See more with both eyes.
Inexperienced astronomers often think of binoculars as poor substitutes for a telescope, something to use only if you donât have a scope. Thatâs a mistake. In fact, a binocular is essential observing equipment even if you own a dozen top-notch telescopes. Experienced observers use binoculars to orient themselves within the constellation when they are locating objects they intend to view with their scopesâto place the object within its context of bright surrounding stars. A binocular is also useful for planning star hops [Hack #21] because the dimmest stars visible in a standard binocular are of similar magnitude to the dimmest stars visible in the optical finder of a typical telescope.
Binocular is properly a single noun. Calling a single instrument a âpair of binocularsâ is incorrect. âMay I use your binocularâ is grammatically correct; âMay I use your binocularsâ isnât, unless you mean two or more instruments. (Actually, asking to use someoneâs binocular is an etiquette faux pas among astronomersânearly equivalent to asking to borrow someoneâs toothbrushâbecause most people have their personal binoculars adjusted to their own vision. If you do borrow a binocular, donât change the diopter adjustment.)
But binoculars are more than just an adjunct to telescopic observing. While the narrow fields of view of telescopes limit you to seeing just the trees, the wide fields of binoculars let you see the whole forest. For viewing large open star clusters, Milky Way star fields, comets, and other large objects, binoculars are often the best choice. On more than one occasion, weâve set up our telescopes only to find at the end of the evening that weâd never used them. Instead, weâd spent the entire observing session using our binoculars to study the heavens.
Binoculars are also useful for more than just astronomical observing, of course. A good binocular has many daytime uses, including sports, birding, and other hobbies.
This section describes the important characteristics of binoculars. Understanding these issues allows you to choose a suitable binocular that fits your budget.
- Magnification and Aperture
Binoculars are designated by their magnificationand aperture. For example, a 7X50 binocular magnifies objects seven times and has 50mm objective lenses. The amount of magnification a binocular provides is important, but the size of the objective lenses is even more important because it determines how much light the binocular gathers.
Magnification, also called power, is specified by a number followed by âXâ and describes the amount of linear image enlargement provided by the binocular. For example, a 7X binocular makes an object appear seven times larger (or, alternatively, seven times closer) than if that object were viewed with the naked eye.
Binoculars are available with magnifications ranging from 4X to 20X or more in âgiantâ models. For hand-held binocular astronomy, the best choice is a binocular in the 7X to 12X range. All other things equal, binoculars with lower magnification show more of the sky, while those with higher magnification allow you to view more detail in celestial objects.
Some models, called zoom binoculars, offer variable magnification, usually in the range of 5X to 8X on the low end to 15X to 30X on the high end. Although there are some (very expensive) exceptions, most zoom binoculars are junkâ mechanically fragile and with very poor optics. We do not recommend zoom binoculars for astronomy, or indeed for any purpose.
Hand-holdability varies from person to person but is affected by the weight and magnification of the binocular. Most people can hand hold a standard 7X binocular steadily; some can hand-hold a 10X binocular with acceptable steadiness; a few can hold-hold a 12X binocular steadily, although we consider 12X to be at the outside limit of hand-holdability.
You can increase the steadiness of your binocular views by lying in a lawn chair or a partially inflated childrenâs swimming pool. With such aid, we are able to handhold even 15X and 20X binoculars with acceptable steadiness. You can also tripod-mount a binocular to eliminate hand-held jitters. Even standard-size binoculars can be tripod-mounted; giant binoculars require it. Most binoculars intended for astronomy provide a standard 1/4X20 tripod socket, which is a feature worth having even if you intend to use your binocular hand-held most of the time.
Aperture is the diameter of the objective lenses of the binocular, the large lenses at the front of the instrument (the end you donât look into). Larger apertures collect more light than smaller apertures, in an amount proportional to the square of their diameters. For example, a 50mm binocular has objective lenses twice the diameter of a 25mm binocular, but gathers 4 times as much light, because 22=4. By the same calculation, a 100mm giant binocular gathers four times as much light as a 50mm binocular, and 16 times as much light as the 25mm model. Light gathering ability is essential because it allows you to see dimmer objects.
Relative to the human eye, a 50mm binocular gathers from 50 to 100 times as much light. Because a 100X difference in brightness corresponds to five stellar magnitudes, using a 50mm binocular may allow you to see objects four to five magnitudes dimmer than those you can see naked eye. For example, if on a particular night from a particular site you can see stars down to magnitude 5.5 naked eye, the 50mm binocular might allow you to see stars down to magnitude 9.5 or dimmer [Hack #13].
To put this in perspective, for the same relative increase in light-gathering ability that you get from using a 50mm binocular instead of your naked eye, you would have to substitute a 14â to 20â telescope for the 50mm binocular.
