An analytical expression for the focal intensity is derived for arbitrary surface profiles and arbitrary groove patterns of compressor gratings. The expression is valid for different compressor designs: plane and out-of-plane compressors, symmetric and asymmetric compressors (compressors composed by two not-identical pairs of grating), and a two-grating compressor. It is shown that the quality requirements for the optics used to write a grating are higher than for the grating. The focal intensity can be maximized by rotating each grating around its normal by 180 degrees. Moreover, it may be increased to maximum by interchanging any two gratings in the compressor, because imperfections of an individual grating do not additively affect the focal intensity. The intensity decrease is proportional to the squared pulse spectrum width and the squared total distortions of the second and third gratings of the four-grating compressor and the total distortions of two gratings of the two-grating compressor.