The Little ABC of Optics

Explore the world of optics

  • Advanced Optics System (AOS)


    For optics manufacturing providing maximum image quality and performance, the glass types previously required had optical properties achieved only by adding lead, arsenic or other aggregates.


    These additions have a high specific density, making the binoculars – especially models with high magnifications and twilight capacity – accordingly heavy. It was crucial to find new solutions. After many years of intensive cooperation with ZEISS optics scientists, Schott Glas, Mainz, a company of the Carl-Zeiss-Stiftung (Carl Zeiss Foundation) and the world’s biggest special glass manufacturer, succeeded in producing glass types without arsenic and lead that have the optical properties required by systems of maximum imaging quality. The new glass types from Schott are markedly lighter, and processing them does not require disposal of environmental pollutants. This success laid the foundation for the Advanced Optics System (AOS) of ZEISS, which has resulted in new optical systems with exceptional qualities.

  • Close-up Range


    For example, a short focusing distance of 2 m will reward the observer with brand-new visual experiences and fascinating images of butterflies, dragonflies, and colorful flowers – close-up and filling the field of vision.


    With objective lenses in riflescopes that are normally set to a range of 100 m, the "close-up range" (depth of field) depends on the power used. Variable-power oscilloscopes can be switched to a lower power, thereby enhancing the depth of field.

  • Contrast (modulation transfer function, MTF)


    The contrast rendition is of utmost importance for the image quality, since it determines whether the object details are still recognizable or not.


    It is measured as a "contrast rendering function" or "modulation transfer function" (MTF) and provides information about how realistically the brightness ratios (contrasts) are reproduced by the optics in question. It is important that the optics in question transmit more than rough object details with high contrast. That is why the binoculars and riflescopes manufactured by ZEISS for use in twilight hours are designed in such a way that they still provide a resolution limit equivalent to that of the eye and a contrast of at least 20%.

  • Eyepieces


    In binoculars, a distinction is made between standard eyepieces and eyepieces for eyeglass wearers. Standard eyepieces have an eye distance – the distance between the exit pupils and the last lens vertex of the eyepiece – of approx. 9 mm, allowing the exit pupils to be placed in front of the eye pupils of the observer in order to be able to survey the full field of vision.


    Eyepieces for eyeglass wearers normally have an eye distance of at least 15 mm and not more than 20 mm, so that eyeglass wearers can enjoy the full field of view as well. By the way, the invention of eyepieces for eyeglass wearers was introduced on the civilian market in 1958 with the ZEISS 8x30B Porro binoculars by Horst Köhler.


    Both standard eyepieces as well as eyepieces for eyeglass wearers are available for standard field of vision or wide-angle field of vision (WA). What stands out here is the lateral field of vision of the eyepieces, which has to be greater than or equal to 60° in wide-angle eyepieces (FV eyepiece = FVm@1000m x magnification / 17.5).


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

  • Exit pupil


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


    The exit pupil is calculated by dividing the lens diameter by the magnification.

    EXAMPLE: In the 8x56 binoculars, this results in an exit pupil diameter of 7 mm. Incidentally, this corresponds to the maximum pupil aperture of the human eye.


    The exit pupil must always be circular, sharply defined, and uniformly bright. If shadows are visible, this is an indication of poor quality.

  • Eye Width


    The eye width is the distance between the centers of the observer’s eye pupils. It is important to adjust the eye width of the binoculars to match the distance between both eyes exactly in order to view directly along the optical axes. This keeps the residual image errors of the optics down to a minimum.

  • Field of view


    The field of vision in binoculars indicates how many meters – e.g. of the edge of a forest – can be seen at one time from a distance of 1,000 m. This specification is thus given in "meters per 100 m".


    The term "field of vision diameter" is also used when the field of vision has a circular shape. The greater the magnification is, the smaller the field of vision usually is. Special wide-angle eyepieces (WA) provides a larger field of vision in binoculars.

  • Focusing Mechanism


    Binoculars need to be focused at different viewing distances. They require a focusing mechanism – so-called "fixed-focus binoculars" do not allow for adjusting and are therefore not recommended. We distinguish between the following types of focusing mechanisms:



    also called Individual Eyepiece Adjustment: this is preferable for binoculars with smaller magnification (7x to 8x) for water sports and public authorities where the focus has to be adjusted for both eyepieces. These binoculars are not suitable for any areas of application where viewing involves more or less rapid shifts between very different distances.*



    Here, the focus is adjusted via a central wheel affecting both halves of the binoculars. Depending on the design, this method moves both eyepieces (eyepiece focusing via central focusing), both lenses (objective lens focusing via central focusing) or internal lenses (internal focusing). What is important is that these binoculars also provide diopter compensation – mostly in the right half of the binoculars – so that the individual defective vision of both eyes can be compensated for as necessary.


