|Optical Hardware |
Binoculars, Telescopes, Monoculars: Technical Guide & Information
It's worth outlining a few technical terms that will help you
make an informed choice of binocular or telescope.
IMPORTANT - Safety Information
Never look at the Sun through binoculars, telescopes or any other
Magnification and Brightness.
Basic binocular specifications are identified by numbers, for example, 8x21.
The first number tells you the magnification, so 8x makes a subject
appear eight times closer. If you are viewing small subjects
at long distances, you will probably need a larger magnification
and for subjects that are close range, you will probably find
a smaller magnification better. You should be aware that more
powerful binoculars will magnify any unsteadiness when you are
holding the instrument (shake), so choosing a smaller magnification
may give an apparently sharper image than a larger magnification.
The second number indicates the diameter of the objective lens
(thatís the large lens at the other end of the binoculars from
the eyepiece). Dividing the diameter of the objective lens by
the magnification of the binoculars gives a measure of how bright
the image will be as this gives the size of the exit pupil ( sometimes called Brightness Index). For example, an 8x21 binocular
has an exit pupil of 21/8 = 2.6mm In daylight, an exit pupil of 2 to 3 is normally
satisfactory, but in low light, 5mm or more (e.g.
8x40, 7x50) will be much better.
Field of View (Angle of View)
If you can see a wide area of the subject, itís much easier to
find the part of the subject you want to look at. Generally,
a more powerful binocular has a narrower field of view, but there
are variations between models. A good 8x binocular should have
an angle of view of 6o or 7o or more, while a 12x model may offer as little as 4o or 5o.
All good binocular lenses are coated in order to reduce internal
reflections and improve the image quality. Itís possible to give
even better viewing, for example, by the use of ruby coloured
anti-UV coatings which help to cut through haze, as well as protecting
your eyes against the harmful effects of ultra violet light. It
should be noted that the ruby coating may not produce the exact
colour of the subject, they often produce a "cold, blue" image.
Some binoculars are constructed with BaK-4 prisms, which utilise
a superior glass to provide brighter, clearer, sharper images
than would be achieved with Ďordinaryí prisms.
Long Eye Relief
If you wear spectacles, you may find long eye relief helpful,
as it will allow you to view a subject in greater comfort, without
pressing your glasses against the eyepieces. Itís also helpful
for short sighted users.
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