Optics ABC




  • Carl Zeiss Advanced Optics System (AOS)

    To build optical systems providing maximum image quality and performance, glass types used to be required whose optical properties are only achieved by adding lead, arsenic or other metals. These additions have a high specific density and the binoculars, especially models with high magnifications and twilight performance, are correspondingly heavy. In other words, new solutions had to be found.
    After many years of intensive cooperation with Zeiss optical scientists, Schott Glas, Mainz, a company of the Carl-Zeiss-Stiftung and the world's biggest special glass manufacturer, has now succeeded in producing glass types without arsenic and lead, providing the optical properties required for systems of maximum image quality. The new glass types from Schott are markedly lighter, and their processing does not require the disposal of environmental pollutants. This success laid the foundations for the Advanced Optics System (AOS) from Carl Zeiss which has resulted in new optical systems with exceptional qualities.

  • Carl Zeiss T* multi-layer coating

    The improvement of image quality achieved with single-layer coatings on lens elements/prisms was increased even further by depositing several different layers. With the Zeiss T* multi-layer coating (T*) on almost all Zeiss binoculars and riflescopes - the relevant models are marked with T* -, maximum transmission and contrast are achieved throughout the entire spectral range.

  • Close range

    A binocular with a short focusing distance opens up totally new dimensions to the viewer. Animal and nature lovers can observe butterflies or other insects from a very short distance. In riflescopes whose objective lenses are usually adjusted for a range of 100m, the "close range" (depth of field) is dependent on the power used. Variable-power scopes make it possible to change to a lower power, thus increasing the depth of field.

  • Contrast (modulation transfer function, MTF)

    An important factor determining image quality is contrast rendition, as it determines whether object details are still recognizable or not. Contrast rendition is measured by the "contrast transfer function" or "modulation transfer function" (MTF) and describes how well the optical system concerned reproduces the existing brightness conditions (contrast). Here, it is of major importance that the optical system does not only image large object details with high contrast. For this reason, the binoculars and riflescopes from Carl Zeiss which are also intended for use in twilight are designed in such a way that they still provide contrast of 20% and more at the resolution limit of the human eye.



  • Eyepieces

    Binoculars are divided into two types – standard eyepieces and high-eyepoint eyepieces for eyeglass wearers.

    Standard eyepieces have a PD – distance from the exit pupil to the last lens vertex of the eyepiece – of approx. 9mm enabling the exit pupil to be aligned with the eye pupil of the viewer, which allows the observer to see the entire field of view.

    High-eyepoint eyepieces for eyeglass wearers usually have a PD of at least 15mm and a maximum of 20mm, enabling eyeglass wearers to have a complete oversight of the field of view.

    On a side note: the invention of high-eyepoint eyepieces for eyeglass wearers was introduced to the civilian market in 1958 by Horst Köhler with the Zeiss 8 x 30B binoculars.

    Both standard eyepieces and high-eyepoint eyepieces for eyeglass wearers are available with standard field of view and wide-angle field of view (WW). It is typical here for the ocular-side field of view (apparent FoV) to be equal to or greater than 60° with wide-angle eyepieces (Apparent FoV = FoVm@1000mx magnification / 17.5 ).

    By the way: wide-angle eyepieces were invented by Heinrich Erfle at Carl Zeiss in 1919.

  • Exit pupil

    The exit pupil (visible in the eyepiece of the binocular/riflescope as a bright disk) is important for twilight vision, as it is its size which determines the brightness of the image formed in your eye - provided the pupil of your eye is as large as the exit pupil or larger.

    The exit pupil is calculated by dividing the objective lens diameter by the magnification. In the 8x56 binocular, this results in an exit pupil diameter of 7 mm. Incidentally, this corresponds to the maximum pupil aperture of the human eye.

    Note: The exit pupil must always be circular and supply uniform brightness. If shadows are visible, this is an indicator of poor quality.


  • Field of view

    The field of view indicates what width of terrain you can see through the binocular at a distance of 1,000 m. In riflescopes, the field of view is given for a range of 100m. The term "field-of-view diameter" is also used, as the field of view is circular in shape. The higher the magnification, the smaller the field of view generally becomes. Special wide-angle eyepieces (Ww) provide an increased field of view in binoculars.

  • Focusing mechanism

    Binoculars must be focused for varying distances. They require a focusing mechanism. So-called "fix-focus binoculars" which do not provide this adjusting possibility are not to be recommended.
    We distinguish between the following types of focusing mechanism:

    • Individual eyepiece focusing (IF): mainly provided in binoculars with a low magnification (7x to 8x) for use in water sports and by national authorities - here, both eyepieces must be focused individually. These binoculars are poorly suited for all fields of application in which a frequent and rapid change in the viewing distance occurs.*
    • Center focus: focusing via a center drive wheel acting on both binocular barrels. Depending on the design used, both eyepieces (eyepiece focusing via a center wheel), both objective lenses (objective lens focusing via a center wheel) or internal lens elements (internal focusing) are moved. It is important that these binoculars also provide a dioptric adjustment - usually on the right eyepiece - to allow the compensation of differences in vision between the left and the right eyes.


