The Binocular Sky


Binocular Basics


1. Aperture, Magnification and Field Size
2. Exit Pupils
3. Coatings
4. Prisms
5. Focusing

1. Aperture, Magnification and Field Size

These are the numbers, usually printed on the binocular, that designate the size of the aperture (in millimetres), the magnification of the binocular and the field of view. Examples from common small and medium astronomical binoculars:

Magnification and Aperture

10×50: This binocular has an aperture of 50mm and a magnification of ×10. Its exit pupil is 50/10 mm = 5mm.

15×70: This binocular has an aperture of 70mm and a magnification of ×15. Its exit pupil is 70/15 mm = 4.7mm.

Field of View

Field 5.5°: This is the true field of view of the binocular in degrees.

420ft at 1000yds: This field of view specification (number of feet at 1000 yds) is common on binoculars for the US market. To convert to degrees, divide the number of feet by 6000, take the inverse tangent of the result, and double your answer:

420 ÷ 6000 = 0.07
tan-1 0.07 = 4.0°
2 × 4.0° = 8.0°

77m at 1000m: This field of view specification (number of metres at 1000 metres) is common on binoculars for the European market. To convert to degrees, divide the number of metres by 2000, take the inverse tangent of the result, and double your answer:

77 ÷ 2000 = 0.0385
tan-1 0.0385 = 2.2°
2 × 2.2° = 4.4°


In recent times, some Chinese budget binoculars have been produced with bizarre numbering, such as 60×90, when they are actually something entirely different, e.g 12×60. Other examples I have encountered in fixed magnification binoculars (there are no good zoom binoculars!) are a "20×50" that is a 10×50 and a "30×60" is a 15×22.

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2. Exit Pupils

The exit pupil (also known as the eye ring) is the position of the image, formed by the eyepiece, of the objective lens. It is the smallest disc through which all the collected light passes and is therefore the best position for the eye's pupil. Ideally, it should be no larger than the eye's pupil.


The size of the eye's pupil varies a great deal between individuals and diminishes with age, as is shown in this table:

If you are concerned that your eye's pupil may vignette the binocular image, you should measure your exit pupil. This is most easily done approximately by standing in front of a mirror at night, waiting 30 seconds or so for your pupils to dark adapt, hold a ruler under the eye, in the same vertical plane as your pupil, photograph the eye in the mirror (use flash, but not anti-red-eye pre-flash), then use image manipulation software to move a section of the ruler over the diameter of the pupil.

Ideally, you will then acquire binoculars whose exit pupis are no bigger than your eye's pupil.

Eye Relief

This is the distance from the eye lens of the eyepiece to the exit pupil. Spectacle wearers require sufficient eye relief to enable them to place the eye at the exit pupil.

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3. Coatings

Binocular coatings are qualitatively described as "coated", "fully multi-coated", etc. There is no universally agreed meaning to these designations, but they are commonly held to have the following meanings:

Coated: At least one glass-to-air surface (usually the outer surface of the objective) has a single layer of anti-reflective coating, usually MgF2; other surfaces are uncoated.
Fully Coated: All glass-to-air surfaces of the lenses (but not the prism hypotenuses) have a layer of anti-reflective coating.
Multi-coated: At least one glass-to-air surface (usually the outer surface of the objective) has two or more layers of anti-reflective coating. The other surfaces may be single-layer coated or not coated at all.
Fully Multi-coated: All glass-to-air surfaces of the lenses (but possibly not the prism hypotenuses) have two or more layers of anti-reflective coating.

More recently, some binoculars coatings have been described as "broadband". Again, there is no industry-wide standard—it can mean anything from 3 layers upward. Some manufacturers are more forthcoming as to the precise nature of their coatings. For example, Kunming Optical, the manufacturer of the popular Oberwerk-branded binoculars in the USA (branded as Strathspey in the UK, Teleskop Service in Germany) provides the following information about its coatings:

Level I: (Equivalent to Fully Coated) Single layer of MgF2 coating on 16 glass-to-air surfaces: 4 for two objectives, 12 (6 per side) for the three optical elements in each eyepiece. The prisms are not coated.
Level II: (Equivalent to a blend of Multi-Coated and Fully Multi-Coated) Broadband multi-coatings of 5-7 layers on the 4 glass-to-air surfaces of the two objectives, and the 4 surfaces of the eye lenses of the two eyepieces. Single-layer MgF2 coating on all other glass-to air surfaces, including the hypotenuses of the prisms.
Level III: Broadband multi-coatings on all the surfaces except the prism hypotenuses, on which there are single-layer MgF2 coatings.
Level IV: Broadband multi-coatings on all the surfaces including the prism hypotenuses.

N.B. There is no guarantee that the coatings have been properly applied. If quality control is poor (as it tends to be with budget binoculars), improperly applied multi-coatings could give worse transmission than a properly applied single coating!

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4. Prisms

Prism Type: Porro or Roof?

The prisms in binoculars serve primarily to correct the inverted and laterally reversed image that would otherwise result from the objective and eyepiece alone. A secondary effect is that they fold the light path, so that the binocular is shorter than it would otherwise be, making it easier to handle. For modern binoculars without angled eyepieces, there are two basic types: the porro prism and the roof prism.

