How Binoculars Work
Essentially, all binoculars and spotting scopes are derived from classical telescopes, which consist, in their most basic fashion, of two lenses. The lens nearest whatever is under scrutiny (objective lens) provides an image, which can then be enlarged by the lens nearest the viewers eye (eyepiece lens), by moving it closer or further away from the objective lens.
A pair of binoculars can be seen as two such telescopes, side by side, which together produce an image which has the depth of field that we are used to, rather than just a large flat image.
Since the light has been refracted (bent) as it has been directed through the lenses, by the time the viewer sees the image it is back to front, and upside-down. To correct this, two prisms are placed inside the binoculars, between the objective and the eyepiece. It is the presence of these four prisms in the shoulders of the binoculars that give them their squat appearance.
Power, Light and Weight
The power of the optics is expressed as two numbers, such as 7 x 35. The first is the number of times magnification, and the second is the diameter of the objective lens. A larger objective lens makes sense during low light conditions, since it can capture more of the available light.
The magnification factor tells you how many times larger the object will be magnified - a number of between 4 and 7 is ample for most applications. Any larger than about 9 or 10, and the natural shake of the human hand will be magnified to such an extent that the image becomes difficult to see, and a tripod will be required.
Glass also has a tendency to reflect as much as 5% of the light that arrives at its surface back towards the light source. A simple coating was devised to prevent this, by allowing more light to pass through the lens, and less to be reflected back. Since the advent of the original coating, the technique has been refined, and there are several grades of lens coating available.
The best result is achieved when multiple layers of coatings are applied, to the front and rear of the lens. Each coating is designed to provide the maximum transmission of light through the lens, and minimum reflection and diffraction, resulting in a brighter, clearer picture than with standard non-coated lens models.
Modern lightweight binoculars have also evolved in terms of the use of roof prisms, rather than the traditional Porro prisms. This means that they have no "shoulders" and look more modern. The lack of superfluous casing makes them easy to carry, and substantially lighter than traditional binoculars, however the price tag for higher power models tends also to be more substantial than for the traditional type of a similar magnification.
When considering how to choose binoculars price is a major consideration. There are several factors that will affect the price. The first is the type of lens and coating that is used; glass lenses, which are coated on each side with multiple layers, will produce a picture at high magnification which is substantially clearer and brighter than that produced by plastic lenses.
Plastic lenses, on the other hand, tend to make the binoculars lighter, but will be substantially more expensive for the same grade of picture quality. If the binoculars are to be used in clear conditions, at a low power, then this may be acceptable. If more variation in lighting (i.e. dusk and night use) is expected, then one should opt for better quality optics, and hence a higher price tag.
The build quality will also affect the price. More rugged, shock-proof binoculars destined for use in harsher conditions (marine or backpacking) will cost more than those which do not need to be waterproof or shockproof.
Uses and Solutions
Before you decide how to choose binoculars you need to consider the solutions for differing environments. Single scopes or spotting scopes, for example, are often used for hunting and bird watching. Here, since they are, in effect, half the size of a regular pair of field glasses, better quality optics can be afforded, as the cost will be proportionally lower.
Bird watching binoculars need to be good in all light conditions, from dawn to dusk, and even have limited night vision. The same goes for hunting binoculars, and in both cases they should be lightweight, but probably with a smaller magnification, and larger objective.
High power spotting scopes, or binoculars, where the power exceeds 10x will need to be mounted on a tripod. The best models will be ones with a very large objective lens, suitable for use in many conditions, but will be too heavy and cumbersome for use on the move.
Finally, when hiking or orienteering, binoculars will be very useful, but it is important to note that optics are very fragile, and so plastic lenses over glass ones, and a rugged case are probably going to be more important than high power, or the ability to use them at night.
As a curio, it is possible to buy, from Zeiss, a pair of binoculars which have a mechanical anti-jog mechanism which allows for extremely high magnification, but without the shake associated with it. They come in at around $4000.
Digital binoculars are a cheap alternative, and can be picked up for considerably less ($200), and usually have a built-in camera. They are not perfect optically, with a resolution of around 3 megapixels, but will suffice for the hobbyist.
There are many different terms that are bandied about when reading descriptions of binoculars and before rushing off to the store, it is worth understanding some of the more esoteric ones.
For example, there are several different descriptions of the coating that has been used (see Power, Light and Weight) to reduce the amount of light reflected back through the lenses during magnification:
· C : Some surfaces coated
· FC : All surfaces coated, except plastic lenses
· MC : Some surfaces have been coated in multiple layers
· FMC : All glass surfaces are coated with multiple layers.
In the last case, one would expect a good quality piece of optics to be able to transmit between 92% and 95% of all available light back to the eye.
The "Exit Pupil" is also important and can be calculated by dividing the power by the objective size and yields a value which is very important - it is the diameter of the light fed to the eye. Given that the average human pupil ranges in size from 2mm to 7mm depending on the available quantity of light, it is clear that, in the midday sun, an Exit Pupil value of 4mm (for example) will mean that 50% of the image returned to the eye is lost.
By a similar token, if the value is smaller than 7mm for a night scope, then it is not taking advantage of the human anatomy. A word of advice - always use night scopes in the dark, to keep the pupil as wide as possible; this means no, or very low, light around the place that you choose to hide out whilst communing with nature.
Finally, if the phrase "Eye Relief" is mentioned, then it refers to the way that the eyepiece is set up with respect to the other optics in the device. Most glasses will come with eye relief between 9mm and 13mm, and is the distance from your eye to the lens before your field of view becomes limited.
If you wear eyeglasses, then eye relief above 14mm becomes desirable, since you will already have a certain amount of distance between your eye and your eyeglass lens, which you can not change. Given this, if your chosen optical device has a small eye relief, then you will have a very restricted field of view, and miss out on most of the picture!
As you can see when you need to decide how to choose binoculars most suitable for you there are many aspects to consider. Hopefully this article will have made your job easier. For more information on the binoculars available you can visit the relevant links.
About the Author
Guy Lecky Thompson is a successful freelance writer offering guidance and suggestions for consumers regarding how to choose binoculars, telescopes , night vision and spotting scopes . His many articles give information and tips to help people save money and make smarter decisions.