A quantitative high resolution transmission electron microscopy (HRTEM) study of amorphous materials has been undertaken in the study-case of amorphous germanium. The analysis consists in a modeling of amorphous germanium, suitable to run multislice calculations, and to simulate HRTEM images, the computed diffractograms of which are numerically compared to experimental ones obtained from a through-focus series taken on a dedicated high resolution microscope equipped with a field-emission gun. The final aim of this work is to quantify the structural information that can be retrieved from HRTEM images of amorphous materials. As a preliminary step, the coefficient of spherical aberration (Cs) of the microscope, as well as the amount of defocus (δf) of each micrograph have to be known. The aim of the first part of this paper is to determine, as precisely as possible, these parameters. A new method is proposed, as an alternative to usual methods based on least-squares fittings of the zeros of the experimental diffractograms: the theoretical diffractograms are computed, and numerically compared, through the minimization of a profile agreement factor depending upon the Cs and δf values, and the thickness of the amorphous film, to the experimental ones. The critical steps of this approach are the modeling of the scattering of the amorphous material, and, more drastically, the estimation of the experimental thickness, which influences significantly the values of Cs and δfs.