    * The statement: "Focus once and your image stays focused" only applies with great limitations - the older the viewer, the greater the limitation.

  • Fungus

    In optical instruments, fungus growth is particularly dreaded. This is mold spreading inside the units and resulting in a permanent clouding and hence in destruction of the optics. All binoculars and riflescopes from Carl Zeiss are provided with a built-in protection against fungus growth. Despite this, it is advisable to store optical instruments – in particular in tropical conditions - in a dry and/or cool place to prevent the growth of fungus.


  • Galilean telescopes

    Galilean telescopes (named after astronomer Galileo Galilei, 1564 – 1642) employ a collecting lens for the objective lens and a diverging lens for the eyepiece. Galilean telescopes produce an upright, laterally correct image. As a result of its design, it does not have an intermediate image plane, and its exit pupil lies in the eyepiece lens. Therefore, high-eyepoint eyepieces for eyeglass wearers – they provide the field of view with or without glasses – are not possible here. Galilean telescopes are limited to a maximum magnification of 4x and are therefore, preferably used for opera glasses.
    The Diadem opera glass from Carl Zeiss, however, is a telescope with erecting prisms and is based on the Kepler telescope.

  • Geometric light gathering power

    Geometric light gathering power is a measure of the image brightness provided. It is calculated as the "square of the exit pupil". For instance, a 10x40 binocular has a geometric light gathering power of 16 - the minimum figure for sufficient image brightness in twilight - and a 8x56 binocular a figure of 49. A comparison: An 8x30 binocular has a geometric light gathering power of 14.1 and thus is less suitable for viewing in twilight.

    Note:The geometric light gathering power is only one parameter among many, it does not say anything about the image quality which is a determining factor in image brightness!


  • High-eyepoint eyepieces (B), wide-angle high-eyepoint eyepieces (Ww)

    A high-eyepoint eyepiece provides a full field of view with and without eyeglasses. Due to a special design of the optics developed by Carl Zeiss, the exit pupil is located at least 15 mm away from the last lens vertex.

    This allows you to bring the pupil of your eye into the exit pupil even if you wear glasses. With Zeiss binoculars, the necessary eye relief is achieved using the following three methods:

    Eyecups with push-pull mechanism *
    Eyecups with rotating mechanism *
    Foldable rubber eyecup.

    If you wear glasses, fold down the eyecups, i.e. flatten them; if you don't, use the eyecups in their extended position.
    High-eyepoint eyepieces provide a standard field of view of 110m to 115m at 1,000m at 8x magnification. In the wide-angle high-eyepoint eyepieces, the field of view is markedly wider - it is 132m to 135m at 1,000m at 8x magnification - and with Zeiss binoculars, of course, this full field of view can be seen with and without eyeglasses. As both the high-eyepoint eyepieces and the wide-angle high-eyepoint eyepieces offer this benefit, all binoculars from Carl Zeiss are provided with them.
    Note: it is not the mechanism or the foldable eyecup which makes a high-eyepoint eyepiece - there are a number of eyepieces with rubber eyecups which provide only 50% of the nominal field of view - but the design of the optics.

    * As far back as 1954, Hensoldt applied for a utility patent for this type of adjustable eyecups and hence played a major role in the development of binoculars at a very early date.


  • Image stabilization (S)

    The Zeiss Classic 20x60 T* S features an image stabilization system - - all-time first for Zeiss.

    This system compensates for hand tremor, thus allowing viewing at 20x magnification without the need for a tripod.

  • Interpupillary distance (PD)

    PD is the distance between the midpoints of the eye pupils of the observer.

    It is important to carefully set the PD on the binoculars so that you can look directly along the optical axis, as the residual aberrations of the optics are minimized here.



  • Kepler telescope

    The simplest Kepler telescopes (named after astronomer Johannes Kepler, 1571 – 1630) consist of a collecting lens for the objective, and a collecting lens for the eyepiece. The objective lens projects an upside-down, reversed image in the intermediate image plane. Cross-line grids can be added here to estimate distance, or data, e.g. from a compass or range finder, can be injected. Because the image is upside down and reversed, a Kepler telescope requires an erecting system (prisms or lens elements) if it is to be used for earth observation. All modern binoculars and riflescopes are Kepler telescopes.



  • Magnification

    The first figure, e.g. 8x, specifies the magnification. In practice this means that you will see an object 100 m away as if you were looking at it with the naked eye from a distance of 12.5 m. In other words, the object appears 8 times closer.


  • Nitrogen filling (N2 filling)

    Optical instruments are filled with nitrogen - it makes up 76% by weight of the earth's atmosphere - to prevent the entrance of moisture and hence internal fogging of the optics. In addition, filling with nitrogen provides the benefit that fungus cannot develop which would destroy the optics. Filling with nitrogen, however, makes only sense if the sealing of the instrument is so good that nitrogen cannot escape to the surrounding atmosphere as a result of pressure and temperature fluctuations.


  • Objective lens diameter

    To reduce color fringes to a minimum, Zeiss mainly uses two objective lens types in its binoculars.

