This work presents a computational model for practical application of the transfer function method for radiographic units evaluation, in order to reduce some experimental constraints involved to its determination. With the proposed algorithm, the complete Optical Transfer Function (Modulation Transfer Function and Phase Transfer Function) can be easily determined as well as the effective focal spot sizes at any field location without using a microdensitometer. All measurements are done from a digitized slit image obtained experimentally at the field center position. The effective focal spot sizes can be calculated by using the Line Spread Function Root Mean Square (RMS) value or by the Modulation Transfer Function (MTF) first minimum. Besides, considering the variation of the effective focal spot size given by the field characteristic equations, all these parameters can also be determined at any location on the radiation field. The computer scheme was used for evaluating slit images obtained from nine different x-ray equipments. Results confirmed the possibility of using the transfer function method for quality evaluation of any radiological system in a simple and automatic way. This computer scheme replaces some of the expensive and specific devices necessary to the experimental MTF evaluation by quite more accessible and low cost equipments.