How to choose Binoculars
There are a number of factors which should be taken into consideration when choosing binoculars. These are primarily: the magnification, the size of the objective lens, the shortest focussing distance, the optical quality, the field of view, the size and weight and the use to which the binoculars will be put.
High magnification may be important for astronomical viewing, while a short focussing distance is important if you plan to observe small wildlife. Observing big game from a moving jeep can be very uncomfortable if you do not have a stabilized image
What do you want to use the binoculars for?
The best binoculars for astronomical use are not necessarily the best ones to take to a football game, nor the best buy for watching small birds.
You need to understand first of all the binocular numbers:
Binoculars are advertised with such numbers as 10 X 30 or 15 X 50.
The first number tells you the magnification,so a 10 X 30 binocular will magnify what you are looking at by a factor of 10, the 15 X 50 will magnify by a factor of 15.
The second number tells you the diameter of the objective lens in mm; the objective lens is the opposite end to the eyepiece and will determine how much light will enter the binoculars an important factor for astronomical viewing for example. A large lens will also imply a larger and heavier pair of binoculars, extra weight you may not want if you are climbing up a mountain for example.
It is also important to know the shortest focussing distance, or in other words, how far you must be from an object in order to bring it into focus. This is an important consideration when observing nature, if you want to observe small animals and insects in great detail it is good to have a pair of binoculars which will focus on an object a relatively short distance away. This is obviously not a factor to consider if you are looking at the stars.
The optical quality is also very important, high quality lenses will produce little chromatic distortion and will give a clear focussed image in the whole field of view, not just in the centre. The better the optical quality the more you are likely to have to pay.
The weight and size of the binoculars will also determine how comfortable they are to use and it is necessary often to trade-off optical quality for less size and weight.
The relationship between the magnification and the size of the lenses will also determine the field of view. A wider field of view can often be vital if you are searching for wildlife, if your view is too restricted this will mean that you too often miss what you are looking for.
Field of view is measured in two ways:
Either as the actual field of view, which is a measurement of the angular amount of sky you can see when you look through the binoculars. This will be given as a small angle between about 3A? and 7A?;
Or as the apparent field of view, which is a measure of how large the image appears. This will be given as a figure somewhere between 50A? and 70A?. If you are sweeping the horizon looking for a rare species a larger value will be what you will appreciate. This factor will not be so important for stargazing but a very small field of view will be more uncomfortable when stargazing with hand-held binoculars.
The higher the magnification of an optical instrument the more noticeable is the problem of optical shake. Hand held binoculars inevitably will have some shake and this will make the images viewed more out of focus and mean that binocular use is rather tiring. Image stabilization technology efficiently removes this problem and increases image clarity and user comfort. Image stabilization technology really comes into its own when you are observing wildlife from a moving vehicle or boat. It also make astronomical viewing at high magnification a much more comfortable and less tiring experience. The higher the binocular magnification the more noticeable is the slight trembling inevitable when the binoculars are hand-held