    * The statement: "“Once adjusted – always in focus" is true only to a very limited extent: the older the user is, the greater the limitation.

  • Fungus


    Fungal infestations are the nemesis of optical instruments. These are molds that spread across the glass surfaces in a mesh, their metabolic products causing lasting turbidity and thus damage to the optics.


    All binoculars and riflescopes manufactured by ZEISS are equipped with built-in protection against fungal growth. Particularly in tropical areas, the devices should be stored in places that are as dry and/or cool possible.

  • Galilei Telescope


    In the Galilei telescope (named after the astronomer Galileo Galilei, 1564-1642), a convergent lens is used as an objective lens and a divergent lens as the eyepiece.


    The Galilei telescope provides an upright and true-to-side image without an erecting system. As a result of its design, there is no intermediate image plane, and its exit pupil is located in the eyepiece lens. That is why special eyepieces for eyeglass wearers – offering a full field of vision, with and without glasses – are not possible. Galilei telescopes are limited to a maximum of 4x magnification and are therefore preferred for use in opera glasses. The Diadem opera glasses manufactured by ZEISS consisted of a telescope with an erecting prism that was based on the Kepler telescope.

  • Geometric Light Intensity


    Geometric light intensity serves as the measure of image brightness.


    It is calculated as the "square of the exit pupil". This is why 10x40 binoculars have a geometric light intensity of 16 – the minimum for sufficient image brightness in twilight – and 8x56 binoculars have an intensity of 49. By comparison: 8x30 binoculars have a geometric light intensity of 14.1 and are therefore unsuitable for observation in advanced twilight conditions.



    Geometric light intensity can serve as a reference point only, since it says nothing about the image quality, which is significantly affected by image brightness!

  • Image Stabilization (S)


    The ZEISS ClassiC 20x60S and the DesignSelection Mono 20x60S have a unique image stabilization system – available at ZEISS for the first time. This system compensates for hand-tremor, thus facilitating 20x magnification without the use of a tripod.

  • Impermeability (ISO standard)


    Here, a distinction is made between "splash-proof" and "water-proof". "“Splash-proof”" means that the device in question can be exposed to rain without the risk of moisture penetrating the interior, which would render the optics useless.


    "”Water-proof”" refers to increased impermeability that does not permit any atmospheric exchange between the interior of the device and its environment. It is important that the inspection is carried out in accordance with the ISO 9022-8 standard for conducting environmental tests on optical devices. Carl Zeiss applies this standard not only to impermeability testing, but also for other environmental tests, such as cold/heat, etc. Here it should be noted that Carl Zeiss also complies with DIN 58 386, which indicates the maximum deviation allowed from specifications (e.g. those in the brochures) for "magnification", "objective lens diameters" and "parameters of the field of vision".

  • Kepler Telescope


    The Kepler telescope (named after the astronomer Johannes Kepler, 1571-1630) in its simplest version consists of one conveyor lens as the objective lens and a second conveyor lens as the eyepiece.


    The objective lens creates an upside down and reversed image in the intermediate image plane. The distance can be estimated with cross-line grids or data from a compass, rangefinder, etc. can be used. Since the image is upside down and reversed, the Kepler telescope requires an erecting system (prisms or lens elements) when it is used for terrestrial viewing. All modern binoculars and riflescopes are Kepler telescopes.

  • Magnification


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

  • MC Multi-Layer Coating


    MC stands for a high-quality multi-layer coating which, depending on the optics, is applied to all glass-to-air surfaces in the Diafun binoculars and the ZM/Z riflescopes.

  • Nitrogen Charge (N2 Charge)


    Optical instruments are filled with dry nitrogen (N2) – the earth’s atmosphere is approx. 76% nitrogen – to prevent moisture from getting inside and damaging the optics.


    One of many benefits of the nitrogen charge is that fungi (molds that render the optics useless) cannot develop. However, the nitrogen charge is useful only if the instrument’s seal is good enough to prevent any atmospheric exchange from taking place with the surrounding environment, e.g. due to pressure or temperature fluctuations.

  • Objective lens diameter


    In order to reduce chromatic aberration to a minimum levels, Zeiss uses two main objective lens types for its binoculars.


    • The achromatic lens already has excellent color correction, though on the edge of the field of vision – especially at higher contrasts – slight color fringing is still visible (secondary spectrum).
    • The superachromatic lens provides good color correction, but its second spectrum is only half as large as that of the achromatic lens.


