Theoretical consideration is given to the error of the optical transfer function caused by noise and truncation of the measured line spread function. The error due to noise is shown to be an increasing function of the scanning range of the line spread function. An optimum scanning range is found to exist that minimizes the total error resulting from noise and truncation. Numerical calculation is presented for the case of an aberration-free circular aperture and the change in total error with varying scanning range and noise level is illustrated.