The image of a hole in an amorphous carbon film illuminated
with a parallel beam showing that
with the objective lens underfocused (0-2) a bright
Fresnel fringe is visible inside the hole;
with the objective lens overfocused (4 - 6) a dark fringe
is visible inside the hole;
at exact focus (3) there is no fringe;
the appearance of the image varies as the x and y correction
(-, OK or +) is altered
1.
Did you try to do the correction at the highest available
magnification?
2. What are the correctors doing to the magnetic field of
the objective lens?