Standard binoculars suitable for astronomy have objective lenses ranging from 35mm to 50mm. Oversized âgiantâ binoculars are popular with some astronomers. These giant binoculars have objective lenses ranging from 56mm up to 80mm or more, and are best used on a tripod or other solid mount. Such instruments are ideal for wide-field views of Milky Way star clouds and other large objects, but they are unsuitable for general use.
There are also so-called âsuper-giantâ binoculars, which more resemble mounted pairs of refractor telescopes than they do standard binoculars. Super-giant binoculars have apertures ranging from 90mm to 6â (152mm) or more, and they are very specialized (and expensive) instruments. They use interchangeable eyepieces, optical finders, expensive mounts, drive motors, and other accessories more commonly associated with telescopes than with binoculars. The largest binocular we know of is the one built by a plucky Australian amateur astronomer using two 20â (508mm) telescope mirrors as objectives.
- Exit pupil
If you hold a binocular up to a light source and look at the eyepieces from several inches or more away, youâll see bright circles in the eye lenses. Those bright circles are the exit pupils of the binocular, which define the point in the optical train where the light is delivered from the binocular into your eyes. You can calculate the exit pupil diameter simply by dividing aperture by magnification. For example, a 7X50 binocular provides an exit pupil of 50/7, or just over 7mm in diameter. A 10X50 binocular provides a 5mm exit pupil, as does a 7X35 or 8X40 model.
Exit pupil is important because the maximum entrance pupil of peopleâs eyes varies in size with age and other factors [Hack #7]. Ideally, you want to match the exit pupils of the binocular to the entrance pupils of your dark-adapted eyes. By doing so, you deliver the maximum possible amount of light and see the brightest possible image. If the exit pupils of the binocular are larger than your entrance pupil, you waste light. For example, if your fully dark-adapted entrance pupil is 5mm, itâs pointless to use a 7X50 binocular because it delivers a 7.1mm exit pupil. In effect, your eye is stopping down your 50mm objective lenses to 35mm. You could instead use a 7X35 or 10X50 binocular, either of which delivers a 5mm exit pupil that matches your entrance pupil, and the images would be as bright as those you see with the 7X50 binocular. Similarly, if you observe primarily from light-polluted locations, where your eyes can never fully dark adapt, a binocular with a 4mm to 5mm exit pupil may be the best choice.
- Eye relief
The eye relief of a binocular is the distance between the outer surface of the eyepiece lens and where your pupil needs to be placed to view the image. Standard binoculars have eye relief ranging from only a few millimeters to 20mm or more. Long eye reliefâat least 17mm to 20mmâis necessary if you wear glasses while using the binocular. Short eye relief is quite acceptable if you use the binocular without eyeglasses or with contact lenses. To decide how much eye relief you need, use the following guidelines:
If your eyeglasses correct astigmatism or other non-symmetric vision problems, youâll need long eye relief so that you can wear your glasses while viewing. Look for a binocular that provides at least 17mm to 20mm of eye relief.
If your eyeglasses correct only for near- or farsightedness, you can simply focus the binocular to accommodate your vision. Any otherwise suitable binocular should be fine, even if it provides minimal eye relief.
If you are nearsighted in one eye and farsighted in the other, as both Robert and Barbara are, you can use the diopter adjustment on one of binocular eyepieces to adjust the binocular to your personal vision. (Make sure that the range of diopter adjustment available on the binocular you choose is sufficient.) To adjust the binocular to your personal vision, close the eye on the side with the diopter adjustment and focus the binocular sharply on the stars using the other eye. Then, close that eye, and using only the eye on the side with the diopter adjustment, turn the diopter adjustment until the focus in that eye is also sharp.
Many observers who routinely wear contact lenses find them unsuitable for astronomical observing and so wear their glasses instead. If you intend to buy a binocular to use while wearing contact lenses, try using another binocular first while wearing your contacts.
- Field of view
Field of view (FoV) quantifies the angular range of the image visible in the binocular eyepieces. FoV is determined by the optical design of the binocular, including its focal length and the type of eyepieces used. FoV may be specified in two ways. Binoculars marketed to astronomers usually specify the FoV angularly, in degrees. Binoculars marketed for birding and other terrestrial uses may specify FoV linearly, in terms of the number of feet visible at 1,000 yards or the number of meters visible at 1,000 meters. (In fact, binoculars sold for astronomy are usually quite suitable for terrestrial use and vice versa, so donât let that worry you.)
