LEICA Geovid 10x42 BRF - Manual (Page 48)

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Distortion The term distortion is used to describe
the effect that causes the image of an object to
be rendered with a non-uniform reproduction ratio.
For binocular observation, unlike in photography,
this effect is applied quite deliberately to reduce
the so-called “globe effect” caused by perspective
observation and by swinging the binocular. To the
observer, the image created in this manner appears
straightened out. There are two kinds of distortion :
pincushion distortion (illustrated on the top), and
barrel distortion (as shown on the bottom). Observa-
tion through a binocular that does not have deliber-
ately implemented judicious distortion is quite un-
pleasant.
Color errors (chromatic aberrations) Every image-forming
component made of glass – such a lens elements – refracts
light rays of different colors at different angles. This results in
the fact that not all the light rays that emanate from a multi-
colored point on the object are re-united at a single point in
the image. This results in color errors for the observer. Color
errors become particularly evident to the observer as color
fringes when high magnifications and long focal lengths are
being used, as they are in spotting scopes. Pictorial examples :
Color errors appear primarily as color fringes around dark
objects in front of bright backgrounds, as illustrated by the
example on the left with the red-fringed feather dress. With
APO correction of Leica (right illustration) such color effects
are no longer discernible.
Optical glasses with anomalous partial dispersion
Light rays of different colors are refracted at different angles
through the lens elements. This effect is called dispersion and
in different types of glass it is present at different levels of
strength. Most types of glass have typical, “normal” charac-
teristics. Special glasses with “anomalous partial dispersion”,
on the other hand, have characteristics that are different in
certain ranges of colors, and this makes a special color error
correction possible that cannot be achieved with normal types
of optical glass. Glasses with anomalous partial dispersion
are used for the enhancement of image quality, and they are
used in all Leica binoculars and spotting scopes.
Glossary
APO color error correction APO stands for apochromatic
color error correction. It is only beneficial for lenses with long
focal lengths and for high magnifications, like in spotting
scopes. Apochromatic color correction is achieved by the use
of special types of optical glass with anomalous partial disper-
sion and with glasses that contain fluorite. Such correction
results in images with enhanced sharpness, increased contrast
and an extremely natural color rendition.
High refraction glasses Lens elements made of high refraction glasses bend light rays more strongly than lenses with the same
shape that are made of standard glasses. Therefore the use of a lens element made of high refraction glass can achieve strong
effect without the need for lenses with excessively strong surface curvatures. This helps to prevent imaging errors, thus leading
to greater imaging performance. High refraction optical glasses are used in all Leica binoculars and spotting scopes in order to
make their short construction possible.
ASPH. ASPH. is an abbreviation that denotes the use of at least one aspherical lens surface in an optical system. Unlike regular
spherical lens surfaces, aspherical lenses have a curvature near their edges that is different from the curvature in the center of
the lens. That makes it possible to influence light rays passing through the edges of the lens differently from light rays passing
through the center portion of the lens. It also makes it possible to achieve several correction goals simultaneously with only one
lens element. Furthermore it helps to reduce the weight and the physical size of an optical system and it makes certain imaging
characteristics possible to begin with. Such surfaces help to increase the image quality or to influence the distortion. The fabri-
cation and the handling of aspherical lens elements is significantly more costly, however, than that of conventional spherical
lenses. In order to assure the extremely high image quality in all models of Ultravid binoculars, aspherical lenses are used effect-
ively in the 25 mm and 20 mm BR / BL models. In the remaining models the high image quality is achieved by means of complex
optical systems that consist of several lens elements.
Astigmatism Because of the naturally curved shape of a lens, not all light rays
converge on the same plane. While the center of the picture is sharp, the edge of
the picture appears to be out of focus. One can cause objects in one or the other
plane to be rendered sharply by refocusing. In addition, astigmatism causes the
sharpness of an object detail to be influenced by its orientation. This effect becomes
stronger towards the edges of the image. For example, if we look at the corner of
a picture of a chain link fence, it will be noticeable (if astigmatism is present) that
the wires that point towards the center of the image are reproduced with a diffe-
rent degree of sharpness than those that are oriented at a right angle to them.
By refocusing, one or the other wire direction can be rendered sharply, but not
both of them at the same time. This effect can be reduced by appropriate optical
design measures, but it cannot be eliminated completely. Astigmatism leads to a
significant impairment of the image quality.