A method of developed photographic emulsion characteristic curve (CC) analytical approximation is considered in this article. This method is a modification of the well known Seidel method. À theoretic base and usage limitations of direct method of CC linearization by SF are proposed. The new method allows to the raise the precision of photographic photometry and rise the speed output of photographic image processing by computers, to expand developed photographic emulsion dynamic range towards the area of low exposures.