In view of the close relationship which exists between image aberrations and the form of the transfer function, the transfer functions of some different types of spiral tomographic movements have been calculated numerically. The results allow prediction of a type of spiral movement which yields blurring of favourable quality. The validity of this approach has been demonstrated experimentally, by comparing the blurring characteristics of tomographs with single and multiple circular movements. When the radii of the circles in the latter case are chosen with reference to the form of the transfer function, a marked improvement in blurring quality is apparent.