from the object is focused symmetrically by a 10 cm converging lens to an intermediate focus, which in turn is focused onto a distant screen by a 20 cm convex lens. A second 10 cm convex lens, the field lens, can be rotated into the beam at the position of the intermediate image. With the field lens out, the image is clear and bright near the center but very dim near the periphery of the image, a phenomenon known as vignetting. When the field lens is inserted, light illuminating the entire image is kept within a small enough angle that it passes through the lens system. Vignetting is virtually eliminated, and the image is uniformly well lighted. The photographs below show the final image with and without the field lens, at the same scale for direct comparison.
Many types of projectors use field lenses to obtain the greatest illumination for the projection bulb used.