Leica 77, Brochure

Page: 97 / 104

94 / 95
Color errors (chromatic aberrations) Every image-for-
ming 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 illustra-
tion) 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.
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.