To convert linear FoV to angular FoV, divide the linear FoV by the conversion factor 52.3598 if the FoV is given in feet at 1,000 yards, or the factor17.4533 if the FoV is given in meters at 1,000 meters. For example, to convert a FoV of 314 feet at 1,000 yards to degrees, divide 314 by 52.3598 to yield a 6.0Â° FoV. To convert a FoV of 131 meters at 1,000 meters to degrees, divide 131 by 17.4533 to yield a 7.5Â° FoV.
For astronomy, a wide FoV is desirableâyou can see more of the sky with a wider fieldâbut a very wide FoV may be problematic. Increasing the FoV beyond a certain point requires optical compromises that cause blurred images at the edges of the field, distortion, short eye relief, and other problems. For 7X or 8X binoculars, something in the 6.5Â° to 8.5Â° range is reasonable. For 10X binoculars, look for something in the 5.0Â° to 7.0Â° range, and for 12X models something in the 4.5Â° to 6.0Â° range.
All other things being equal, a binocular with a FoV on the low end of our recommend range will probably provide better edge performance and eye relief than a similar model with a wider field. High-end binoculars can push the limits by using complex (and expensive) eyepiece designs that provide wider fields while maintaining optical quality and long eye relief.
- Interpupilary distance
Interpupilary distance is the distance between the centers of your two pupils. Standard binoculars are adjustable to accommodate different interpupilary distances, typically within a range of 60mm to 75mm. That suffices for most people, but some women and many children have interpupilary distances too short for a standard binocular to accommodate. Unfortunately, the only answer is often compact binoculars, which are unsuitable for astronomy because of their small objective lenses.
- Prism type
Binoculars use prisms to present a correct-image view, right-side up and not reversed left-to-right. Two types of prism are commonly used, Porro prisms and roof prisms.Ironically, although roof prisms are generally considered âbetterâ than Porro prisms, we consider roof-prism binoculars less than ideal for astronomical use.
Itâs easy to identify the two types of binocular visually. Roof-prism binoculars align the objective lenses and eyepieces on a common center optical axis, giving roof-prism binoculars their characteristic single-piece âHâ shape. Porro-prism binoculars are available in the two-piece âZâ or âGermanâ shape or the single-piece âBâ shape. All Porro-prism binoculars, regardless of shape, align the objective lenses and eyepieces in a stepped or offset arrangement, with the objective lenses farther apart than the eyepieces. Figure 1-5 shows two inexpensive Orion Scenix 7X50 binoculars, which are typical Porro-prism models.
Roof-prism binoculars are generally much more expensive than Porro-prism binoculars of equal optical and mechanical quality. Thatâs true both because the roof prisms themselves are more expensive to produce than Porro prisms and because roof-prism binoculars are much less forgiving of collimation (alignment) errors. Roof prisms are typically collimated to an angular accuracy of two arcseconds. For equivalent image quality, Porro prisms must be collimated to an accuracy of only 10 arcminutes, or 1/300th as precise. These strict tolerances mean that roof prisms are ordinarily affixed permanently to a solid metal plate and laser or interference collimated before assembly, which is a time-consuming and expensive procedure. The upside is that because roof prisms are permanently affixed to a solid substrate, they seldom require recollimation.
With very few exceptions, binoculars must be collimated at the factory or a service center. Binoculars are generally pretty rugged. Weâve dropped numerous binoculars through the years and have never knocked one out of alignment. If you do knock a binocular hard enough to require recollimation, though, expect an expensive trip back to the factory and a long wait before you see the binocular again.
The possibility of miscollimation is a good reason to buy your binocular locally, or at least from an online source that has a no-questions-asked return policy. Even premium binoculars are sometimes miscollimated when they leave the factory, and that problem is even more likely to occur with inexpensive models.
The other disadvantage of roof prisms for astronomy is that they transmit less light than Porro prisms. The optical design of roof-prism binoculars requires that one surface of a roof prism be semi-silvered to transmit part of the light and reflect part of it. This arrangement causes light loss of 12% to 15%, which doesnât matter for daytime use but is a severe drawback for astronomy, where every photon counts when viewing dim objects.
Accordingly, we recommend avoiding roof-prism binoculars for astronomy. Stick to the tried-but-true Porro-prism binoculars. Youâll see brighter images and save money as well.
Two types of glass are used to make Porro prisms. Cheap Porro prisms are made from inferior BK-7 borosilicate flint glass. Better Porro prisms are made from superior BaK-4 barium crown glass. Although nearly any binocular that uses BaK-4 prisms advertises that fact, itâs easy enough to check for yourself. Simply hold the binocular with the eyepieces several inches from your eyes, pointing at the sky or another evenly lit light source, and look at the exit pupils. If the prisms are of BaK-4 glass, the exit pupils are round and evenly illuminated. If the prisms are of BK-7 glass, youâll see a square inside the circle, with the area inside the square brightly illuminated and the area outside the square dimmer.
