What is a binocular?

The purpose of binoculars is optically to overcome distance and bring distant objects closer by magnification.

What is the purpose of the focusing wheel on the eyepiece?

It allows diopter compensation. What is the purpose of that? Being human we are not precise! Our eyes are dissimilar and differ in their focal properties. Differences of up to one diopter can be compensated by the brain without optical equipment.
But that doesn't work when it comes to using a binocular. Here the difference must be compensated, and that is the function of the diopter compensation.
Why then is there only one diopter compensation wheel? Simple answer: You need only one, because it's only the difference between the eyes that needs to be compensated.
A little example: The left eye is short-sighted to the extent of -0.5 dpt., the right eye is long-sighted to the extent of +0.5 dpt. Thus the diopter compensation required at the right eyepiece is -1 dpt. to correct this imbalance.

Why does the binocular have eyecups that can be twisted out?

The eyecups ensure the optimum distance between eyepiece and the eye.
Why is that necessary? Every binocular has what is called an eye relief. This point gives the optimum distance between the eye and eyepiece. Only when the eye relief coincides with the pupil can one observe with a relaxed eye, so that the eye does not have to refocus and the ciliary muscle for the eye lens is relaxed. That is the precondition for relaxed observation over extended periods. For most PENTAX binoculars the eye relief lies between 12 and 25 mm.
This relatively large eye relief value is important for those who wear glasses. Since the glasses now come between the eyepiece and eye, the large eye relief value is necessary to ensure that the eye relief can coincide with the pupil or eye lens.
In practice that means that when glasses are not being worn the eyecups will be twisted out, when glasses are being worn they will be twisted in.

What must be particularly borne in mind when cleaning a binocular?

Put briefly: A binocular should be cleaned in such as way as not to cause any scratches.
To clean the body, use a gentle cleaning agent. Since many PENTAX binoculars are provided with a protective rubber armoring, do not use chlorine-based cleaning agents, since these attack rubber.
To clean the lenses, use a soft cloth. We advise against using commercially-available glasses cleaning cloths, since over time these contribute to removal of the important lens coating. We also advise against using glass cleaners, since they contain chemicals which can attack the lens coating.
Micro-fiber cloths (obtainable from opticians and specialist dealers) cannot do any harm when cleaning lenses.

What does the "Snow / Rain / Fog / Water" label mean?

This label indicates characteristics that do not necessarily apply to all binoculars.
These binoculars are protected against practically all weather conditions: Snow, rain, water, fog.
That means that they are specially sealed, in general with a double O-ring. This means that they have two "O"-shaped rubber seals, one behind the other, giving double protection against the ingress of moisture.
But that alone is not sufficient to offer optimum protection. In order to prevent internal fogging, binoculars that carry the corresponding indication are nitrogen purged. That means that the air within the binocular is replaced with nitrogen.
How does nitrogen prevent internal fogging of the binocular? Moisture has the characteristic that it always condenses on the coldest surface. Normal air always contains a certain amount of residual moisture. When this residual moisture is enclosed in a binocular and is subjected to large changes in temperature, it condenses on the lenses (the coldest surface). That means that the lenses become fogged and for a time the binocular cannot be used.
The nitrogen within the binocular contains no residual moisture. Therefore the binocular does not fog, even when subjected to large changes in temperature.

Can those who wear glasses take their glasses off when using a binocular?

As explained in the answer to question 2, during extended periods of observation it is absolutely necessary that the eyes should be relaxed. For those who wear glasses, this can be ensured only by wearing their glasses.

At what minimum distance must an object be in order to see it in sharp focus through a binocular?

This depends on the design and purpose of the binocular that is being used.
In general, binoculars are selected for a particular purpose, to observe a particular thing. A distinction must be made here between "Close observation" and "Distant observation". Binoculars are optimized for their intended purpose.
If a binocular is optimized for distant observation, the close observation point is generally further away from the binocular. If they are optimized for close observation, the close observation point is generally closer to the binocular.
A further determining factor is the magnification of a binocular. For technical reasons, a binocular with 20 times magnification cannot achieve the same close observation distance as one with 6 times magnification.

How can the quality of a binocular be checked?

Put briefly, by observing through them for an extended period. This will indicate the quality. Can observation through a binocular be sustained for a long period without the eyes becoming tired?
A further quality criterion is the optical quality. How is the image rendered? Is it clear, is the contrast correct, is the color rendering correct, is any chromatic aberration evident?
Does the binocular lie comfortably in the hand, is it easy to operate?
Are all the lenses in the binocular multiple coated? That is important for image quality!

Can binoculars be mounted on tripods?

