The prism system of a binocular reduces the size needed to contain a long optical path and turns what would be an upside-down image right-side-up. there are two types of prism systems, roof and porro.
Roof Prism Systems
In roof prism binoculars the prisms overlap closely, allowing the objective lenses to line-up directly with the eye piece. The result is a slim, stream-lined shape in which the lenses and prisms are in a straight line. Roof prism binoculars are less bulky and more rugged than an equivalent porro model.
Porro Prism Systems
In porro prism binoculars the objective or front lens is offset from the eye piece. Porro prism binoculars provide greater depth perception and generally offer a wider field-of-view.
Waterproof / Fogproof
Some binoculars are O-ring sealed and nitrogen-purged for total waterproof and fogproof protection. These models can withstand complete immersion in water and stay dry inside. The interior optical surfaces won't fog due to rapid temperature change or humidity.
Binoculars are often referred to by two numbers separated by an 'x'.
For example: 8 x 32. The first number is the power or magnification of the binocular. With an 8 x 32 binocular, the object being viewed appears to be eight times closer than you would see it with the unaided eye.
Objective Lens Size
The second number in the formula (8 x 32) is the diameter of the objective or front lens. The larger the objective lens, the more light that enters the binocular and the brighter the image.
Most optical prisms are made from borosilicate (BK-7) glass or barium crown (BaK-4 glass. BaK-4 is the higher quality glass yielding brighter images and high edge-to-edge sharpness.
Lens surface coatings reduce light loss and glare due to reflection for a brighter, higher-contrast image with less eyestrain.
Types of coatings:
Coated - A single layer on at least one lens surface.
Fully Coated - A single layer on all air-to-glass surfaces.
Multi-Coated - Multiple layers on at least one lens surface.
Fully Multi-Coated - Multiple layers on all air-to-glass surfaces.
The side-to-side measurement of the circular viewing field or subject area. It is defined by the width in feet or meters of the area visible at 1000 yards or meters. A wide-angle binocular features a wide field-of-view and is better for following action. Generally, the higher the magnification, the narrower the field-of-view.
Refers to the size of the circle of light visible at the eyepiece of a binocular. The larger the exit pupil, the brighter the image. To determine the size, divide the objective lens diameter by the power (an 8 x 32 model has an exit pupil of 4mm).
The distance a binocular can be held away from the eye and still present the full field-of-view. Extended or long eye relief reduces eyestrain and is ideal for eyeglass wearers.
Eyeglass Wearers - Eyecups
Binoculars come with twist-up, pop-up or soft rubber fold down eyecups which go down for eyeglass wearers. These options allow everyone to see the entire field-of-view.
A 'fine focus' adjustment ring usually provided around one eyepiece to accommodate for vision differences between the right and left eyes.
Found on the best roof prism binoculars, this chemical coating is applied to the prisms to enhance resolution and contrast. Would not provide an advantage on porro prism models.
Rubber armour provides multiple benefits. It helps protect the binocular from the bumps and scratches that come with day-to-day use. it provides a comfortable gripping surface for making them easier to hold on to. It's easy to wipe clean after a tough day on the field. And it suppresses noise if the binoculars bump aluminium or other non-rubber surfaces, which might otherwise spook wildlife.
What do the letters mean?
The symbols or letters that you will find often follow the numbers e.g. XCF also provide important
information about the specifications of the binoculars.
This is just a brief guide as to what some of them mean.
Z or B – This usually means that the binoculars have a porro prism body type.
These types gather the light from lenses further apart than your pupils thus giving a stereoscopic effect.
This counter acts the foreshortening of perspective caused by magnification.
D – This denotes roof prism binoculars. These binoculars are more compact than the porro prism binoculars
but do have a tendency to be more expensive.
CF – This means that the binoculars are Centre Focusing
IF – This indicates that the binoculars are Eyepiece Focusing. You will find that most IF binoculars are
7x magnification giving a large depth of field, which eliminates the need to focus at distances over 100m.
WP – This means that the binoculars are Weather Proof or Water Proof. You will need to look at the full specification of the model you are looking at to check which it is.
W or WA – This indicates that the binoculars give you wide angled or wide field vision.
FP – This denotes that the binoculars are Fog Proof
FM – Indicates that the lenses are fully multicoated. Coating a lens surface helps to reduce the amount of light that is reflected back or dissipated from the lens, thus ensuring bright images.
Multi-coating further reduces the loss of brightness - providing the sharpest, clearest images.
L – This means that they have Long Eye Relief
Is there anything else I should consider when choosing binoculars?
Another point to consider, after you have found the right specifications for you, is the weight.
If you choose a pair of binoculars which are too heavy, it may be the case that more often than not
they get left at home. A lightweight pair of binoculars can be taken anywhere and will get far more general use than a heavier pair.
How much should I pay?
Porroprism binoculars are less expensive to construct than roof-prisms, and will usually be optically superior when compared with roof prism models at similar price levels. This highlights the necessity to pay more money for roof-prism designs of comparable image quality. As a general rule, low price roof-prism models will not provide the same performance as equivalently priced porro designs.
For general Birdwatching, the lower magnifications tend to be better (8x30 or 8x40). If you intend to use a binocular in low light such as dawn or dusk and in woodland, consider a 7x42. The best compromise for hide use would be the higher powered instruments with adequate objective lens diameter (10x40 or 10x42).
For walks etc a compact pair (8x20 or 10x25) may suit as these can be carried in a pocket or handbag and will be lighter to carry.
Care and Use
Looking after binoculars
Binoculars are valuable tools, so it is worth taking a little advice to learn how to achieve the best results and keep your binocular in good working order. Some models may represent a considerable financial investment. With the correct care you might well expect it to last a lifetime and with the good advice you can maximize it’s performance.
The basics After using a binocular in the rain, always wipe it dry. If moisture is apparent on the internal optic surfaces, leave it in a warm place to dry. Never dismantle it and do not put it back in the case until it is fully dry.
When a binocular is manufactured, the optics are very carefully aligned using special equipment, so that you see only one image.
The most common problem experienced is misalignment of the optics and the consequent double image, as a result of the instrument being knocked or dropped. This can be rectified but only by specialist repairers with similar equipment to that used by the manufacturer. Some people may not notice slight alignment faults, so it is best to get someone else to check your binocular at regular intervals..
How to use your binocular
When you first look through a binocular you should:
Adjust it for the width between your eyes.
Close your right eye or cover right hand objective lens & focus using centre focus wheel until clear.
Open right eye or uncover right hand objective lens.
Close left eye or cover left hand objective lens & Adjust the individual eyepiece for your eyesight.
You have now set up your binoculars for both eyes & only the centre focussing wheel need be used.
How to use your monocular
Adjust the individual eyepiece for your eyesight.