We demonstrate experimentally the ability of a wafer stepper to produce micron and submicron features with an aspect ratio of 2 or more. Vertical profiles developed in the resist layer cannot be explained by the usual assumption of vertical incidence, especially if high numerical apertures are used. Under spatially coherent illumination, we explain image formation and vertical profiles by interferences of 3 wavefronts (or more) issued from the discrete spectrum of the object in the Fourier plane of the lens : the surfaces of equal intensity resulting from these interferences are vertical planes in a domain ± Az over and under the best focus plane. A focus shift Az, like an aberration, disturbs phase relationships between wavefronts : if zero-order and first-order diffraction wavefronts present a phase shift equal to half-wavelength, lines and spaces are interchanged. This phenomenon is known as "contrast inversion". We assume the depth of focus as being proportional to this depth of inversion. We present a focus wedge technique giving an experimental evaluation of the ratio between depth of focus and depth of inversion, for 4 degrees of pupil coverage : 0,15 ; 0,29 ; 0,58 ; 0,87. The effect of wafer reflectivity is considered.