Feature Explanation - Westfalia Wetekom 90 01 59 Instruction Manual

Feature Explanation

Magnification
Prismatic binoculars are defined to a combination of numbers, here 9x60 mm.
The 9 indicates the magnification and the 60 is for the diameter of the front lens in
millimetres. This diameter determines how much light can enter into the optical
system.
Twilight Output
The twilight output is an important factor to quantify the ability how much details
you can see in dim light. The higher the value, the more you can see details in
twilight. The factor is found by multiplying the objective lens diameter by the
magnification and then finding the square root of that product. In this case the
value is 23. With this feature the night binoculars is perfect for viewing objects in
dim light conditions.
Exit Pupil
The exit pupil is the size of a virtual aperture, from which rays pass through
binoculars. To find the exit pupil size, divide the diameter of the objective lens by
the magnification, in this case approx 6.5 mm. You can see the exit pupil if you
hold the night binoculars approx. 20 cm away from the eyes and look into a bright
afterimage. You will see a bright circle in the centre of the lens. The larger the exit
pupil is, the brighter the image will be while observing the object.
Angle of Vision
The FOV (field of view) is indicated in a distance of 1000 m. The unit has a FOV
of 95 m at 1000 m distance and an angle of vision of approx. 5.5º
FOV
Angle of Vision
6