The invention relates to a collimator with adjustable focal length, especially for use in X-ray testing devices whose operating principle is based on diffraction phenomena in an object. Fixed focal length collimators used in such X-ray testing devices have to be displaced over a large range. The aim of the invention is to reduce the range of displacement. For this purpose, the collimator has at least two diaphragms having respective substantially circular slots arranged about a common center axis, wherein at least one diaphragm can be displaced along the center axis.