THE KEY FIGURES IN THE DESIGNATION
How much we see with binoculars depends on the magnification and the diameter of the lens. The first number of binoculars indicates the magnification, the last number the lens diameter. Thus the Bresser Pirsch ED 8x42 binoculars have a magnification of 8 times and an objective lens diameter of 42 mm. For observations in twilight the relation between objective diameter and magnification is important. However, one cannot say the higher the magnification, the better the binoculars are, as the detail recognition is greatly reduced when using binoculars with high magnification. This may be due to the "co-magnified holding movements" or the poor image quality.
In contrast to the magnification, the objective lens diameter is very much a quality criterion of binoculars, because the higher the one objective lens diameter, the greater the incidence of light and therefore the brighter the image. However, you should always keep the actual purpose of the binoculars in mind. A diameter of 21 mm (8x21) is sufficient for daytime observations. However, if observations have to be made during twilight, the binoculars should have a diameter of at least 50 mm.
These characteristics are only interesting for a comparison of different types of binoculars (8x30, 7x50, 16x50 etc.). They result in all the formulas needed for comparison. They do not say anything about the quality of a pair of binoculars and do not take into account any increase in performance due to coating or higher quality glass types.
THE EXIT PUPIL
The exit pupil is the bright image point which hits the eye of the observer at the eyepiece side. This exit pupil is of immense importance for twilight vision.
The larger it is, the brighter the image in your eye appears. It is calculated by dividing the objective diameter by the magnification. With 7x50 binoculars this would be:
Calculation
(Objective lens diameter/magnification)
for example: 50 : 7 = 7.14
The maximum opening of the pupil of the human eye depends on the eyes and the age of the observer. The ability of the pupils to enlarge in the dark decreases with age. The pupils of a young observer can still dilate to about 7 mm, but in a 50-year-old observer only up to about 5 mm. In this case the nature observer cannot use the wider exit pupil of 7x50 binoculars and should better choose a lighter 8x40 binocular with an exit pupil of 5 mm. However, it can also be an advantage for older people to choose binoculars with a high exit pupil. When observing "out of motion" or on a boat (moving location), the observer's pupil (which only widens to about 2 mm during the day) does not get outside the exit pupil diameter so quickly and "shadowing" of the field of view is avoided.
THE TWILIGHT FACTOR
Most of the time it is animals that can only be observed during twilight and especially at this time of day the lighting conditions are not ideal for observations with the naked eye. Binoculars must therefore be all the brighter so that details can be seen even at dusk in the morning or evening hours.
Binoculars must already have a magnification of at least 8x and a corresponding objective lens diameter of at least 56 mm to do justice to this "twilight performance". The power to be able to make observations even at twilight is indicated in the twilight index.
This is a standardised value for the comparison of optical devices with regard to detail recognition, especially in poor lighting conditions. The higher the twilight factor, the more light the device emits.
The twilight factor is calculated from the square root of the product of magnification and lens diameter. This means with 7x50 binoculars:
Calculation
(square root of magnification x lens diameter)
for example: root of 7x50 = 18.71
THE EXIT PUPIL DISTANCE
It indicates how far away the eye may be from the eyepiece without losing sight of the full field of view. The interpupillary distance can be changed with almost all binoculars by folding rubber eyecups or by rotating or pulling out eyecups. This means that binoculars with these so-called "spectacle wearer eyepieces" can also be used by people who wear glasses. It is important that the exit pupil of these eyepieces is positioned as far back as possible (13.5-20 mm).
THE FIELD OF VIEW
In most binoculars, the field of view size (in addition to magnification and lens diameter) is also printed on the body of the binoculars. In Europe, the size of the field of view is usually given in m at a distance of 1000 m or otherwise as an angle. If binoculars have a field of view of 101m/1000m, the observer will see a 101m wide or round field of view at a distance of 1000 m.
The higher the magnification, the smaller the field of view. With binoculars that do not have any special systems (e.g. mirror prism system) but nevertheless have a very high field of view, the edge sharpness can sometimes "suffer". This can be checked when observing fine continuous structures (walls or fences) by comparing the sharpness from edge to edge (you must stand straight in front of the object of observation when checking). If the field of view is given in degrees, the corresponding "metre value" can be calculated by multiplying by 17.45. A field of view of 101 m thus corresponds to 5.79 degrees.
THE FOCUS
A distinction is made between binoculars with single eyepiece adjustment and those with central drive (central focusing). Binoculars with central drive can be adjusted faster and moving objects can be followed more quickly. Prism binoculars with single eyepiece adjustment are mostly used for marine applications. However, waterproof models with central focusing are now also available. Because of the speed of focusing, birdwatchers prefer to use central focusing or also called centre drive focusing.
THE RUBBER ARMOURING
Binoculars with rubber armouring are recommended for tough, professional use. The rubber armouring protects the binoculars against impact, falling and splashing water. The rubber armouring of all BRESSER binoculars is lightfast under normal use and insensitive to sunlight.
