Methods for determining the transfer function of a signal-processing system that do not require a known input signal. The methods are based on two representations 1(x) and I2(x) of an object, which the system has produced from differently scaled input signals originating from the object, or from a representation I1(x) of a first object and from a representation I2(x) of an object that is geometrically similar thereto but has been scaled differently. The representations are either given or are produced at the start of the method. According to the invention, the representations are transformed into a working space, and sections that relate to the same region of the object are selected in each case. The quotient of the functions corresponding to these two sections in the working space from which the unknown input signal comes makes it possible to clearly determine the transfer function sought. Various methods are indicated for this determination. The method can be used, in particular, to improve images from electron microscopes for which there are no suitable test structures for determining the transfer function.