When selecting a new pair of binoculars there is a myriad of features and options to choose from. Whilst some specifications such as magnification, waterproofing, size and weight are simple enough to choose between, some features are more difficult to know which to go for. Binocular coatings and prisms are two such aspects that are often taken for granted or overlooked when selecting a suitable pair.

All binoculars available today, apart from the very cheapest models, have some degree of coatings applied to the optics within the binocular. As light enters or leaves glass (such as binocular lenses and prisms) about 5 percent of the light is reflected back. Binoculars can have 16 air-to-glass surfaces - with light lost at every surface, by the time the image gets to the eye it has lost about half of its light. With an un-coated binocular, not only will the image be 50 percent dimmer, the lost reflected light will bounce around in the binocular causing a dim, fuzzy image. This is never really a problem with modern binoculars which all have some sort of coating to limit this problem, now it's choosing what level of coating you need or can afford.

In the 1940s it was discovered that by applying a thin layer of magnesium flouride to the outside of the glass it would allow more light through. This single layer technology when applied to one or more glass surfaces on a binocular is usually referred to as a 'coated' binocular. This coating will reduce the light loss from 5 to about 1.5 percent at each surface. Binoculars referred to as 'fully coated' are when all air-to-glass surfaces have a single coating - this reduces light loss to about 15 percent. This is about the best you can expect from budget priced models. You can compensate for light loss by going for a binocular with a larger objective lens (e.g. 8x40 instead of 8x20) but this will result in bulkier/heavier binoculars.

The next level is 'multi-coated' optics - this is where up to 15 layers of coats are applied to some of the surfaces within the binocular. The chemicals used vary from brand to brand - they can give a visual green, blue, purple or red tint to the lenses. But this gives no indication of the quality or type used - this can only be checked by manufacturer specifications. The top level is 'fully multi-coated' optics which have all surfaces covered with multiple layers of coats. This method will reduce overall light loss down to only 5% but it is the most costly option. Standard multi-coated binoculars achieve somewhat less reduction in light lost.

Quality of coatings can be an issue when it comes to durability. Top quality coatings are now harder and resistant to scuffs and scratching. Lower priced coated binoculars may have lower quality coatings which are quite soft and need to be protected from wear and tear.

Colour rendition can vary from one brand of coating to another. Specialty coatings can screen out specific wavelengths and thereby enhancing desired colours - tailoring the binocular to a certain purpose. For example, some screen out the blue end of the spectrum and enhances reds, oranges and yellows (including browns and tans) which is very useful for trying to spot brownish animals in green foliage. Some are the other way round, enhancing blues and greens which improves contrast between light and dark areas, giving the image extra 'sharpness'. Colour-neutral (maintaining natural colours) coatings are the most suitable for bird watching when it is crucial to see the natural colours of the bird for identification. Ruby-red coatings which are advertised to protect from UV and glare off water are not good for wildlife/bird watching. They tend to merely be an attractively coloured coating to catch a customers attention - they sometimes even reduce light levels. The bright red lenses of such binoculars act as a beacon to any colour-sighted creatures you are trying to observe.

The function of prisms in binoculars is to give a properly orientated image. Without them the view would be upside down and reversed. There are two types; porro and roof prisms. Porro prism systems have two right-angled pieces of glass set apart from each other, which means the objective lenses have to be further apart than the eyepieces - this gives the classic 'dog'leg' binocular shape. Roof prism systems have the angular pieces of glass closely overlapping one another giving a more compact, streamlined shaped binocular.

The difference between binoculars with these different prism systems becomes obvious when handled. For the casual user, comfort is often the determining factor when choosing a binocular. Porro prism binoculars give a larger but steady platform for viewing and are often preferred by people with larger hands. The more streamlined design of roof prism binoculars allows the user to hold them with their arms tucked closely to the body for maximum steadiness. It also places the focus wheel conveniently at your fingertips for more comfortable, faster focusing - often preferred by people with smaller hands. Comfort is obviously a big issue when using your binoculars more regularly. But for extended or more professional use (especially when cost is a consideration) the prism systems need to be looked at more closely.

There are two types of glass used in binocular prisms, BAK-7 (made from borosilicates) is often used in cheaper binoculars. BAK-4 (crown barium) glass is of a higher density which greatly reduces light scattering within the prism, giving sharper and better defined images.

Of the two types of prisms, porro prism binoculars provide the better quality images for your money. This is due to the way the different prism systems reflect the light within the binocular - porro prisms lose less light in so giving the better image quality. You may see advertised roof prism systems with 'PC or phase-corrected coatings' - this is a special coating to greatly reduce light loss. But these binoculars come at a price: although the images they produce are as good as the best porro systems, they are some of the most expensive binoculars on the market.

Porro prisms also have a better stereoscopic or 3-dimensional view of the image than roof prism binoculars. But porro prism binoculars do have the disadvantage of requiring larger housing to fit the prisms in hence bulkier, heavier binoculars. There is often a trade off between mid-prices binoculars for the sharper images of porro or lighter, more comfortable roof prism - it's really down to your personal preference. When making a final decision on a binocular you will need to look at the other features of the model such as lens coatings, magnification and so on.