Home   Contact Us  

How Binoculars Work

All binoculars are essentially based on classical telescopes, which consist, in their most basic fashion, of two lenses. The objective lens, the ones further from to the eyes provide an image, which is then enlarged by the eyepiece lens or the ones nearest the viewers eye by moving it closer or further away from the objective lens.

Because binoculars are two telescopes, side by side, they produce an image which has the depth of field that we are used to, rather than just a large flat image. As light enters the binoculars it is refracted (bent) and directed through the lenses the image becomes back to front, and upside-down. To correct this and allow the viewer to see the image the right way, two prisms are placed inside the binoculars, between the objective and the eyepiece. The presence of these four prisms in the shoulders of the binoculars is what gives them their squat appearance.

Power, Light and Weight

The optical power of the binocular is expressed by two numbers, such as 7 x 35. The first representing the number of times magnification, and the second is the objective lens diameter. A bigger objective lens captures more of the available light making it more suitable for low light conditions

The magnification factor tells you how many times larger the object will be magnified or how much closer it will appear – a number between 4 and 7 is suitable for most applications. If magnification is larger than 9 or 10 the natural shake of the human hand will be magnified to such an extent that the image becomes difficult to see, making it necessary to use a tripod.

As glass has a tendency to reflect as much as 5% of the light that arrives at its surface back towards the light source, simple coatings have been devised to prevent this. There are several grades of coatings available that allow more light to pass through the lens, and less to be reflected back. Better quality binoculars have multiple layers of coatings applied to the front and rear of the lens. These coatings are designed to allow the maximum transmission of light through the lens, resulting in a brighter clearer picture along with and minimum reflection and diffraction.

New lightweight binocular models have evolved with the use of roof prisms, rather than the traditional Porro prisms. This means that they have no ‘shoulders’ and look more modern and streamlined. Without the superfluous casing it makes them easier to carry, and substantially lighter than traditional binoculars. However the price tag for these advanced models is significantly more than for the traditional types with similar magnification.

Jargon Explained

There are many different terms used in describing binoculars so and before rushing off to the store, it is worth becoming familiar with the more basic ones. For example, there are several different terms to describe the coatings used to reduce the amount of light reflected back through the lenses during magnification:

In the last case, one would expect a good quality set of optics able to transmit between 92% and 95% of all available light back to the eye.

The term “Exit Pupil” is very important as it indicates the diameter of the light fed to the eye. It is calculated by dividing the power with the objective lens size. Considering 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 therefore it would be better to have a smaller Exit Pupil if the device is going to me used mainly during the day. Similarly if the value is smaller than 7mm for a night instrument, then it is not taking advantage of the human anatomy. A word of advice – always use binoculars with a large Exit Pupil in the evening or dark, to keep the pupil as wide as possible whilst communing with nature.

Finally the phrase ‘Eye Relief’ refers to the way that the eyepiece is set up with respect to the other optics in the device. Most binoculars come with eye relief between 9mm and 13mm, this refers to the distance from your eye to the lens before your field of view becomes limited. If you wear eyeglasses, then eye relief above 14mm is more desirable. This is because you will already have a certain amount of distance between your eye and your eyeglass lens, which you can not change. If the eye relief is to small then the field of view will be restricted causing you to miss out on most of the picture!

Uses and Solutions

Before you decide how to choose binoculars you need to consider the environments where you plan to use them. It may even be more suitable to have a single scopes or spotting scopes, for example, when hunting for example. Since these scopes 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.

Hunting or birding glasses need to be good in all light conditions, from dawn to dusk. In general they should be lightweight, but probably with a smaller magnification, and larger objective. High powered binoculars, where the power exceeds 10x will need to be mounted on a tripod. The most powerful 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 without a tripod.

If you are going to do hiking while hunting or birding, it is important to note that optics are very fragile, and so plastic lenses instead of glass ones, and a rugged case are probably going to be more important than high power, or the ability to use them at night.

The best binoculars will seem to disappear from your awareness while you're using them, so that your attention is on the bird or animal, not the binoculars. The best binoculars make you feel as if you are simply seeing through your own eyes, only closer. You can view through good optics all day long without any sense of strain. With inferior optics, you will feel a subtle sense of relief when you stop looking through them. Therefore it could pay you to invest in the instrument that gives you most enjoyment. You will never be sorry you bought the best binoculars.

When choosing binoculars price is usually going to be major consideration. There are several factors that will affect the price including the type of lens and coating that is used. Multi coated glass lenses will produce a picture at high magnification which is substantially clearer and brighter than that produced by glass or plastic lenses without the extra coatings. Plastic lenses, on the other hand, tend to result in lighter binoculars; however they will be substantially more expensive for a similar grade of picture quality.

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.


Portable Solar Power Gear
www.flexopower.com
Johannesburg • South Africa • Tel +27 (0)11 465 0022

Rita Liotta is a successful freelance writer offering guidance and suggestions for consumers regarding camping gear, camping equipment, tents and GPS.