underfocus
underfocus
"Underfocus" means that the excitation of the objective lens in a TEM is slightly decreased from that at the in-focus (focused on a specimen). At this excitation, the image produced on the selector aperture is the specimen image when the objective lens is focused on a position above the specimen. To enhance contrast in a bright (or dark) field image (to make the outline of the image clear), the image is often taken at a slight underfocus, instead of at the in-focus.
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