There are two basic modes of TEM operation, namely the bright-field mode, where the (000) transmitted beam contributes to the image, and the dark-field imaging mode, in which the (000) beam is excluded. The size of the objective aperture in bright-field mode directly determines the information to be emphasized in the final image. When the size is chosen so as to exclude the diffracted beams, one has the configuration normally used for low-resolution defect studies, so-called diffraction contrast . In this case, a crystalline specimen is oriented to excite a particular diffracted beam, or a systematic row of reflections, and the image is sensitive to the differences in specimen thickness, distortion of crystal lattices due to defects, strain and bending.