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The Most Important Optical Parameters
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In birding, the use of binoculars or spotting scopes for bird watching is essential. They help the observer to identify the bird without disturbing it, to study behaviour without interfering with it, and to simply enjoy watching it. In order to achieve an optimal result, good equipment as well as some necessary know-how is required.

We at ZEISS would therefore like to give you an understanding - from a technical point of view - of what is important when it comes to bird observation. In this and the articles that will follow over the coming months, we provide you with more and more information that we hope will help you to better understand ZEISS products, as well as their function.

The Most Important Optical Parameters

1.1 Magnification (M)

The first digit in the type designation indicates the magnification factor. With 10x56 binoculars, the observed object appears 10x larger than with the naked eye. To put it into simpler terms, the distance to the object shrinks by the magnification factor, i.e. a deer 400 m away looks like it is only 40 m away when you look through 10x binoculars.

Two different interpretations:

  1. The object appears e.g. 10 x larger
  2. The distance appears e.g. 10 x smaller. The object appears 40 m instead of 400 meters.

For anyone interested in precise mathematical descriptions: the magnification is defined as the ratio between the tangent of the angle at which the height of the object appears with binoculars, to the tangent of the angle at which the height of the object appears without binoculars (see the diagram above). In other words:

M = tan(viewing angle with binoculars) / tan(viewing angle without binoculars)

Typical binoculars for free-hand observation feature 7x or 8x to 10x magnification. Higher magnifications deliver a larger image and, in theory, better detail recognition, but this benefit quickly becomes the opposite due to the effect of hand tremor increasing significantly. This is something that everyone should test themselves. When in doubt, a lower magnification is more enjoyable and the better choice in the long run. Apart from a steadier image, lower magnifications (e.g. 8 x 42 compared to 10 x 42) also offers additional benefits:

  • The field of view is larger and you have a better overview of the surroundings.
  • The exit pupil is larger, resulting in a brighter image at dawn.
  • The depth of field, i.e. the area in the foreground and background that is still perceived as sharply focused, is also larger. You do not have to refocus for minor range changes constantly.

1.2 Lens Diameter

The second digit in the product designation indicates the diameter of the objective lens (that is the lens at the opposite end of the binoculars to the eyepieces) in millimeters. For example, 8 x 56 binoculars have an aperture of 56 mm. The higher this value the more light enters the binoculars. On the flip side, a larger lens diameter also means more weight. However, in recent years, the use of state-of-the-art housing materials and thinner lens elements has resulted in a noticeable reduction in weight.

Unfortunately, on lower-quality binoculars the effective aperture, i.e. the effective entrance pupil, is occasionally much smaller than the stated diameter. This may be due to the small dimensions of the prisms, lens elements, or other components that obstruct the flow of light inside the binoculars. For example, if only 3 mm of the incidental light is cut all around the outer edge of an 8 x 56 model, the effective entrance pupil is 50 mm only. These are now effectively only “8 x 50” binoculars therefore, and have about a 20% smaller light transmission than expected and 20% less light reaches the eye.

1.3 Exit Pupil

In addition to the objective lens aperture, the magnification plays a key role in image brightness. The larger the image on the retina, i.e. the larger the area on which available light is distributed the darker the image.

The key factor is the exit pupil which takes these two effects into consideration. It is the “window” or the “ circular light exit” from which light emerges from the binocular eyepieces. The larger it is the more light that can reach the eye and, therefore, the brighter the image.

The diameter of this exit pupil is calculated on the basis of the following influencing parameters:

Exit pupil = lens diameter / magnification

8 x 56 binoculars includes an exit pupil with a diameter of 7 mm, 10 x 30 binoculars 3 mm only. If you reach out your arm and hold the binoculars against a light surface and look at the eyepiece, you can clearly see the exit pupils. These are absolutely round with crisp edges and evenly bright on high-quality binoculars.

The exit pupil can be interpreted as the diameter of the luminous flux that exits the binoculars. The eye also plays a vital part in the perceivable image brightness: only the part of the luminous flux that enters the eye can contribute to image brightness. If the user’s pupil is only open 2 to 3 mm during the day, the 7 mm exit pupil of 8 x 56 binoculars cannot be fully utilized. The image through an 8 x 56 model is then no brighter than through 3 x 10 binoculars with a 3 mm exit pupil – assuming all other quality features are identical.

To take full advantage of the emerging light intensity of a binocular the eye pupil has to be at least as large as the exit pupil of the binoculars.

The exit pupil remains the decisive factor in suitability for use at dawn and dusk. In this case, the exit pupil on binoculars should be as large as possible but no less than 4 mm. If the exit pupil is larger than the pupil of the eye, the theoretical brightness of the binoculars is not fully utilized but the eye still has a certain amount of flexibility. Despite shaking hands or binoculars not placed exactly in front of the eyes, you still remain “in the picture” literally. The maximum aperture of the eye pupil largely depends on age.

The pupils of children can open up to more than 8 mm, while they seldom exceeds 4 mm in old age.

Average maximum pupil diameter (at night) in relation to age.

While a user’s pupil may only be 2-3mm on a bright day, so they would not be utilizing the full exit pupil of even an 8x32 binocular (4.0mm), it should be remembered that the exit pupils of popular binoculars such as 8x32, 8x42 and 10x42 can still be useful if the day becomes dark due to cloud and rain, or if the user stays out unexpectedly longer, into the twilight of dusk.


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