The resolving power of a photographic material may be defined qualitatively as the ability to show fine detail in the picture. It is defined numerically as the number of lines and spaces per millimeter which it resolves. This definition, however, is rather inadequate since resolving power depends on many factors, such as the ratio of the width of the line to the width of the space, the color temperature of the light image, or the wavelength where monochromatic radiation is in question, and the contrast in the object.
The present paper gives some results of an experimental investigation of the dependence of the resolving power upon the contrast in the test object, where contrast is defined as the ratio of the photographic intensities of two adjacent small images to be resolved.
The method of investigation was to photograph in a reducing camera a series of parallel line test objects differing only in contrast, and by microscopic exmaination of the developed photographic images to determine the maximum resolving power for the respective objects.
The results show that the resolution changes very rapidly with contrast at low contrast values. Thus, with no resolution at unit contrast, the resolving power reaches approximately 65% of its maximum value for a test object density of 0.5, that is, a transmission through the opaque spaces of 31.5% or a contrast of 3.17; and 87% of its maximum value when the test object density is 1.0, transmission 10% and contrast is 10. The maximum value of resolving power is reached when the test object has a contrast of approximately 100 to 200.
OTTO SANDVIK, "THE DEPENDENCE OF THE RESOLVING POWER OF A PHOTOGRAPHIC MATERIAL UPON THE CONTRAST IN THE OBJECT," J. Opt. Soc. Am. 16, 244-252 (1928)