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Exit pupil measurement is obtained by dividing the diameter of the objective lens by the power of the binoculars. This will give the measure of how bright the image will be when looked at through the binoculars, but as we get older our pupils tend to get smaller and so binoculars with large exit pupils may not be as useful. If the beam of light from the binoculars is wider than the pupil of the eye, the excess will not get in and so the eye is unable to see it. In low light conditions a young person's pupil may expand up to a 7 or 8mm diameter whilst the pupil of a 30 to 40 year old may only expand to about 4 or 5mm. Thus a pair of 7x35 binoculars with an exit pupil of 5mm (35 divided by 7) would be more than adequate for an older person but inadequate for a young observer
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| Prism glass in most binoculars, are manufactured from borosilicate (BAK-7) glass or barium crown (BAK-4) glass. BAK-4 is the higher quality glass providing a brighter image and better edge to edge sharpness.
The prisms in a pair of binoculars bend the light in order to reduce the size that a straight optical path would require. It also turns the orientation of the image from upside down to the right way up.
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| Porro prism binoculars are perhaps the easiest design to manufacture, and therefore has the advantage of being cheaper to produce. The objective lenses are typically spaced further apart than that of the eyepieces but this is not always the case and they are sometimes closer together in compact models. They are typically brighter than comparable roof prism models but are usually much bulkier. Porro prism binoculars also generally provide greater depth awareness.
| ![More Technical Information about Binoculars - Prisms Porro]() |
| Roof prisms are a newer design with the advantage of being smaller and more compact than porro prisms. This allows them to be fitted into smaller housings making for more compact binoculars or spotting scopes. Alignment is often more problematic as miniscule alignment errors will cause a degradation of image. As already mentioned, they are not as bright as Porro prisms as the mirrored surfaces tend to lose some light. The exception to the rule is roof prism binoculars from the likes of Zeiss, Leica, Nikon and Swarovski. The binoculars sold by these manufacturers are likely to be too expensive for all but the most dedicated observer.
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| Coatings are an essential part of almost all optical systems. Whenever light enters or leaves a glass element, between 4 - 5% per cent is reflected back. With many binoculars having as many as sixteen glass to air surfaces, less than half the light got through to the eye in early binoculars. This lost light ended up bouncing around inside the binoculars making the image very hazy and hard to see. With modern anti-reflective coatings, more light reaches your eyes and as a result the image is brighter and clearer.
Coated, or single coated binoculars have a single layer of coating on at least one lens surface. Fully coated binoculars mean that every glass to air surface has one layer of coating. Multi-coated are perhaps the most common coating system in binoculars today with some glass to air surface being coated, usually at least the front and rear lenses, with multiple layers. Finally the fully multi-coated binoculars being the best level of coating available, with every glass to air surface covered with multiple layers of high quality coatings.
Eye Relief can be a major consideration when buying your first pair of binoculars, for instance people who wear glasses should aim to buy binoculars that offer better "eye relief" as having to take your glasses off to use them can be frustrating. Eye relief is the measurement of how far back from the eyepiece your eye can be whilst still seeing the whole field of view. Any further back and you will see only the centre part of the image. Eye relief for binoculars tend to be from about 10 to 17mm. Binoculars with fold down or screw style eyecups let spectacle wearers get closer but you will still need binoculars with a long eye relief of 14mm or more.
Eyecups allow the user to just comfortably touch their face whilst helping to block out lateral light. They are typically rubber eyecups that can be folded down, or the type that either twists or slides up and down on the eyepiece. Spectacle wearers always have the binocular eyecup rolled down, thus allowing the widest possible field of view.
Field Of View is the horizontal measurement of the circular area visible at 1000 yards (or sometimes metres) High power binoculars have a narrower FOV and, as already mentioned above, makes following fast moving subjects difficult.
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