All binoculars are marked with their power and objective diameter(big front lens) ie. 7x50 or 8x42 for instance. Anything up to 10x can be hand held but above that a tripod is really required.
The second number is the diameter of the objective which by letting more light in generally increases the resolution and the brightness of the image.
Low light performance is largely dependent on the exit pupil size. This is calculated by dividing the objective size by the power eg. 7x35 = 5mm exit pupil diameter. During the day your pupils are on average 3mm diameter but at night they maybe as big as 7mm diameter. From this we can see that if a binocular with an exit pupil of 7mm is used during the day when your pupils are only 3mm diameter the difference is wasted. Not ideal at all! At night when our pupils are at their largest (up to 7mm) we need a corresponding binocular exit pupil to make the image as bright as possible. Brightness is affected greatly by the quality of the lens coating too!
FIELD OF VIEW
The field of view is size of the image you actually see though the eyepiece. It is usually expressed in either degrees or width at a certain distance (ie. 430 feet at a 1000 yards). A wide field of view means it will be easier to track a Buzzard or see all of Orions belt if you are an astronomer.
EYE RELIEFEye relief is the optimum distance from the eyepiece your eye must be to be able to see the whole field of view the binocular offers. A long eye relief is very important if specticles are worn. Eye relief is usually around 10mm but nearer 20mm is ideal for specticle warers. Most manufactures will specify this figure!
LENS COATINGS
On average, about 95% of light passes through the binocular to your eyes meaning that 5% of the light is relected back. With poor coating this figure will be far greater. Obviously, the more money you spend on a binocular the better the coating will be and hense the brighter and more contrasty image you will get. More expensive Roof Prism types now get phase coating on their prisms so quality is up to or even surpassing porro type binoculars. Below are the descriptions of coating types:
C - Coated (one or more surfaces coated)
FC - Fully Coated (All air to glass surfaces coated)
MC - Multi Coated (One or more surface is multi-layer coated)
FMC) - Fully Multi Coated (All air to glass surfaces multi-layer coated)
PORRO PRISM AND ROOF PRISM
Porro prism are the Z shape old fashioned looking units that you often see in old films and it was in the 60s when the straight through roof prism types first appeared which were more compact and easier to hold but due to the complex manufacturing process involved were very expensive. Over the years, manufacturers have succeeded to produce high end roof prism binoculars and although still expensive are very popular. A lot of people still buy porro due to their lower prices some people state, better stereo imaging.
BINOCULARS FOR BIRDING
Binoculars are already good enough for hunting, they are good enough for watching sports or general nature observation and they are good enough for surveillance too. Are they not good enough for birding though! Birding places demands on optics that no other use does. There is more detail in a bird's plumage, especially when you consider the range of colors involved, than you will find anywhere else in nature. Birders use binoculars in a wider range of conditions, and at a wider range of distances, than almost any other users. No one else is likely to demand such perfection from optics makers. For these reasons it is most important when choosing your birding binocular.
BINOCULARS FOR ASTRONOMY
Binoculars are getting more and more popular for use in astronomy. Obviously you can not see the rings of Saturn or study a small crater on the moon. What they are extremely good at though is broad sweeps of the Milky Way and picking out some deep sky objects. Another advantage is that no set up time is required. When choosing a binocular for astronomy, there are these factors to consider:
LIGHT GRASP
The size of the objective lens will determine the maximum resolution available which will give more detail. A 7x50 binocular has an exit pupil of 7mm but to take full advantage of this you must have an equal size pupil when fully dilated. On average, adults reach about 5-6mm so it might be better to choose a binocular like a 8x40 which has an exit pupil of 5mm and will also be lighter to hold. Something else to consider is that although your view will be brighter with a 7mm exit pupil so will any light polution, i.e. the sky will not be black (reduced contrast!) unless you are at a dark site. There is something nice about seeing stars standing out against a black background which you get with a smaller exit pupil.
WEIGHT
Hand holding a heavy binocular is hard to keep steady and your arms will soon ache. Generally, anything above ?x50 will be better if used with a tripod or other aid. It is possible to hand hold with sizes up to ?x70 but only for a short amount of time or by lying on a sun lounger or similar.