IMPORTANT - Safety Information
You should NEVER look directly at the Sun through binoculars, telescopes, monoculars, cameras or any other optical equipment.
 Understanding binoculars can seem like a daunting task, if you have little knowledge or experience. It certainly puzzled George Dubya Bush - he didn't know he had to remove the lens caps! He isn't the only one - Shimon Perez struggled too! But seriously, we'll try here to demystify some of the terms and make your decision easier. It's worth outlining a few technical terms that will help you make an informed choice of binocular, monocular or telescope and include some advice about tripods.
Magnification and Brightness
Any pair of binoculars, except the most basic toy, will be designated with some numbers and sometimes letters, for example 10x25DCF. Their basic and most important characteristics are identified by the numbers. The first number tells you the magnification, so 10x makes any subject appear ten times closer than your naked eye will see it. If you are viewing small subjects at long distances, you will possibly need a larger magnification number. For subjects that are at close range, you will possibly find a smaller magnification number better. One crucial point is that more powerful binoculars will magnify any unsteadiness (shaky hands) when you are holding the instrument, so choosing a smaller magnification number may give an apparently sharper image than a larger magnification.
The second number indicates the diameter of the objective or final lens - the large lens at the opposite end of the binoculars from the eyepiece. The size of the objective lens has important effects on the brightness of anything you see through 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 the Brightness Index). So 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
This term describes the amount of image you can see either side of the subject and everything around it. If you can see a wide area around the subject, it’s much easier to find the part of the subject you want to look at and to follow a moving subject. Generally, a binocular with more powerful magnification 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. It is a common attitude among buyers to want high magnification, only to find they become frustrated later when trying to follow a moving subject. Another case in life where less can be more.
Lens Coatings
All good binocular lenses are coated in order to reduce internal reflections and improve the image quality. It’s possible to get even better viewing 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.
The Important Issue of Prisms
It surprised me to learn some time ago that even small in-line binoculars, the kind made from one straight tube, need prisms. If you think about childhood experiments with lenses, the image would be upside dow wouldn't it? Actually there are 2 main kinds of binoculars, from the prism's point of view - Roof Prisms and the stranger sounding Porro Prisms, named atfer their inventor Ignazio Porro. Roof Prisms are used in the straight-line or in-line binoculars and Porro Prisms (which I always remember as 'bent' prisms) in binoculars with the angled or dog-leg bodies. Some binoculars are constructed with BAK4 prisms, which utilise a superior glass to provide brighter, clearer, sharper images than would be achieved with ‘ordinary’ prisms. Having BAK4 prisms can increase cost dramatically. Binoculars utilising BAK7 prisms can be thoroughly adequate and cost much less. It really depends what you want the binoculars for.
What is Eye Relief?
If you wear spectacles, you may find it helpful to buy binoculars boasting "long eye relief", as you will be able to view a subject in greater comfort, without pressing your glasses against the eyepieces. It’s also helpful for short sighted users. |
