5 OPERATION
5-36
EM210F-1
5.3.7b Using Fresnel Fringe
1.
Manipulate the OBJ COARSE (R1-
⑥
) to obtain the sharpest possible image
(coarse focusing).
2.
While observing a portion of the image having the greatest contrast (a film
hole or dust particle is ideal for practice purpose) through the binoculars, turn
the OBJ FOCUS-FINE knob (R1-
⑥
) slightly counterclockwise and clockwise
and obtain an in-focus.
If a film hole is used, a bright, high contrast fringe (Fresnel fringe) forms around the
edge of the hole, as shown in Fig. a below, when the FINE knob is turned counter-
clockwise. This is referred to as the under focus condition, a condition in which the
contour width increases as the amount of under focus is increased. When the FINE
knob is turned clockwise, a dark fringe (Fresnel fringe) appears slightly apart from
the edge of the hole as shown in Fig. c below. This is referred to as the over-focus
condition, a condition in which the distance between the fringe and the edge of the
hole increases as the amount of over-focus is increased. In-focus lies between
under-focus and over-focus. As the knob is gradually turned clockwise from the
under-focus condition, the width of the bright fringe forming the hole contour
decreases. When the fringe disappears and contour contrast is minimum as shown in
Fig. b below, the image is in-focus. As the knob is turned further clockwise, the
image assumes an ambiguous state and appears as either an in-focus or
over-focused image. Immediately after the image has assumed such a state, a dark
fringe appears slightly away from the edge of the hole (over-focus), as the knob is
turned further clockwise.
a: Under-focus
b: In-focus
c: Over-focus
Fig. 5.35
3.
Set the OBJ FOCUS-FINE knob (R1-
④
) to the in-focus position or slightly
under-focus position (obtained by turning the knob slightly counterclockwise
from the in-focus position).
In the case of low-contrast specimens (such as very thin biological sections), the
image should be slightly under-focused in order to obtain a sufficiently sharp image.
In order to distinguish the focus suitable for photography from in-focus, it is called
‘optimum’ focus.
