The geometrical optics design of a focusing grating coupler (FGC) is discussed for the situation in which the shape of the guided and free-space pencil and the diffraction geometry are general. A function that describes the so-called optical path-length defect in a general point of the FGC yields the spatial frequency components after differentiation. The wavefront diffracted by the FGC is analyzed at large values of the numerical aperture of the free-space wave by means of ray tracing. Spatial variation of the wave amplitude over the wavefront due to the progressive diffraction of the guided wave is not taken into account. Tolerances on the wavelength drift of the source, the waveguide properties, and the source position are given for an integrated optical disk pickup equipped with an FGC.