Technical terms binoculars and spotting scopes


The magnification value of a lens is one of the most important terms since magnification is directly connected with the use of an optical device. The specified magnification (e.g. 6x) denotes that the distance to the object is reduced by that factor, i.e. an object at a distance of 60 m viewed with a 6x magnification appears as close as viewed without magnification at 10 m distance. However, it must be noted that the larger the magnification the darker the object appears.


The entrance pupil is the freely operant lens diameter through which the image/light enters the binoculars.
The exit pupil is the diameter of the circle of light that exits the eyepiece. The exit pupil relates to the entrance pupil based on magnification.

Eyepieces for eyeglass wearers

Binoculars made for eyeglass wearers feature a large exit pupil diameter. Eyeglass wearers con use the optimal field of view by folding the eyecups either up or down.

Dioptre compensation

The balancing of various eyesight values by adjusting the eyepiece.

Geometric light intensity / twilight factor

While the geometric light intensity is a measuring unit for the apparent brightness with which the object being observed is seen in the scope, the twilight factor is a unit of measurement for the details still discernible in the twilight. These, however, are simply numerical values resulting from the ratio of magnification to lens diameter. These values are less important for practical use.

Field of view

This value denotes the diameter of a section of an image 1,000 m away. Most users describe an large field of view as pleasant because the „camera shake“ has less influence on the quality of the picture. Optical devices labelled WW (wide angle) have a particularly large field of view.


Coating is a treatment in which lenses are coated with a layer that reduces reflexion. This is necessary since otherwise reflections occur on each glass-air-surface of the device causing a loss of light of 4% per surface. Some modern optical devices feature a large number of air-glass-surfaces. This thin layer reduces the loss of light to 0.5% per glass-air-surface. High quality long range lenses come with a standard multi layer coating.


By filling binoculars with one of these noble gases you prevent fogging of the inside optics when the outside temperature changes.

Roof prism

A set of prisms made from roof prisms allows for a particularly sleek design. The optical and precision engineering effort is higher than with a porro prism of equal quality.

Porro prism

Binoculars with porro prisms are generally wider than comparable roof prism models but they are not as long. Since the lenses are further apart, porro prisms offer a somewhat more three-dimensional image at shorter observation distances. This kind of binoculars is also very popular because of the excellent depth of focus.


The focusing of the lenses onto the object that’s being observed.

HDF Systemes

All models with the HDF (High Definition Fluoride) designation feature a glass that is especially difficult to process. These lenses contain calcium and have a very high refraction index. These lenses are used frequently when colour rendition is important.

Internal Focusing

For internal focusing only lenses inside the binoculars are moved to focus the object. These binoculars are generally sealed tightly thus avoiding penetration of dust, air or humidity.


The exact assembly of both halves of the glass is key for relaxed observation with binoculars. It is important that both are absolutely parallel otherwise image doubling or blurred edges quickly emerge. Adjusting the lens centres it precisely in the path of rays and both sides have the same magnification.

Central focusing device

A central focusing device used for binoculars makes it possible to focus both halves of the binoculars simultaneously. A dioptre compensation can be achieved independently, even if the user has different eyesight in his left and right eye.


The lenses facing the object during observation. These represent an important performance feature of binoculars. The bigger the lens the brighter the image. In twilight optical devices with large lenses are preferred.

Variable lenses

A variable telescope allows for continuous adjustment of various magnifications.


The lens system at the eye-space end of the scope featuring various magnifications.

Phase correction

A very elaborate coating on a roof prism. This coating reduces light diffusion and is necessary for outstanding imaging quality.