A general 2-D linear shift-variant operation can be replaced by a 4-D convolution. If a 'variation band-limited' point spread function is assumed, this 4-D convolution may be carried out as a sequence convolution, i. e. a discrete set of 2-D convolutions, which can be performed very effectively by special convolving networks or coherent optics. The underlying principle of that technique is reviewed. If the point spread function is further restricted to either rotationally or radially symmetrical functions, instead of the 4-D convolution a certain 3-D convolution is sufficient and thus the sequence convolution is drastically simplified. Both special cases are discussed. Experimental results are shown.