The Fresnel fringes in TEM images are affected by astigmatism of objective lens (OL). If the objective lens field is astigmatic (namely, not cylindrically symmetric), the focus will be different along different directions. For instance, those asymmetric Fresnel fringes present around the edge of a round hole in carbon (C) film. In this case, the fringes will disappear at some points before others. To correct the OL astigmatism, it is the best to find a small hole in the C film and then correct the OL astigmatism by making the Fresnel fringes symmetric. This lens for the correction function is called objective astigmator that is adjusted until the fringe is uniform around the hole and disappears uniformly when approaching focus. Note that this adjustment is always performed at the highest possible magnification using the binocular to view the image. Anticontaminator is often used to minimize contamination which increases the thickness of the film and thus makes the observation of the fringe more difficult.
The residual astigmatism in TEM can be examined by measuring the maximum and minimum fringe widths, given by,
Equation 1930 can be used to evaluate the astigmatism effect on TEM resolution.
Xmax -- The maximum fringe width,
Xmin -- The minimum fringe width,
Δfa -- The residual astigmatism.