 Solution

For bright-field transmission electron microscopy, place the objective aperture in the back focal plane around the transmitted beam, excluding most diffracted beams. Adjust the intermediate lens to image the objective's real-space image plane; projector lenses enlarge that image onto the screen or camera. Regions removing electrons from the admitted beam tend to be darker, although phase contrast can also contribute.

For selected-area electron diffraction, put the selected-area aperture in the objective image plane to choose a specimen region. Remove or open the objective aperture and change the intermediate-lens excitation to image the objective's back focal plane onto the camera. The projector then magnifies the diffraction pattern, rather than the specimen image. Thus the two apertures act in different conjugate planes.