Understanding Binoculars

Binoculars are really two mini telescopes aligned to give you stereo vision which is a whole lot more comfortable to the eyes and has the advantage of allowing better depth perception. Try viewing a fast moving object through a telescope to see the advantage binoculars have over telescopes. The performance of an individual binocular depends on several factors among them the brightness, magnification power, prism quality and the coatings used in manufacture.

Binocular specifications can be confusing with many specifications quoted, here’s a guide to help understand it all.


Aperture: The diameter of the objective lens (nearest the object you’re viewing) in millimeters. That 8x 40 binocular has a 40mm objective lens. Broadly speaking a larger aperture means a brighter clearer image as the binocular is allowing more light in.

BaK4 & BaK7 Glass: The two types of glass most commonly used, BaK4 being Barium Crown glass and considered superior to the alternative Boro Silicate glass.

Coatings: Modern binoculars have varying degrees of special coatings on their lenses and prisms in order to reduce loss of light through reflection back out of the unit. The coatings instead help to retain as much light as possible through to the eyepiece. Binoculars may be marked as:

  • Coated, meaning at least some surfaces have been coated
  • Fully coated (FC) meaning all surfaces have at least one coating or
  • Fully multi coated (FMC), meaning all lenses and prisms have been treated with multiple layers of coatings.
  • Phase coatings are used in good quality roof prism binoculars to enhance color and clarity.
  • Rubicon coatings are sometimes used on marine binoculars in order to reduce glare.

Close Focus: The minimum distance away that objects must be in order to be able to view them in focus. This may seem a strange concept given that most of us use binoculars for far away objects but certain users such as birdwatchers sometimes use binoculars at very short distances to see details like plumage patterns.

Diopter: Most center focus binoculars allow for adjustment of the focus on one eyepiece, usually the right one, in order to allow for the small differences in eyesight between an individuals two eyes.

Exit Pupil: The diameter of the shaft of light that comes through the eyepiece to your pupil. Divide the objective lens size in millimeters by the magnification to calculate, thus a 7x 35 binocular will have an exit pupil of 5mm. The human pupil size varies with light (and age) between about 2.5mm and about 6 mm. In a perfect world we would use an exit pupil which matched our pupil size exactly but in practice an exit of 4 to 6mm is just fine for pretty much everybody.

Eye Relief: The distance from the eyepiece to your eye where the image is viewed. If you wear glasses while using binoculars you’ll need to allow for the extra distance to your pupil caused by having your glasses in between the eyepiece and your pupil. This is known as long eye relief and might be above 14 or 15mm.

Field of View (FOV): The width of the view at 1000 yards. If your binocular says something like 300′ at 1000 yards it means that when looking at something 1000 yards away the side to side view is 300 feet wide. Some binoculars quote an angle instead of distance, the larger the angle the greater the field of view. To convert them multiply the angle in degrees by 52.5 to get feet at 1000 yards.

Focus Type:

  • Center focus is the old standard method of focusing using a finger wheel mounted between the two sides of the binocular.
  • Individual focus means each eyepiece can be focused independently.
  • Focus free types use an optical trick to be permanently focussed to all distances greater than a minimum point. These are really useful for fast moving sports action because you avoid the need to keep re-focusing as the action moves to and fro at varying distances.

Interpupillary Distance (IPD): Binoculars fold to allow for adjustment to how far apart the eyepieces are. When properly adjusted to your size you should see just one circular image, if you are seeing two images you need to adjust this further. If you have particularly close set eyes you should find roof prism binoculars are a better match.

Image Stabilized Binoculars: A type of binocular which uses gyroscopes and motors to allow the internal prisms to ‘float’ and compensate for handshake. These produce excellent images even at relatively high magnification factors. They are relatively expensive but are gaining popularity in both astronomy and marine fields.

Magnification: The number of times an object appears to be brought closer, signified with an ‘x’ to show its a multiplier. For example an 8x 40 binocular makes objects appear 8 times closer. A word of caution here – more is not necessarily better, in fact it can mean a dimmer, fuzzier image unless you have large enough aperture.

Nitrogen Purged: The air inside the binocular has been replaced with nitrogen to eliminate moisture in order to prevent mould or mildew forming on the surfaces. It also make them fog-proof as there is no water vapour to condense on the glass.

Objective Lens: The lens closest to the object you are viewing, see aperture above.

Ocular lens: The eyepiece lens.

Prism Type: Porro prism binoculars are the traditional styles with the objective lenses stepped out and wider apart than the eyepieces. Roof prism styles are the sleek styles with objective lenses in line with the eyepieces.

Relative Brightness: A measure sometimes used to compare brightness of image between binoculars. The diameter of the exit pupil squared, the higher the number the brighter the image.

Transmittance: The percentage of light which makes it through the binocular to your eye. The better the anti reflective coatings the more light makes it through.

Twilight Factor: A measure used when comparing performance in low light conditions. Calculated by multiplying the objective size in millimeters by the magnification then finding the square root of the result. The larger the number the better the low light performance.