<s>
In	O
video	B-Device
projection	I-Device
terminology	O
,	O
throw	O
is	O
the	O
distance	O
between	O
a	O
video	B-Device
projector	I-Device
lens	O
and	O
the	O
screen	O
on	O
which	O
it	O
shines	O
.	O
</s>
<s>
It	O
is	O
given	O
as	O
a	O
ratio	O
(	O
called	O
throw	B-General_Concept
ratio	I-General_Concept
)	O
,	O
which	O
describes	O
the	O
relationship	O
between	O
the	O
distance	O
to	O
the	O
screen	O
and	O
the	O
width	O
of	O
the	O
screen	O
(	O
assuming	O
the	O
image	O
is	O
to	O
fill	O
the	O
screen	O
fully	O
)	O
.	O
</s>
<s>
Throw	B-General_Concept
ratio	I-General_Concept
is	O
a	O
characteristic	O
of	O
the	O
lens	O
of	O
the	O
projector	O
(	O
although	O
"	O
projector	O
throw	O
"	O
and	O
"	O
lens	O
throw	O
"	O
are	O
often	O
used	O
synonymously	O
)	O
.	O
</s>
<s>
Some	O
lenses	O
are	O
fixed	O
at	O
a	O
specific	O
throw	B-General_Concept
ratio	I-General_Concept
,	O
while	O
most	O
are	O
adjustable	O
(	O
zoomable	O
)	O
to	O
a	O
range	O
of	O
throw	B-General_Concept
ratios	I-General_Concept
.	O
</s>
<s>
Although	O
it	O
is	O
often	O
stated	O
as	O
a	O
single	O
value	O
(	O
or	O
range	O
of	O
values	O
)	O
,	O
throw	B-General_Concept
ratio	I-General_Concept
is	O
a	O
comparison	O
of	O
D	O
:	O
W	O
.	O
To	O
reduce	O
this	O
to	O
a	O
single	O
number	O
(	O
as	O
is	O
typically	O
seen	O
in	O
projector/lens	O
specifications	O
)	O
,	O
start	O
by	O
dividing	O
both	O
sides	O
by	O
W	O
,	O
leaving	O
us	O
with	O
D	O
/	O
W	O
:	O
1	O
.	O
</s>
<s>
A	O
video	B-Device
projector	I-Device
(	O
lens	O
)	O
with	O
a	O
throw	B-General_Concept
ratio	I-General_Concept
of	O
2.0	O
(	O
or	O
"	O
2.0	O
:	O
1	O
"	O
)	O
would	O
need	O
to	O
be	O
positioned	O
at	O
a	O
distance	O
that	O
is	O
twice	O
the	O
width	O
of	O
the	O
screen	O
.	O
</s>
<s>
A	O
video	B-Device
projector	I-Device
(	O
lens	O
)	O
with	O
a	O
throw	B-General_Concept
ratio	I-General_Concept
of	O
0.4	O
or	O
less	O
would	O
be	O
positioned	O
relatively	O
close	O
to	O
the	O
screen	O
,	O
and	O
would	O
be	O
considered	O
a	O
"	O
short	O
throw	O
projector	O
"	O
.	O
</s>
<s>
A	O
video	B-Device
projector	I-Device
that	O
must	O
be	O
positioned	O
very	O
far	O
from	O
the	O
screen	O
would	O
need	O
a	O
"	O
long	O
throw	O
lens	O
"	O
with	O
a	O
throw	B-General_Concept
ratio	I-General_Concept
like	O
7.5	O
.	O
</s>
<s>
While	O
some	O
lenses	O
have	O
a	O
fixed	O
throw	B-General_Concept
ratio	I-General_Concept
(	O
as	O
in	O
the	O
examples	O
above	O
)	O
,	O
most	O
are	O
adjustable	O
.	O
</s>
<s>
This	O
is	O
referred	O
to	O
as	O
a	O
zoom	O
lens	O
,	O
and	O
the	O
throw	B-General_Concept
ratio	I-General_Concept
is	O
given	O
as	O
a	O
range	O
(	O
e.g.	O
</s>
<s>
This	O
provides	O
flexibility	O
in	O
positioning	O
the	O
projector	O
,	O
since	O
the	O
lens	O
can	O
be	O
adjusted	O
to	O
accommodate	O
any	O
throw	B-General_Concept
ratio	I-General_Concept
within	O
that	O
range	O
.	O
</s>
<s>
EXAMPLE	O
:	O
A	O
video	B-Device
projector	I-Device
with	O
a	O
throw	B-General_Concept
ratio	I-General_Concept
of	O
1.2	O
-	O
3.0	O
is	O
to	O
be	O
used	O
on	O
a	O
screen	O
that	O
is	O
100	O
"	O
wide	O
.	O
</s>
<s>
CAUTION	O
:	O
In	O
some	O
projector/lens	O
specifications	O
,	O
the	O
ability	O
of	O
a	O
lens	O
to	O
adjust	O
(	O
zoom	O
)	O
its	O
throw	B-General_Concept
ratio	I-General_Concept
is	O
given	O
as	O
yet	O
another	O
ratio	O
.	O
</s>
<s>
A	O
greater	O
zoom	O
ratio	O
means	O
that	O
the	O
lens	O
is	O
able	O
to	O
adjust	O
more	O
,	O
but	O
it	O
says	O
nothing	O
about	O
the	O
specific	O
throw	B-General_Concept
ratios	I-General_Concept
the	O
lens	O
is	O
designed	O
for	O
.	O
</s>
<s>
Another	O
lens	O
with	O
a	O
throw	B-General_Concept
ratio	I-General_Concept
range	O
of	O
2.4	O
-	O
6.0	O
would	O
have	O
the	O
exact	O
same	O
zoom	O
ratio	O
as	O
the	O
previous	O
example	O
.	O
</s>
<s>
The	O
"	O
zoom	O
ratio	O
"	O
is	O
more	O
useful	O
for	O
marketing	O
purposes	O
,	O
while	O
the	O
actual	O
"	O
throw	B-General_Concept
ratios	I-General_Concept
"	O
are	O
necessary	O
for	O
designing	O
a	O
projection	O
system	O
.	O
</s>
<s>
A	O
relatively	O
small	O
percentage	O
of	O
lenses	O
have	O
no	O
zoom	O
capability	O
(	O
fixed	O
lenses	O
)	O
,	O
so	O
their	O
actual	O
throw	B-General_Concept
ratio	I-General_Concept
would	O
be	O
given	O
as	O
a	O
single	O
value	O
.	O
</s>
