<s>
In	O
photography	O
,	O
acutance	B-Algorithm
describes	O
a	O
subjective	O
perception	O
of	O
sharpness	O
that	O
is	O
related	O
to	O
the	O
edge	O
contrast	O
of	O
an	O
image	O
.	O
</s>
<s>
Acutance	B-Algorithm
is	O
related	O
to	O
the	O
amplitude	O
of	O
the	O
derivative	B-Algorithm
of	O
brightness	O
with	O
respect	O
to	O
space	O
.	O
</s>
<s>
Due	O
to	O
the	O
nature	O
of	O
the	O
human	O
visual	O
system	O
,	O
an	O
image	O
with	O
higher	O
acutance	B-Algorithm
appears	O
sharper	O
even	O
though	O
an	O
increase	O
in	O
acutance	B-Algorithm
does	O
not	O
increase	O
real	O
resolution	B-Algorithm
.	O
</s>
<s>
Historically	O
,	O
acutance	B-Algorithm
was	O
enhanced	O
chemically	O
during	O
development	O
of	O
a	O
negative	O
(	O
high	O
acutance	B-Algorithm
developers	O
)	O
,	O
or	O
by	O
optical	O
means	O
in	O
printing	O
(	O
unsharp	B-Algorithm
masking	I-Algorithm
)	O
.	O
</s>
<s>
In	O
digital	B-Application
photography	I-Application
,	O
onboard	O
camera	O
software	O
and	O
image	O
postprocessing	O
tools	O
such	O
as	O
Photoshop	B-Operating_System
or	O
GIMP	B-Application
offer	O
various	O
sharpening	O
facilities	O
,	O
the	O
most	O
widely	O
used	O
of	O
which	O
is	O
known	O
as	O
"	O
unsharp	B-Algorithm
mask	I-Algorithm
"	O
because	O
the	O
algorithm	O
is	O
derived	O
from	O
the	O
eponymous	O
analog	O
processing	O
method	O
.	O
</s>
<s>
As	O
the	O
transition	O
is	O
instantaneous	O
,	O
the	O
line	O
is	O
as	O
sharp	O
as	O
can	O
be	O
represented	O
at	O
this	O
resolution	B-Algorithm
.	O
</s>
<s>
Acutance	B-Algorithm
in	O
the	O
left	O
line	O
was	O
artificially	O
increased	O
by	O
adding	O
a	O
one-pixel-wide	O
darker	O
border	O
on	O
the	O
outside	O
of	O
the	O
line	O
and	O
a	O
one-pixel-wide	O
brighter	O
border	O
on	O
the	O
inside	O
of	O
the	O
line	O
.	O
</s>
<s>
The	O
actual	O
sharpness	O
of	O
the	O
image	O
is	O
unchanged	O
,	O
but	O
the	O
apparent	O
sharpness	O
is	O
increased	O
because	O
of	O
the	O
greater	O
acutance	B-Algorithm
.	O
</s>
<s>
Artificially	O
increased	O
acutance	B-Algorithm
has	O
drawbacks	O
.	O
</s>
<s>
Several	O
image	B-Algorithm
processing	I-Algorithm
techniques	O
,	O
such	O
as	O
unsharp	B-Algorithm
masking	I-Algorithm
,	O
can	O
increase	O
the	O
acutance	B-Algorithm
in	O
real	O
images	O
.	O
</s>
<s>
Low-pass	B-Algorithm
filtering	I-Algorithm
and	O
resampling	O
often	O
cause	O
overshoot	B-Algorithm
,	O
which	O
increases	O
acutance	B-Algorithm
,	O
but	O
can	O
also	O
reduce	O
absolute	O
gradient	O
,	O
which	O
reduces	O
acutance	B-Algorithm
.	O
</s>
<s>
Filtering	O
and	O
resampling	O
can	O
also	O
cause	O
clipping	O
and	O
ringing	B-Algorithm
artifacts	I-Algorithm
.	O
</s>
<s>
An	O
example	O
is	O