Not as initially delivered. However some PENTAX binoculars can be attached to a tripod by means of a tripod adapter.
But you have to ask the question, why would you ever want to mount a binocular on a tripod. In general you wouldn't want to mount a binocular on a tripod, it would simply be impractical for hiking or in the theater to lug a tripod around with you.

But of course there are exceptions, such as:
  1. Observations at high magnification.
    When binoculars have a high magnification they can no longer be held steady by hand. That's because a tiny movement of the binocular corresponds to an enormous displacement of the (far distant) observation object.
  2. Observations over an extended period: For instance wildlife watchers and bird watchers often lie for hours in their hide in order to catch the desired animal at a particular moment. In such circumstances it can be very stressful to hold the binocular in the desired position by hand.
  3. Astronomical observation: Perhaps less often now than previously, but many hobby astronomers started their careers with a binocular. Since one is generally working at high magnifications and of course the objects are far distant, the use of a tripod is recommended.

How does a binocular work?

The basis of a binocular is the Kepler telescope. The drawing hows the principal layout of a Kepler telescope. As the drawing shows, a telescope consists of two groups of lenses (here shown symbolically as two lenses). The front lens group is called the objective, the second the eyepiece.
Let's look a little more closely at this: For the sake of simplicity we shall consider the functions of the objective and eyepiece separately. The function of the objective is to capture an image of the object that is being observed. The objective forms the image at what is called the intermediate image plane (in the drawing this is the place where the rays cross). The eyepiece now prepares the intermediate image for the human eye. This process now enlarges the image.
One further comment for photographers: Neither binoculars nor telescopes have a stated focal length. There's a reason for this: They are what are called afocal systems. That means that the system has no focal point as such, and hence no focal length. But of course both the objective and the eyepiece each has a focal length. The designation for this is therefore the magnification. This is derived as follows: Magnification factor= focal length (objective) / focal length (eyepiece).

What different designs of binoculars are available?

We differentiate two designs of binoculars: The Porro prism binocular and the roof prism binocular.
The question then arises, in what way do these two designs differ?
Before we start to delve into the inner life of binoculars, let us consider them externally. If you pick up a Porro prism binocular in your hand you immediately notice that is larger and bulkier. That is because of the arrangement of the prisms.

The drawing on the left shows the principal layout of a Porro prism system. You can easily see that it is spatially quite demanding. It is this which gives a Porro prism binoculars their characteristic shape the widely spaced objective tubes with their tapered offset relative to the eyepieces. For decades, this has been the iconic shape of binoculars.
The advantage of a Porro prism binocular is that they allow a high optical quality without undue complexity. The prisms can be manufactured without too much trouble, and because of the path of the rays there is no necessity for coating the prisms.
The second group of binoculars are the roof prism binoculars. Roof prism binoculars are characterized by a slimmer design. This results from the special design of the roof prism. (see illustration on the left)

This design means that the ray of light appears to travel straight through the binocular. In fact the ray is diverted within the prism so as to give this effect.

This prism permits even the "smallest" designs of binoculars.
But the benefits come at a cost. The production of the prisms is significantly more complex. They must be ground and polished more precisely.
Because of the more complex path of the ray of light through the prism, special coatings are required to avoid reductions in the image quality.
The tolerances for optimum results are extremely tight. That means that economically priced roof prism binoculars often cannot match the image quality of equivalently priced Porro prism binoculars.

What is the angle of view?

The angle of view is the inclusive angle of the field of view. The angle is measured between the left and right edges of the field of view.
The angle of view depends directly on the magnification and the optical aperture of a binocular. It can be said that the greater the magnification and the larger the optical aperture of a binocular, the smaller its angle of view.
The advantage of stating the angle of view is that the field of view can be calculated for any specific distance. What is the field of view? The field of view is also widely used as reference data for binoculars. It denotes however the width of the image at 1000 yards (1000 m in continental Europe).
In order to get a feel for the angle of view of a binocular it is necessary to know that a person has a normal angle of view of 120°, within which he can consciously see everything. A binocular with 10 times magnification and an optical aperture of 50 mm has a angle of view of about 5.0° (for instance the PENTAX DCF ED 10x50).

What does (for instance) 10x50 stand for?

These numbers describe the binocular. What then does the first of these numbers tell us? The first number (10x) describes the magnification of the binocular. In this example the binocular has a magnification of 10. The rule of thumb is: Objects viewed at a distance of 1000 meters appear as if only 100 meters distant.
The second number denotes the diameter of the optical aperture, which is the diameter of the objective lens. This is important because it gives an indication of how well the binocular will perform under low light conditions. The greater the optical aperture, the more light can be "gathered" and presented to the observer, resulting in a brighter image.
These two values serve as the basis for calculating a whole range of values that are of greater or lesser significance, such as the twilight factor, relative light intensity, exit pupil.
Neither of these values tells us anything about the quality of the binocular. They give us no kind of information about the sort of glass used, which lenses are coated, what sort of prisms are used, how they were processed, etc.

