Last Updated
：2025/11/20
focal length
Noun
(optics)
A
measure
of
how
strongly
an
optical
system
converges
or
diverges
light,
the
inverse
of
optical
power.
In
the
simple
case
of
a
single
convex
lens,
it
is
the
distance
along
the
optical
axis
from
the
principal
plane
of
the
lens
to
the
lens's
focal
point
(where,
for
instance,
the
film
would
be
placed
in
a
camera).
For
more
complex
or
multi-element
optical
systems,
focal
length
need
not
correspond
to
any
obvious
measurable
physical
distance.
Japanese Meaning
光学系において光がどれだけ集光または拡散されるかの尺度を示す値。単一の凸レンズの場合、主面から焦点（例えばカメラのフィルムが位置する点）までの距離となり、複雑な光学系では必ずしも目に見える物理的距離に対応しない場合がある。
Sense（1）
(optics)
A
measure
of
how
strongly
an
optical
system
converges
or
diverges
light,
the
inverse
of
optical
power.
In
the
simple
case
of
a
single
convex
lens,
it
is
the
distance
along
the
optical
axis
from
the
principal
plane
of
the
lens
to
the
lens's
focal
point
(where,
for
instance,
the
film
would
be
placed
in
a
camera).
For
more
complex
or
multi-element
optical
systems,
focal
length
need
not
correspond
to
any
obvious
measurable
physical
distance.
（ plural ）
Quizzes for review
(optics) A measure of how strongly an optical system converges or diverges light, the inverse of optical power. In the simple case of a single convex lens, it is the distance along the optical axis from the principal plane of the lens to the lens's focal point (where, for instance, the film would be placed in a camera). For more complex or multi-element optical systems, focal length need not correspond to any obvious measurable physical distance.
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See correct answer
focal length
To capture distant subjects more clearly, photographers often increase the focal length of their lenses.
See correct answer
To capture distant subjects more clearly, photographers often increase the focal length of their lenses.
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