Optical image formation (and processing) is strongly affected by film grain noise. The noise effects depend on the spatial coherence of the illumination. In this paper we shall analyse these phenomena in the case of Kehler illumination. The effective transmittance of any photographic image is instrument dependant due to diffusion by the grain (Callier effect). Thus the question arises : what is the meaningful physical quantity in image formation ? The Callier effect directly influences the contrast of weak modulations, which happens to be maximum in symmetric partially coherent illumination. Ultimately, the detection of weak modulations is not limited by the contrast but the signal to noise ratio. Using reasonable statistical assumptions for the noise behaviour, the optimal conditions for weak modulation detection will be discussed. This study is primarily devoted to'imaging problems, but it results may be partly extended to double-slit microdensitometry. In that case however, the illumination setup makes the complete study more difficult than in the K8hler illumination case.