We have derived analytical relations necessary for interpreting piezooptic effect and calculation of all piezooptic coefficients on the basis of interferometric technique, which take a micro-wedge shape of samples into consideration. The analysis is presented for the tetragonal crystals of point symmetry classes 4, 4 and 4/m. The relations valid for the remaining symmetry classes 422, 4mm, 42m and 4/mmm represent simple particular cases of the general relations.