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Contrast and resolving power Contrast is defined as the difference in brightness between light and dark areas of the image. The
greater this difference, the higher the contrast. An image that is rich with contrast is subjectively perceived to be sharper and more
brilliant. Resolving power is the extent to which an optical system is capable of reproducing the finest structures. The greater the
resolving power, the better are the finest details rendered, even under difficult light conditions. Pictorial examples : In optimally
rendered images (left), both contrast and resolution are high. If the contrast is too low, the image is flat (center). If the resolution
is too low, the image is unsharp, one can no longer discern details in the feathers (right).
Depth of field Depth of field is the distance between the nearest and the furthest away
objects that can still be perceived as being sharp at a given focus setting without the need for
refocusing. The depth of field is dependent upon the magnification of a binocular or spotting
scope. The lower the magnification, the greater the depth of field. Therefore, if one wishes to
see several objects at different distances but with the same sharpness (as illustrated on the
left), one would give preference to a binocular with a 7x or 8 x magnification, which would
also have great advantages for observation at twilight or in poor light conditions, because
one would not have keep refocusing to maintain a sharp image. The greater the magnifica-
tion, the smaller the depth of field. Greater magnifications are advantageous when one wishes
to discern more details or smaller objects, as illustrated on the right.
Reflections and flare When light rays are reflected by lens surfaces and/or structural
components or scattered by lens mounts, they arrive at a location in the image that is
different from the one where they are intended to arrive. Undesired reflections and light
areas that can degrade the image significantly and that can impair the visual impression
occur especially when the sun shines directly into the front lens. To prevent reflections
and flare caused by stray light rays, Leica takes numerous dedicated steps. To begin with,
the shapes of lens surfaces, mounting components and light traps are already optimized
during the design stage so that no extremely disturbing effects are to be expected.
Furthermore, a significant reduction in reflections and stray light components is achieved
by means of special coatings of lens elements (vapor deposition of special reflection-
reducing layers) and by the matte black finish of internal mechanical components.
Optical quality Imaging errors (aberrations) In order for a sharp image to be created, light from a point on the subject must re-converge as a
point in the picture. As a rule, a single lens element is not adequate for this purpose, because it has inherent deviations (imaging
errors or aberrations), that are described a follows : By selecting appropriate types of optical glass and lens coatings, and by
combining the right lens elements and by converting the computed lens design into reality a accurately as possible, Leica suc-
ceeds in keeping all the residual aberrations at a very low level. To that end all phases, from optical design to fabrication, are
optimized for the always highest possible imaging quality.
Aperture errors – spherical aberration The closer to the edge light rays pass
through a lens, the more they will tend to arrive away from the actual picture
point. Because this effect becomes stronger as the front lens diameter increases,
it is referred to as aperture error, also called spherical aberration. Aperture
error causes a loss of sharpness and contrast in the image. In extreme cases,
flare becomes noticeable – halos are formed around point sources of light, as
illustrated in the pictorial example above.
Coma When coma is present, light rays will deviate to one side of their intended
picture point. The picture point will gain a tail like a comet. This effect occurs
more towards the edges of the picture and not in its center. Strong coma leads
to a loss in sharpness and contrast, in extreme cases the coma tail becomes
noticeable in point sources of light, as illustrated in the enlarged section of a
picture of a star. This section was cropped from the left upper corner of the
moon picture.
Glossary For illustration purposes, the pictorial examples show strongly exaggerated
effects of the various types of imaging errors or aberrations.