For what purposes can binoculars be used?

The main purpose of a binocular is of course observation. But even here there are a multitude of differences. It may be a hunter on a high perch waiting in the twilight for wild boar, or a may be a bird watcher watching a thrush in a thicket under the midday sun. Those are just two examples, both in natural surroundings, but posing very different requirements for the binocular.
The hunter on his high perch essentially needs a binocular which will give him an overview of the clearing. So he needs a binocular with a wide angle of view in order to maintain a view over a wide area. Since the game will wait until twilight before emerging from the shelter of the forest, he needs a binocular with a high optical aperture so that he can discern movements even under low light conditions.
The ornithologist on the other hand requires a binocular with a high magnification so that he can see every detail of the bird. Because often he may have to walk some distance through the countryside in order to reach his observation point, he will also prefer a relatively light binocular.
Irrespective of the purpose to which binoculars are put, they all should have in common the following features: The best possible quality of the optics. That applies across the board, be the user a hunter, a bird watcher, a rambler or an astronomer. They all want to the best possible quality, which can be achieved only by guaranteeing the use of appropriate glass, prisms and coatings.

Is there such a thing as a universal binocular?

Based on the answer to question 13, there really is no such thing as a universal binocular.
But of course that is by reference to special cases. Yes there are general-purpose binoculars for "John Everyday". Here it's a question of price and also the size that he would like.
If you want a binocular for leisure activities such as walking, then the recommendation is for small, compact binoculars. These are not heavy, they fit in a jacket pocket and they often offer surprisingly good optics.

What is meant by Phase Coating?

Phase Coating is a special coating of the roof prisms which allows optimum image reproduction.
What's the secret? In contrast to a Porro prism, with a roof prism we have some challenges to face. Firstly, a roof prism generates more reflections than a Porro prism and because of the reflection angle does not achieve total internal reflection. That means that light is lost as it passes through the prism. One of the tasks of the phase coating is to prevent this.
The other task is to salvage the image quality through the prism. It is generally known that light is a wave. Waves have the characteristic that when they are reflected they do not maintain their optical time, i.e. they go out of phase. As mentioned above the light is reflected many times on its path through a roof prism, and at each reflection the phase shift becomes ever more pronounced. Without phase coating the images would lack contrast, the color reproduction would be poor and a certain fuzziness would be evident.
As you would expect, the coatings of the prisms also differ according to the price and quality of the binocular. The quality of the coatings differ in respect of the materials and processes used. The materials used for a coating are aluminum and silver. Since silver exhibits a higher reflectivity than aluminum it is more effective in correcting the two errors that were listed. For higher and medium priced binoculars the materials are applied to the prism by vapor deposition.
Prisms of binoculars in the higher price bracket are also coated with silver, however

I must leave the question open at present, since I honestly don't exactly know how much further I can take it.

How heavy is a binocular?

There are major differences here depending on the design and size.
The weights of compact binoculars lie between 250 grams and 300 grams. That's logical, since these binoculars are intended to be kept to hand at all times. That means they must not be too heavy, otherwise they are awkward in the pocket and tedious to carry round at all times.
The next size, called semi-compact binoculars, are larger and thus heavier. These weigh between 350 grams and 500 grams. The advantage of this design is that it strikes a compromise between weight and optical performance. The improvement in optical performance results from the use of high-quality optical lenses, which due to their manufacture are inherently heavier. The optical aperture is also larger than for compact binoculars, with corresponding larger lenses which also add to the weight.
The top line, the "full-size" binoculars are the heaviest binoculars of all. Here the weight starts around 660 grams and carries on from there. At PENTAX the top weight is about 1500 grams. This is consequence of the relatively large optical apertures (up to 60 mm) and high magnifications (up to 20 times). These devices require more lenses, which means more weight!

What is the purpose of the adjustment wheel between the eyepieces?

This adjustment wheel is for focusing the binocular.
To understand this more precisely, one must know that there are, as there always have been, fixed focus binoculars. At first glance there appear pleasant, in that the irritating work of focusing no longer arises.
But that means the eye must take over the job of focusing. This means, as explained in the answer to question 2, that observation with the eye relaxed is no longer available under all circumstances. This in turn leads to headaches during long periods of observation.
The provision of focusing overcomes this problem. Irrespective of how far away the object being observed is, it can always be observed with a relaxed eye.

How many lenses are there in a binocular?

This too depends on the binocular. Depending on the price bracket and the design there may be between 6 and 12 lenses, combined into groups of lenses.

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