When we form images in TEM, we either form an image using the central spot, or we use some or all of the scattered electrons. The way we choose which electrons form the image is to insert an aperture into the back focal plane of the objective lens, thus blocking out most of the diffraction pattern except that which is visible through the aperture. We use the external drives to move the aperture so that either the direct electrons or some scattered electrons go through it. If the direct beam is selected we call the resultant image a bright-field image, and if we select scattered electrons of any form, we call it a dark-field image.

List several advantages of dark field imaging for crystalline specimens.