Porro Prisms. The Porro prism assembly consists of two isosceles right-angled prisms mounted with their hypotenuses facing each other but with their long axes exactly perpendicular. Porro prisms are usually considered to offer the best value for money in astronomical binoculars; they are certainly less expensive than roof prism binoculars of equivalent optical quality. The light path in Porro prisms is like this:

Roof Prisms. The Pechan roof prism (above) is a combination of a semi penta prism (45° deviation prism) and a Schmidt roof prism: The combination is a compact combination that results in an almost "straight through" light path. The consequence is a very compact binocular. There is, of course, a limit to the aperture of roof prism binoculars that is imposed by the "straight through" light path because, the centres of the objectives cannot be separated by more than the observer's inter-pupillary distance.

Although the roof-prism configuration is physically smaller and thus uses less material in its construction, it has to be made to a much higher tolerance (e.g. 2 arcsec for the roof) than is acceptable for Porro prisms (10 arcmin), i.e. 300 times as precise!

Any thickness or irregularity in the ridge of the roof will result in visible flares, particularly from bright, high-contrast objects, i.e. many astronomical targets. Additionally, a result of the wave nature of light is that interference can occur when a bundle (or "pencil") of rays is separated and recombined, as happens with a roof prism. The consequence is a reduction in contrast. This can be reduced by the application of a "phase coating" to the faces of the roof. Binoculars with phase coatings usually have "PC" as part of their designation (see below).

The demand for better quality of the optical elements and their coatings in roof-prism binoculars means that they will inevitably be more expensive than Porro prism binoculars of equivalent optical quality. They do, however, offer three distinct advantages:

It is a matter of personal judgement whether these advantages warrant the extra expense.

There is a common misconception that roof-prism binoculars are "birding binoculars" and that Porro-prism binoculars are inherently better for astronomy. Whereas roof-prism binoculars are advantageous for birding (lighter, easier to waterproof) and Porro-prism binoculars generally offer equivalent optical quality at a lower price and are not aperture-limited because of the design, either can be used for either activity, where the one with the better optical quality will generally perform better.

Prism Glass: BaK4 or BK7?

A consideration is the glass used for the prism. Normal BK7 glass has a lower refractive index than the optically denser BaK4 glass that is used in better binoculars (but see Note 3 below!). A higher refractive index results in a smaller critical angle, 39.6° in BaK4 as compared to 41.2° in BK7), so there is less light likely to be lost because of non-total internal reflection in the prisms:

The difference is more noticeable in wide angle binoculars whose objective lenses have a focal ratio of f/5 or less. The non-total internal reflection of the peripheral rays of light cone from the objective results in vignetting of the image. This effect can easily be seen by holding the binocular up to a light sky or other light surface and examining the exit pupil. The exit pupil of a binocular with BaK4 prisms will be perfectly round, whilst that of a binocular with BK7 prisms will have tell-tale blue-grey segments around it:

Note 1: The BK7 image was taken from a slight angle in order to show the nature of the vignette segments. Viewed from directly behind the exit pupil, there is a square central region with vignette segments on four sides.

If the segments are black, this is an indication of undersized prisms, a cost-cutting measure in budget binoculars.

Note 2. There is no industry-wide standard for coating names. The designations I have been using above are those from Schott AG, a German manufacturer of optical glass. However, Chinese BaK4 is not the same as Schott BaK4; it's not even Barium Crown glass! It has a refractive index (and, hence, critical angle) that is between those of Schott BK7 and BaK4, and the tolerance for "bubble count" is significantly less stringent. It does have the advantage that it is less dispersive, but this is unlikely to be a critical factor in budget binoculars, where colour dispersion will result from other optical components as well.

Note 3: Whilst the type of prism glass does make a difference, there are much more important considerations for binocular choice, such as crispness of focus, flatness of field, edge distortion, chromatic aberration, build quality, smoothness of focus ... I could go on ... and on. The first binocular I ever used for astronomy, a 1960s or '70s vintage Zeiss 10x50, had BK7 prisms and single-layer coatings — overall it was optically and mechanically superior, and hence better suited to astronomy, than many of today's budget fully multi-coated efforts with BaK4 prisms.

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5. Focusing Mechanisms

There are three different types of focusing mechanism commonly found on binoculars:

Centre Focus (Porro Prism)

The eyepieces are connected to a threaded rod in the central hinge. An internally threaded knurled wheel or cylinder causes the rod to move, thus moving the eyepieces.

The advantage is that the eyepieces can be focused simultaneously, which is a consideration for general terrestrial use, but not for astronomy, where refocusing is rarely necessary during an observing session. The disadvantages are that there is almost always some rocking of the bridge, which leads to difficulty in achieving and maintaining focus, the focusing system is difficult to seal, so dirt can enter, and the optical tubes are extremely difficult to water-proof, resulting in increased likelihood of internal condensation.

The right hand eyepiece is usually independently focusable in order that differences in focus of the observer's eyes can be accommodated; this facility is often called a 'dioptre adjustment'.

Centre Focus (Roof Prism)

Like the Porro prism centre focus system, there is an external focus wheel and an independent helical focuser (dioptre adjustment) for the right eyepiece; the similarity ends there. The mechanism is internal and focusing is achieved by changing the position of a focusing lens between the objective lens and the prism assembly. It has the dual advantages of permitting simultaneous focusing of both eyepieces and allowing relatively simple dust- and water-proofing. The disadvantage is that there is an extra optical element that must be accurately made, which absorbs a tiny amount of light, and whose movement during focusing alters the field of view slightly.

Independent Focus

The eyepieces each have a helical focuser. This is much more robust than a centre focus system and is easier to make dirt-and water-proof. The best quality astronomical (and marine and military) binoculars have independent focusing. The disadvantage is that the eyepieces cannot be focused simultaneously, but this is not an issue for astronomical observation, where refocusing is not normally necessary once good focus has been attained.

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