    Objective lenses of the "superachromat" type are also termed "fluorite objective", "ED", "HD" or "EDX".

    • The achromat provides good color correction - however, at the field-of-view edge minor color fringes continue to be seen, particularly with pronounced black/white contrast (secondary spectrum).
    • The superachromat provides very good color correction, as its secondary spectrum is only about half that of the achromat.

  • Objective lens (achromat, superachromat)

    The second figure, e. g. 56, specifies the diameter of the objective lens in millimeters. This figure indicates how much light can enter the binocular/riflescope. For daytime viewing, an objective lens diameter of 20 mm is sufficient at a magnification of 8x. In twilight, the objective lens should be large enough to optimize the amount of light entering the binocular, and this can only be achieved with a large diameter.


  • PD

    PD is the distance between the exit pupil and the last lens vertex of the eyepiece(s).

  • Phase correction (P-Coating) (P)

    In binoculars with roof prisms (Abbe-König and Schmidt-Pechan), interference results in an unfavourable intensity distribution impairing resolution. This becomes particularly apparent at high magnifications and small exit pupils.

    The deposition of dielectrical layers on the roof surfaces (P coating) corrects this effect in such a way that the contrast and resolution provided by Carl Zeiss roof prism binoculars are markedly increased. All Zeiss binoculars containing roof prisms are provided with this phase correction coating which significantly improves resolution.

  • Prisms

    Four types of reversal prism are used which determine the design of Zeiss binoculars

    1. Porro 1: Wide-bodied binoculars with a low height, e.g. 7x50B/GA 2. Porro 2: Used in the monocular and binocular 20x60 T* S 3. Schmidt-Pechan: compact binoculars, e.g. all pocket binoculars from Carl Zeiss 4. Abbe-Koenig: Long, slim binoculars, e.g. Victory 8 x 56 B T*. The generic term used to describe Pechan and Abbe-Koenig prisms is roof prisms.



  • Rubber armor (GA)

    The rubber armor on the binocular's housing is primarily used to protect the surface and to mute noise. The rubber armor does not have any (positive) effect on the sealing of the binocular concerned.


  • Sealing (ISO standard)

    Here, we distinguish between "spraywater-proof" and "water-proof". "Spraywater-proof" means that the unit concerned can be exposed to rain without any moisture creeping into the interior. "Water-proof" indicates increased sealing which prevents an interchange between the atmospheres inside and outside the unit. Here, it is important that the leakage test is performed in compliance with the ISO 9022-8 standard for environmental testing. Carl Zeiss applies this standard not only to the leakage test, but also to other environmental tests, such as cold/heat, etc.

    It is important to note that Carl Zeiss also complies with the DIN 58 386 standard which allows a maximum deviation of 5% from specifications (e.g. in brochures) for the "magnification", "objective lens diameter" and "field of view " parameters.

  • Spotting scope

    Spotting scopes are high-magnification telescopes for earth observation. Many models have an extendable housing – retractable telescopes – the majority, however, have a rigid housing. For this, there are generally models with straight and inclined viewing available.
    In part – as with the Diascope 65 T* FL and 85 T* FL spotting scopes – you have the option of a set magnifying and vario eyepiece.

    By the way: stability of the stand and the mobility of the associated video head are just as important as image quality with spotting scopes.

  • Straylight

    Straylight is caused by reflections from the housing, lens edges and mounts and other components. As straylight is superimposed on the image, it results in markedly reduced image brilliance. In Zeiss binoculars and riflescopes, straylight is kept to a minimum by a large number of measures/precautions. These precautions not only include the careful selection of the glass types used and a special treatment of the inner surfaces of the housing or the painting of the lens edges, but also the use of special procedures for the lens and prism mounts and the reversal systems in binoculars/riflescopes to reduce straylight to below 2%, if possible.


  • Transmission

    This is the amount of light in % which can pass through an optical system. Here, it is not only important that it is as high as possible - 90% is standard in binoculars and riflescopes from Carl Zeiss - its maximum must also lie in the right spectral range, an important factor in binoculars to be used in low light conditions. As the sensitivity to blue of the human eye increases in twilight, an image with a yellow or pink tinge in daylight indicates a low transmission in the blue spectral range and hence poor detail recognition in low light conditions.

  • Twilight factor/Twilight performance

    The twilight factor makes it possible to compare the performance of binoculars in low -light conditions. It is calculated by first multiplying the magnification by the objective lens diameter and then finding the square root of the result. In a 7x42 binocular, the twilight factor is therefore 17.2 - the minimum for sufficient detail recognition in twilight - and an 8x56 binocular has a twilight factor of 21.2. A comparison: An 8x30 binocular, on the other hand, has a twilight factor of 15.5 and is therefore less suitable for viewing in very low light conditions.

    Note: The twilight factor is only one parameter among many, it does not say anything about the image quality which is a determining factor in detail recognition in twilight (twilight performance)! Twilight performance is mainly determined by as high a transmission as possible in the right spectral range, as low a straylight portion as possible, as high contrast as possible and as high a resolution as possible. Only if all these requirements are met at the same time - and only then - can the twilight factor be used a measure of the twilight performance in binocular viewing.