We see diffraction contrast in an image for two reasons: either the thickness of the specimen varies or the diffraction conditions change across the specimen: the t effect and the s effect! The
effect: when the thickness of the specimen is not uniform, the coupling (interference) of the direct and diffracted beams occurs over different distances, thus producing a thickness effect. Don’t confuse diffraction contrast due to thickness changes with mass-thickness contrast discussed in the previous chapter. The effects are very different. The diffraction contrast changes with small changes in tilt, but the mass-thickness contrast doesn’t.