5 OPERATION
EM210F-1
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5.3.8 Focusing with Fine Structure
Utilizing the background structure in conjunction with the Fresnel fringe (over-focus or
under-focus) is very effective for increasing the focusing accuracy when focusing images
at magnifications of 100,000 times or more. that is to say, focus the image as precisely as
possible with the fringe at under-focus, and then finely focus the image (in-focus) so as
to minimize background structure contrast.
A further point to keep in mind when focusing images at high magnifications is that,
since the possibility of mistaking the apparent fine structure for the actual structure is
large, it is necessary to photograph several images while changing the focus very slightly
(through-focus method) and to select the optimum-focus image from among the ones
photographed (
F
Sect. 5.5.3).
Fig. 5.31 shows micrographs of perforated film obtained by the through-focus method.
These micrographs clearly indicate that the background structure varies according to
defocusing (Fresnel fringe variation).
a: Under-focus
b: In-focus
c: Over-focus
Fig. 5.36 Fresnel fringe and background structure variation
