ON TARGET (2/5): Optical Dimensions

July 16 2013

#Rifle scopes #SpottingScopes #Binoculars

ON TARGET (2/5): Optical Dimensions
ON TARGET (2/5): Optical Dimensions
ON TARGET (2/5): Optical Dimensions
ON TARGET (2/5): Optical Dimensions
ON TARGET (2/5): Optical Dimensions
ON TARGET (2/5): Optical Dimensions
  • ON TARGET (2/5): Optical Dimensions
  • ON TARGET (2/5): Optical Dimensions
  • ON TARGET (2/5): Optical Dimensions
  • ON TARGET (2/5): Optical Dimensions
  • ON TARGET (2/5): Optical Dimensions
  • ON TARGET (2/5): Optical Dimensions
  • Product Specifications

    In general, magnification, objective lens diameter and, if relevant, the object-side field of view (in degrees) are marked on all optical systems.

    Binoculars marked with "8,5x42, 7,6°" means 8,5x magnification, 42 mm objective lens diameter and 7,6° objective side field of view in degrees.

    Spotting scope 30x75: 30x magnification, 75 mm objective lens diameter. Spotting scopes can usually be connected to eyepieces that offer a fixed or variable magnification.

    Rifle scope 2-12x50: variable magnification between 2x and 12x, 50 mm objective lens diameter.

    Good to know: When buying a product, you should not only compare geometrical data. Image and manufacturing quality also play a vital role. The quality of the coatings, HD optics, colour reproduction and ergonomic build are all just as important as the product specification when making your choice.


    Magnification is usually regarded as the most important dimension, indicating how many times closer an object appears to be. The higher the magnification, the closer the object appears but the field of view will become narrower. A differentiation is made between binoculars, which usually have a fixed magnification, and telescopes and riflescopes, which usually have a variable magnification (zoom function).

    Binoculars with a 10x magnification will optically magnify a deer that is 100 metres (110 yds) away, making it appear as if it were only 10 metres (11 yds) away.

    Objective Lens Diameter

    The objective lens diameter determines the size of the optical system’s entrance pupil. The bigger the objective lens diameter, the more light the system can capture. However, the size of the objective lens does not determine the size of the field of view.

    Exit Pupil

    The size of the exit pupil is determined by the objective lens diameter and the magnification.
    If you look at the eyepiece from a distance of around 30 cm (11.8 in), the exit pupil appears as a bright disc. Measure this exit pupil and you can very easily check the basic dimensions (magnification and objective lens diameter).

    For calculating the exit pupil the formula is: Exit pupil = objective lens diameter / magnification

    The larger the exit pupil, the more light will reach the eye. As a comparison, an 8x56 binocular will have an exit pupil of 7 mm (0.28 in) but an 8.5x42 binocular will only have an exit pupil of 4.9 mm (0.19 in).

    Twilight Factor

    The twilight factor defines the optical system’s performance in poor light. The statement “the greater the twilight factor, the better the suitability for twilight” only applies if the exit pupil is larger than or at least as big as the eye’s pupil.

    The pupil in the human eye can only open to around 8 mm. As we get older, our eyes become less flexible, which limits our ability to see things in twilight or at night. Therefore the binoculars’ exit pupil cannot always be fully utilised.

    For calculating the twilight factor the formula is: Twilight factor = root of ( magnification x objective lens diameter ).

    Good to know: Spotting scopes have extremely high twilight factors because of their high magnification and large objective lens diameter. But their small exit pupil makes them rather unsuitable to use in twilight. In practice, the twilight factor is of limited significance to hunters because it does not take into account the optical devices’ image and manufacturing quality. These are factors that have a key role to play, particularly in twilight conditions. Therefore, two separate binoculars with the same twilight factor can be extremely different in terms of their suitability for use in twilight.

    Field of View

    The field of view is the size of the circular section of the area which can be observed when you look through a long-range optical device. With binoculars and spotting scopes the field of view is given in metres at a distance of 1,000 metres (ft/1,000 yds), e.g. 141 m/1,000 m (423 ft/1,000 yds). In the case of rifle scopes, it is specified at a distance of 100 metres (100 yds), e.g. 42.5 m/100 m (127.5 in/100 yds). As an alternative to metres, the field of view can also be given in degrees (e.g. 6.6°).

    It is always desirable to have the largest possible field of view for long-range optical instruments. But the technically feasible size for the field of view is essentially determined by the magnification. The higher the magnification the smaller the field of view.

    Manufacturers face a major challenge to achieve a combination of the widest possible field of view and an edge-to-edge sharp image. A distinction needs to be made between the objective (relating to the object) and the subjective (relating to the user) field of view.

    Good to know: It is ultimately the purpose for which the long-range optical devices are used that determines the choice of magnification. When rifle scopes are used for driven hunting, a large field of view is crucial for quick target acquisition, so you will need to select a low magnification. For hunting in the mountains, where more time is available for taking the shot, often fired over long distances, a higher magnification is required, meaning the field of view is of secondary importance.

    Stay connected to this blog for parts 3 to 5 of ON TARGET - An introduction to high quality hunting optics!

    Learn more about high-quality hunting optics:

    ON TARGET (1/5): Structure of an optical System

    ON TARGET (3/5): Image Quality

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