A method is presented to estimate the influence of the atmospheric turbulence and the telescope on the mean power spectra of the granulation brightness distribution at positions outside the disk centre, derived using the determination of Fried's parameter. The procedure is based on the differential degradation suffered by the directions perpendicular and parallel to the solar limb; the former one is more affected because it contains more power at high frequencies, due to the geometrical foreshortening effect. A certain spectral ratio is defined, which can be evaluated after the observed power spectra and can be described directly by a Korff's function for a given value of the Fried parameter r0. The best fit of the measured attenuation to the theoretical one is calculated, allowing to obtain the adequate Modulation Transfer Function to reconstruct the original power spectra. A test has been performed by evaluating the mentioned ratio after restored spectra showing that these present the required elongation up to a resolution of about 0".4.