bicubic	B-Algorithm
interpolation	I-Algorithm
,	O
widely	O
used	O
in	O
image	B-Algorithm
processing	I-Algorithm
for	O
resizing	O
images	O
.	O
</s>
<s>
One	O
definition	O
of	O
acutance	B-Algorithm
is	O
determined	O
by	O
imaging	O
a	O
sharp	O
"	O
knife-edge	O
"	O
,	O
producing	O
an	O
S-shaped	O
distribution	O
over	O
a	O
width	O
W	O
between	O
maximum	O
density	O
D1	O
and	O
minimum	O
density	O
D2	O
–	O
steeper	O
transitions	O
yield	O
higher	O
acutance	B-Algorithm
.	O
</s>
<s>
Summing	O
the	O
slope	O
Gn	O
of	O
the	O
curve	O
at	O
N	O
points	O
within	O
W	O
gives	O
the	O
acutance	B-Algorithm
value	O
A	O
,	O
</s>
<s>
More	O
generally	O
,	O
the	O
acutance	B-Algorithm
at	O
a	O
point	O
in	O
an	O
image	O
is	O
the	O
gradient	O
of	O
the	O
density	O
(	O
or	O
intensity	O
)	O
at	O
that	O
point	O
,	O
a	O
vector	O
quantity	O
:	O
</s>
<s>
Thus	O
the	O
acutance	B-Algorithm
of	O
an	O
image	O
is	O
a	O
vector	O
field	O
.	O
</s>
<s>
Perceived	O
sharpness	O
is	O
a	O
combination	O
of	O
both	O
resolution	B-Algorithm
and	O
acutance	B-Algorithm
:	O
it	O
is	O
thus	O
a	O
combination	O
of	O
the	O
captured	O
resolution	B-Algorithm
,	O
which	O
cannot	O
be	O
changed	O
in	O
processing	O
,	O
and	O
of	O
acutance	B-Algorithm
,	O
which	O
can	O
be	O
so	O
changed	O
.	O
</s>
<s>
Properly	O
,	O
perceived	O
sharpness	O
is	O
the	O
steepness	O
of	O
transitions	O
(	O
slope	O
)	O
,	O
which	O
is	O
change	O
in	O
output	O
value	O
divided	O
by	O
change	O
in	O
position	O
–	O
hence	O
it	O
is	O
maximized	O
for	O
large	O
changes	O
in	O
output	O
value	O
(	O
as	O
in	O
sharpening	O
filters	O
)	O
and	O
small	O
changes	O
in	O
position	O
(	O
high	B-Algorithm
resolution	I-Algorithm
)	O
.	O
</s>
<s>
Coarse	O
grain	O
or	O
noise	B-Algorithm
can	O
,	O
like	O
sharpening	O
filters	O
,	O
increase	O
acutance	B-Algorithm
,	O
hence	O
increasing	O
the	O
perception	O
of	O
sharpness	O
,	O
even	O
though	O
they	O
degrade	O
the	O
signal-to-noise	O
ratio	O
.	O
</s>
<s>
The	O
term	O
is	O
sometimes	O
heard	O
(	O
by	O
analogy	O
with	O
critical	O
focus	O
)	O
for	O
"	O
obtaining	O
maximal	O
optical	O
resolution	B-Algorithm
"	O
,	O
as	O
limited	O
by	O
the	O
sensor/film	O
and	O
lens	O
,	O
and	O
in	O
practice	O
means	O
minimizing	O
camera	O
shake	O
–	O
using	O
a	O
tripod	O
or	O
alternative	O
support	O
,	O
mirror	O
lock-up	O
,	O
a	O
cable	O
release	O
or	O
timer	O
,	O
image	O
stabilizing	O
lenses	O
–	O
and	O
optimal	O
aperture	O
for	O
the	O
lens	O
and	O
scene	O
,	O
usually	O
2	O
–	O
3	O
stops	O
down	O
from	O
wide-open	O
(	O
more	O
for	O
deeper	O
scenes	O
:	O
balances	O
off	O
diffraction	O
blur	O
with	O
defocus	O
blur	O
or	O
lens	O
limits	O
at	O
wide-open	O
)	O
.	O
</s>
