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 optical equipment.
Magnification and Brightness.
Basic binocular specifications are identified by numbers, for example, 8×21. 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 (that’s 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 8×21 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. 8×40, 7×50) will be much better.
Field of View (Angle of View)
If you can see a wide area of the subject, it’s 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 7oor 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. It’s 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 ‘ordinary’ 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. It’s also helpful for short sighted users.