Binocular Terms
Included here are definitions for Binocular terms and some tips on how to use them in purchasing an item suited to your needs.
Angle of View / Apparent Angle of View:
- The Angle of View is the angle between the left and right edges of the field of view at 1000 yards from the binocular's point of view. The Apparent Angle of View is this same angle with the binocular's magnification figured in, the Apparent is simply the (magnification x Angle of View). For a 10x magnification and a 5° Angle of View the Apparent would be 50°. This is directly related to the Field of View, but is often used to compare binoculars.
- Tips: The Angle of View is just another way to express the Field
of View. The higher the magnification the smaller the Angle of View.
Here is a picture describing Angle of view:
Image courtesy of Canon
Brightness Index:
- This is the square of the exit pupil diameter. A 3mm exit pupil binocular would have a brightness index of 9. A common way to measure the brightness of an instrument.
Dimensions:
- Here is a diagram explaining what the Height, Width, and Length fields are referring to. If there is no
Height included you can get a pretty good idea of what it is from the Objective Diameter. Just add on
about 10mm to account for the binocular body and convert to inches, 25.4mm = 1 inch.
Image courtesy of Canon
Diopter/dioptric correction:
- A focusing adjustment for the eyepiece, to allow a perfect focus for both eyes, independent of each other. Many people's left and right eye focus differently and require individual accommodation.
- Tips: This is an important feature but is present in almost all binoculars.
Eco Glass:
- Eco Glass - Nikon only: An environmentally friendly top quality optical glass from Nikon that is manufactured without the use of arsenic or lead.
Exit Pupil:
- The size of the image as seen from the binocular's eyepiece. This is the circle of light seen in the eyepiece seen from about 10 inches away. Commonly used as a measurement of how bright the binoculars are. You can figure out the exit pupil diameter by dividing the objective lens diameter by the magnification power. (10x50mm binoculars would have a 5mm exit pupil.)
- Tips: A binocular works best when the exit pupil diameter matches your eye's pupil diameter. A smaller diameter of around 2-3mmis good for binoculars for bright daytime use. Larger exit pupils of around 7mm are good for low light conditions and star gazing.
Eye relief:
- This is the distance that the binocular can be held from your eyes and still provide a good image. Eyeglass wearers will need a longer eye relief because the binoculars will be held against their glasses- further from their eyes.
- Tips: A longer eye relief is generally preferable but a simple, effective solution, though not always used, is having adjustable eye cups. These can usually make up for shorter eye relief, which is common in lower end binoculars.
Field of View:
- The width of the area seen at a certain distance, usually 1000 yards.
- Tips: A large field of view is good for a general surveying of an areas- good for hunting. A large field of view is always accompanied by a lower magnification. A high magnification is good for spotting specific objects but bad for quick, easy searches.
- Here is a picture describing Field of view:
Image courtesy of Canon
Focusing:
- Most binocular have a central focusing wheel to make objects at varying distances visible. The central wheel focuses both lenses simultaneously. Most binoculars provide also provide diopter adjustment on the eye pieces. Internal focusing is an important feature for waterproof binoculars- this means all the moving parts are internal and therefore more secure from dust, fog, rain, etc.
- Tips: Some binoculars don't have a focusing wheel instead everything from a certain distance away is already in focus. The only problem is that the minimum focusing distance is relatively far - commonly 60 ft. Other factors include the wheel size and place for easy adjustment while wearing gloves.
Interpupillary Distance (ID):
- The distance between the centers of a person's pupils. Every binocular has a range of movement, by pulling the barrels inwards/outwards, to accommodate different IDs - e.g. "56-72mm" means the minimum ID is 56mm and the maximum ID is 72mm. If your ID is 54mm you will never be able to see properly through such a binocular because the barrels will never line up with the centers of both your eyes.
- Tips:Most binoculars have an adequate interpupillary distance range for most people. However, if children will be using the binoculars often you may want a binocular with a minimum ID of around 56mm or below.
Lens Coatings(Multi-layer, Multi-coated, etc.):
- These are coatings put on the lenses of the binoculars. Every brand has its own type of varying quality. This affects most directly the light transmission of the binoculars. Which means of all the light coming through the objective lens- how much comes out the other end into your eye. The coatings mainly prevent light reflection from happening on the lenses and prisms. Top end binoculars can transmit better than 95% of the light let in the objective lenses.
- The four main types in increasing order of quality are: Coated, Fully Coated, Multi-Coated, and Fully Multi-Coated. The difference between Coated and Fully Coated is the coated binoculars have antireflective coatings only on the Objective and eyepiece lenses, where the fully coated binoculars are coated on every air to glass surface. The Multi-Coated binoculars have multiple antireflective coatings on each surface to improved light transmission further.
- Tips: A large objective lens affects how much light is let into the binocular and the coatings affect how much light gets through to your eyes. Multi-coated lenses are important as a rule but top quality multi-layer lens coatings are also usually expensive. An economic buyer will find a good balance between objective lens size and lens coating.
Magnification:
- The magnification of a binocular refers to how many times larger an object appears. For example, with a x10 magnification an object 1000 yards away would appear to be only 100 yards away. The most common magnification is 8x.
- Tips: The lower the magnification the wider the field of view is and the easier it is to hold a steady image. High magnification is useful for birding and star gazing but a wide, steady field of view is more useful for hunting and general use.
Minimum Focusing Distance:
- This is the minimum distance that the binoculars can render a clear image.
- Tips: The short minimum focusing distance is nice when viewing nature, going to the museum, etc. It is not of paramount importance in buying binoculars but as with everything else, it depends on how you plan to use them.
Objective Lens:
- This is the larger of the two outer lenses in the binocular. This lens is what gathers all the light that you see- the bigger the lens the more light allowed in. Also, a larger objective lens generally provides better resolution and more detail than a smaller one. The sizes range from a tiny 15mm for theater glasses to 50mm and larger for low-light and spotting binoculars.
- Tips: The larger the lens the more light allowed in but also the heavier the item is. The amount of light is also determined by lens coatings and the overall quality of the lens.
Phase-corrected coating(P-coating):
- Applied to roof-prism binoculars to eliminate distortion along the roof edge of the prism. Without a coating it is impossible for the image from roof-prism binoculars to be clear from edge to edge.
- Tips: A good P-coating is usually expensive, for a more economical quality binocular it may be better to choose a porro prism design.
Prisms:
- The main prism systems in binoculars are roof prisms and porro prisms. Without prisms the image seen through binoculars would be upside down and reversed. Roof prisms provide for a slender, lightweight body but require phase correction coating to provide a clear image. Porro prism binoculars are more economical but still highly effective. Porro prism design also provides for better three dimensional viewing. BaK-4 or BK7 glass is generally used, BaK-4 is of much better quality.
- Tips: A roof prism design is lighter and more compact but to get the same performance as a porro prism, you will probably pay more for it. Still many of the new high end binoculars are of roof prism design.
- Here is a picture of the two most common prism layouts:
Stabilization:
- A relatively new innovation in binoculars offer by some brands. The image through the binoculars is stabilized so that involuntary hand shaking, etc. does not ruin a view. This is important for high magnification binoculars which are more sensitive to shaking.
- Tips: This comes in handy for bird watching, star gazing, and especially marine use. When you need a steady image at a high magnification and you can't use a tripod, these binoculars will probably do the trick.
