Introduction

In the preceding chapters, we have studied how fast electrons interact with a crystalline thin foil. We have introduced the multi-beam dynamical diffraction equations in Chapter 5, solved the two-beam equations in Chapter 6, provided numerical methods to compute the multi-beam amplitudes for systematic row and near zone axis orientations in Chapter 7, and explained various defect contrast features and diffraction effects in Chapters 8 and 9. What we have not done yet is to describe how the microscope affects (i.e. changes) the electron wave function as it travels down the column. Actually, we did describe part of the influence of the microscope when we talked about two-beam microscopy; in a typical bright field–dark field observation, we introduce an aperture in the OL back focal plane, and this determines which component of the wave function reaches the observation screen. Moving the aperture around (or, more practically, tilting the incident electron beam) changes the image contrast, and this is indeed an example of how the microscope settings affect the image contrast. Another example is the excitation current of the objective lens: when we change the current, the image goes from under-focus to over-focus, and at the in-focus condition we observe an interpretable image. It is important to realize that the same information is present in all images, regardless of the focus condition; the human brain is just better at interpreting the contrast of the in-focus image, and we do not usually care too much about the out-of-focus condition.