To show what a field lens does.◙
Bright point source with condenser lens, object transparency, 10cm focusing lens, 10cm field lens, and 20cm◙imaging lens.
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
Lecture Demonstration Record Form. ◙Ludwik Kowalski, On Field Lenses, TPT 30, 366-367 (1992).