are mainly due to
sometimes reversed
effect of superior
on at any image
° example at “0.2”.
ects mainly due no
ig. 7 are replotted
r is shown for low
t. Some resolving-
ion | Ratio:
b/a
2.6
1.8
| 2.7
aged by a perfect
s the low contrast
The 6” lens is not
is C.T. is low and
6" lens cross the
the latter and the
is correspondingly
mulsion is greater
is a log luminance
zero frequency.
possible out to the
)0 lines/mm region
ng it at the same
t by Selwyn. With
of frequencies we
d resolving-power
lenses of the same
curve for a slow
ontrast its resolu-
the ratio is nearly
ater sensitivity of
high contrast tests or
the futility of using
them for low contrast
subjects). It will be
clear that the differ-
ence between the lens-
es would be smaller
on an emulsion such
as Super XX, or on
detail of even lower
contrast than shown,
such as occurs in air
photography.
Fig. 10 shows the
effect of different fo-
cus settings on a given
lens, at high and low
contrast, referred to
the threshold curves
for slow and fast
emulsions. Focus set-
ting (A) gives better
modulation at lower
frequencies with a
sharp cut-off at about
100 lines/mm, while
(B) gives lower mod-
ulation in the 10-100
line region, with some
response out to several
hundred lines/mm. At
high contrast the two
focus settings give e-
qual resolving power
on the slow film, but
(B), with the lower
cut-off, gives higher
resolution on the fast
film. At low contrast
(B) is superior to (A)
on both emulsions. (A)
would obviously be
superior to (B) on an
emulsion having an
Fig.10. Two focus set-
tings of a 2" f/2 lens,
at high and low con-
trast, with threshold
curves for fast and
slow emulsions.
LOG M
LOG M
THE PHOTOGRAPHIC IMAGE, BROCK
15
o
-
—— t":
—
- 20
vn
p—
1.0
LOG R(LINES/mm.)
2.0
Fig.9. Two nominally identical lenses, (32 mm f/4.5) at low
and high contrast, with threshold curve for a slow emulsion.
o
I
|
1.0
LOG R(LINES/mm.)
20
