In Gaussian focus one sets the defocus to zero, the sample is in focus. As a consequence contrast in the image plane gets its image components from the minimal area of the sample, the contrast is "localized" (no blurring and information overlap from other parts of the sample). The contrast transfer function becomes a function that oscillates quickly with "Csu4". This means that for certain diffracted beams with a spatial frequency "u" the contribution to contrast in the recorded image will be reversed, thus making interpretation of the image difficult.