By a consistent argument, based on the characteristic curves of photographic plates, the optimum conditions for the following spectrographic problems are derived: weak emission line without background; weak absorption line; weak emission line on a given continuous background; prefogging; measurement of relative intensities. The effect of the grain on the resolving power is minimized by averaging over the length of the line. This technique gives to the very sensitive plate (Kodak type I-O), in spite of its coarse grain, the same high resolving power as that of a very fine grain plate which requires a 90 times longer exposure.
O. OLDENBERG, "Conclusions from the Characteristic Curves of Photographic Plates, Minimum Effect of Grain," J. Opt. Soc. Am. 46, 300-302 (1956)