Midway image equalization means any method giving to a pair of images the same histogram, while maintaining as much as possible their previous gray level dynamics. In this paper, we present an axiomatic analysis of image equalization which leads us to derive two possible methods. Both methods are then compared in theory and in practice for two reliability criteria, namely their effect on quantization noise and on the support of the Fourier spectrum. A mathematical analysis of the properties of the equalization methods is performed. Their algorithms are described and they are tested on such typical pairs as satellite image stereo pairs and different photographs of a same painting.