INSTRUMENTS
Key Points to keep in mind:
Aberrations
and Distortion:
There
are
six
kinds
of
aberrations
which
cause
image
defects
in binoculars.
They
comprise
Seidel's
five
aberrations (spherical, astigmatism,
curvature
of field,
coma
and distortion),
and
chromatic
aberration.
In
line
with
its
belief
that
brightness
and
imageSharpness are the hallmarks of quality in binoculars, Good make strives
hard to reduce aberrations in its renowned optics. The result: sharp images with
a minimum of aberrations, the godd make way!
Spherical
aberration:
Spherical
aberration
is
defined
as
any
deviation
from
the
light
source
("P"
in
the
figure)
along
the
optical
axis
at
the
focal
point
of
paraxial
rays
("P'"
in
the
figure).
This
kind
of
aberration
causes
a
noticeable
softness
of
the
image.
Entire
image
appears
out
of focus.
Astigmatism:
|
In
this
case,
horizontal
stripes
appear
in
focus
and
vertical
stripes
out
of focus,
or
vice
versa.
Astigmatism
is
most
noticeable
in
the
periphery
of
the
image
in
wide
field binoculars.
Some
so-called
wide
field
binoculars
on
the
market
feature
no
astigmatism
correction. Wide
field
of
view
binoculars
provide clear,
crisp
images
right
up
to
the
edge
of
the
view
field. |
Original
|
Vertical
lines
appear
out
of
focus |
Horizontal
lines
appear
out
of
focus |
|
Curvature
of field:
A
plane
object
at
right
angles
to
the
axis
of
an
optical
system
does
not
appear
as
a
plane
image.
With
binoculars
affected
by
this
aberration,
you
can
focus around
the
center
of
the field,
but
the
periphery
of
the
field
will
appear
out
of focus,
or
vice
versa. Binoculars
7x50
and 10x70 for example,
incorporate
optical
systems
which
fully
compensate
for
image curvature.
These
binoculars
are
particularly
well
suited
to
astronomical
observation.
The
periphery
of
the
field
appears
out
of focus.
Coma: (comatic
aberration)
|
Points
present
a
comet-shaped appearance.
|
Coma
aberration
is
manifested
by
an
image
with
a
comet-like
extension.
As
coma
increases
toward
the
exterior
of
the lens,
it
affects
the
periphery
of
the viewfield. |
Distortion:
When
the
image
differs
geometrically
from
the object,
you
have distortion.
There
are
two
types
of distortion:
positive
and negative,
both
caused
by
variations
in
the magnification.
As
distortion increases,
although
the
image
appears
in
focus
over
the
entire field,
the
size
of
the
image
differs
at
the
center
and
periphery
of
the
field
of view.
|
| Negative
distortion |
Positive
distortion
|
|
Chromatic
aberration:
Chromatic
aberration
is
caused
by
a
difference
in
light wavelength.
The
focal
length
(Fig.
1
)
or
magnification
(Fig.
2)
of
a
lens
varies
according
to
the
wavelength
of
each
type
of
incident
light. Accordingly,
if
you
look
at
an
image
through
a
lens
with
chromatic
aberration,
the
image
is
blurred
and
appears
colored
at
the fringes.
In general,
the
higher
the magnification,
the
bigger
the
chromatic
aberration.
Good Make have
developed original glass (Extra-low Dispersion) featuring
minimized
chromatic
aberration. The glass
fitted
fieldscopes
produce clear,
brilliant
imaqes
with
true color.
Fig.
1
Axial
chromatic
aberration |
Fig.
2
Magnification
chromatic
aberration
|
The-image
is
blurred
and
appears
colored
at
the fringe.
Achromatic, apochromatic
The achromatic systems of lenses eliminate
the chromatic defects for two wavelengths. The good binoculars prefer the
apochromatic system. These lenses are made up various types of special glass
assembled in a completely transparent way. Consequently, the chromatic
aberrations are eliminated in almost all the spectrum from the light.
French
