## Objective and intermediate lens Consider the behavior of electron beams passing the sample when operating in conventional TEM mode. Two **critical planes** have insertable apertures -- back focal plane, and first intermediate plane of the objective lens. And these apertures are called selected area electron diffraction aperture, and objective aperture. They controls three most important [[Capture modes|modes]] in TEM -- bright field, dark field, and electron diffraction. ### Back focal plane After the beam passing the specimen, each parallel wave is focused to a crossover point in back focal plane of objective lens. This is shown in the figure below. At this plane, we may see the diffraction pattern, and here we may insert the objective aperture, to selected bright field (spot by center, diffracted beam) or dark field (diffraction beam). ### First intermediate image plane At this plane, all electrons together finally form a magnified image of the specimen in the first intermediate image plane. At this plane, we have the image visible, so we may insert the selected area aperture, let the diffraction pattern of only the selected area in our sample projected onto the screen and captured by the camera.