The paper presents a theoretical development of the optical characteristics of photometers employing lenses. Both in-focus and out-of-focus operation are analyzed in detail for three types of photometers: a photometer employing fixed apertures, a photometer employing a simple lens, and a photometer employing a combination of a simple lens and a fixed aperture. The range of satisfactory operation of such instruments is carefully defined, as well as the gross errors which may occur in special applications.
DOMINA EBERLE SPENCER, "Out-of-Focus Photometry," J. Opt. Soc. Am. 55, 396-396 (1965)