    Objective lenses of the "superachromat" type are also designated as "fluorite objective lenses", "ED", "HD" oder "EDX".

  • Objective lenses (achromatic, superachromatic)


    The second figure of the characteristic data – e.g. 56 – indicates the objective lens diameter in millimeters. It is a measure of the amount of light that can enter the binoculars/riflescopes.


    An objective lens 20 mm in diameter with 8x magnification is sufficient for observation by day. At twilight, the objective lens needs to absorb as much of the fading light as possible, which can be achieved only with a large diameter.

  • PD


    PD is the distance between the exit pupil and the last lens vertex.

  • Phase Correction Coating


    Interference effects in binoculars with roof prisms cause a reduction in resolution capacity (Abbe-König and Schmidt-Pechan).


    This is especially true for higher magnifications with a small exit pupil. This is prevented by applying a "phase correction coating" (P-coating) on the roof surface by means of vacuum deposition, thereby increasing detail resolution. All binoculars with roof prisms manufactured by ZEISS are equipped with such phase correction.

  • Prism system


    There are four types of erecting prisms that influence the design of ZEISS binoculars.


    1. Porro 1: Binoculars with a wide profile and lower height, e.g. Marine 7x50B/GA
    2. Porro 2: Used with monocular and binocular 20x60S glasses
    3. Schmidt-Pechan: Compact binoculars, e.g. all pocket binoculars manufactured by Carl Zeiss
    4. Abbe-König: Long, thin binoculars, e.g. Victory 8 x 56 B T*


    The generic term used for referring to Pechan and Abbe-König prisms is roof prisms.

  • Rubber Coating (RC)


    The rubber coating of the binocular housing serves primarily to protect the surface and provide noise reduction. The rubber coating has no (positive) influence on the impermeability of the binoculars in question.

  • Spotting scope


    Spotting scopes are high-power telescopes for terrestrial observation. Some of them have telescoping housings – so-called extendable telescopes – although most have fixed housings.


    The models available here generally have straight and angled eyepieces. Sometimes – as with the DiaScope 65 T* FL and 85 T* FL spotting scopes – it is possible to choose between eyepieces with fixed magnification and those with varying magnification. By the way, the stability of the tripod and the mobility of the integrated video head is equally as important in spotting scopes as the imaging quality.

  • Stray light


    It is created by reflexes stemming from the housing, lens edges and mounts, as well as other components. Among other things, it results in significantly reduced image brilliance due to image overlap.


    ZEISS binoculars and spotting scopes keep stray light as low as possible thanks to numerous measures. These include more than just the careful selection of glass types, special treatment of the interior housing surface, and lens edge coating. In addition, the sockets and prism mountings as well as the erecting systems in binoculars/riflescopes undergo special procedures so that stray light is kept below 2% if possible.

  • Transmission


    It is the amount of light expressed in a percentage that is transmitted by the optics. In this case it is not merely important that it be as high as possible – 90% is the standard for ZEISS binoculars.


    Their maximum has to lie within the proper spectral range, particularly for binoculars designed for twilight use. Since the eye becomes extremely sensitive to blue hues at twilight, images tinted yellow or pink during the day indicate that transmission in the blue spectral range is too low, resulting in poor detail recognition in twilight conditions.

  • Twilight Parameter,
    Performance at Twilight


    The twilight parameter gives a reference value for performance in low-light conditions. It is calculated by multiplying the magnification by the lens diameter and then extracting the square root from the product.


    7x42 binoculars have a twilight parameter of 17.2 – the minimum for sufficient detail recognition in low-light conditions – while 8x56 binoculars have a twilight parameter of 21.2. By comparison: Since 8x30 binoculars have a twilight parameter of 15.5, they are not as suitable for viewing in low-light conditions.



    The twilight parameter is only one of many and does not say anything about the actual image quality. However, this is the decisive factor for detail recognition at twilight (performance at twilight). The twilight performance is determined mainly by properties such as the maximum possible transmission, low veiling glare, high contrast, and the highest possible resolution. Only when these requirements are simultaneously met can the twilight parameter be used to measure the twilight performance of a set of binoculars.

  • T* multi-coating


    The improvement of image quality achieved with single-layer coatings on lens elements/prisms was increased even furtherby applying several different layers.


    With the Zeiss T* multi-layer coating (T*) on almost all ZEISS binoculars and riflescopes – the corresponding models are labeled with T* – maximum light transmission and contrast are achieved throughout the entire spectral range.

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