<s>
In	O
photography	O
,	O
a	O
bellows	B-Algorithm
is	O
the	O
accordion-like	O
,	O
pleated	O
expandable	O
part	O
of	O
a	O
camera	O
,	O
usually	O
a	O
large	O
or	O
medium	O
format	O
camera	O
,	O
to	O
allow	O
the	O
lens	O
to	O
be	O
moved	O
with	O
respect	O
to	O
the	O
focal	O
plane	O
for	O
focusing	O
.	O
</s>
<s>
Bellows	B-Algorithm
are	O
also	O
used	O
on	O
enlargers	O
.	O
</s>
<s>
The	O
bellows	B-Algorithm
provides	O
a	O
flexible	O
,	O
dark	O
extension	O
between	O
the	O
film	O
plane	O
and	O
the	O
lens	O
.	O
</s>
<s>
Bellows	B-Algorithm
may	O
be	O
part	O
of	O
a	O
camera	O
or	O
come	O
as	O
an	O
optional	O
accessory	O
.	O
</s>
<s>
Two	O
kinds	O
of	O
bellows	B-Algorithm
are	O
commonly	O
used	O
on	O
cameras	O
;	O
bag	O
bellows	B-Algorithm
are	O
normally	O
used	O
with	O
a	O
lens	O
of	O
short	O
focal	O
length	O
,	O
and	O
accordion	O
bellows	B-Algorithm
with	O
a	O
much	O
longer	O
range	O
of	O
extension	O
.	O
</s>
<s>
For	O
large	B-Algorithm
format	I-Algorithm
cameras	I-Algorithm
,	O
“	O
double	O
extension	O
”	O
refers	O
to	O
bellows	B-Algorithm
that	O
extend	O
to	O
a	O
length	O
equal	O
to	O
about	O
twice	O
the	O
focal	O
length	O
of	O
a	O
standard	O
lens	O
,	O
e.g.	O
</s>
<s>
“	O
Triple	O
extension	O
”	O
for	O
the	O
same	O
format	O
indicates	O
bellows	B-Algorithm
extension	O
of	O
450	O
to	O
500mm	O
.	O
</s>
<s>
Bellows	B-Algorithm
allow	O
movements	O
that	O
can	O
be	O
used	O
to	O
correct	O
distortion	O
in	O
a	O
photograph	O
and	O
to	O
avoid	O
converging	O
or	O
diverging	O
verticals	O
.	O
</s>
<s>
Use	O
of	O
a	O
bellows-based	O
camera	O
can	O
ensure	O
that	O
parallel	O
elements	O
in	O
a	O
scene	O
remain	O
parallel	O
in	O
the	O
final	O
photograph	O
.	O
</s>