Because of their design, roof-prism binoculars, can use BK-7 glass without suffering the edge dimming and vignetting that occurs with BK-7 Porro prisms.
A typical binocular has from 6 to 10 lenses and prisms in the optical path between the object you are observing and your eye. Each of those optical elements has at least two surfaces, and each surface reflects a small percentage of the light rather than transmitting 100% of the light that strikes it.
An uncoated surface may reflect as much as 4% of the light that strikes it, which doesnât sound excessive until you run the numbers. If a binocular has 10 optical elements, each with two 4%-reflective surfaces, its overall light transmission is 0.96 raised to the 20th power, or 0.442. In other words, that binocular transmits only 44.2% of the light that strikes its objective lens. The remaining 55.8% of the incident light is reflected and scattered, which reduces both brightness and contrast dramatically.
Optics makers use interference coatings on lenses and prisms to reduce these reflections. The simplest and least expensive coating is a thin single layer of magnesium fluoride (abbreviated MgF or MgF2), which can increase transmission to 0.985. A 10-element binocular with single MgF coatings on all surfaces transmits (0.985)20, or about 73.9% of the incident light, a great improvement over an uncoated binocular.
Reflection can be further reduced by using multi-layer interference coatings, a process known as multicoating. Although a multicoated optical element may have literally hundreds of coatings, three to seven layers is more typical. A properly multicoated surface may have transmission ranging from 0.995 to 0.999, which translates to overall transmission of 90.5% to 98.0%. Obviously, good multicoatings make a huge difference, even compared to standard coatings.
But coating costs money, and multicoating more so. To cut costs, some optics makers coat (or multicoat) only some of the surfaces. A standard terminology has arisen to describe the levels of coatings used on binoculars and other optical equipment.
Single-layer MgF coatings have been applied to some but not all of the optical surfaces, typically only to the external surfaces of the objective lens and eyepiece lens. We consider a coated binocular unsuitable for serious astronomical use. Only the cheapest binocularsâwe are tempted to call them toysâare in this category.
- Fully coated
Single-layer MgF coatings have been applied to all of the optical surfaces. We consider a fully coated binocular the minimum acceptable for serious astronomical use. Most of the inexpensive binoculars sold by Wal*Mart and similar big-box retailers are in this category.
Multi-layer coatings have been applied to some but not all of the optical surfaces, typically only to the external surfaces of the objective lens. Usually, but not always, the other surfaces have had single-layer MgF coatings applied, which is sometimes described as âfully coated and multicoated.â Most inexpensive binoculars suitable for astronomical use, such as the Orion Scenix models (http://www.telescope.com), are in this category.
- Fully multicoated
Multi-layer coatings have been applied to all of the optical surfaces. Binoculars in this category are the best choice for astronomical observing, but are not inexpensive. Low-end, fully multicoated models, such as the Orion UltraViews, start at $150 and go up from there.
All coatings are not of the same quality. Applying top-notch multicoating is a very expensive process that, if done properly, can exceed the cost of making the lenses themselves. On the other hand, if all you care about is being able to claim that your optics are multicoated, you can slap on multicoating relatively cheaply. Optics from Zeiss, Nikon, Fujinon, or Pentax (to use just a few examples) have superb multicoating. Optics from second-tier Japanese makers, such as Vixen, have good multicoating, but not as good as that of first-tier makers. Cheap multicoated optics from Chinese makers, well, theyâre multicoated, but thatâs about the most you can say about them. Avoid any binocular with âruby redâ or other strange coatings.
Binoculars are available in a wide range of prices. For convenience, we classify standard-size binoculars by price range as inexpensive (<$75), midrange ($75 to $250), premium ($250 to $500), and super-premium (>$500). As you might expect, itâs not difficult to get a good binocular if you are willing to pay premium or super-premium prices. Even midrange binoculars are generally excellent.
In the sub-$250 range, each doubling of price generally buys you substantially better optical and mechanical quality, all other things being equal. For example, a $100 7X50 binocular is not twice as good as a $50 7X50 binocular, but it is likely to be built more solidly and to provide noticeably superior images. A $200 7X50 binocular provides a similar relative improvement over a $100 binocular. Once you get into the $300 range, although you can spend much more, the improvements become incremental, not to say invisible. For example, other than by examining the name plate, few people would be able to detect any difference in optical or mechanical quality between a $300 10X50 binocular and a $600 10X50 binocular.
Even some inexpensive binoculars provide surprisingly high bang-for-the-buck. When Barbara first became interested in astronomy several years ago, Robert bought her a $90 Orion Scenix 7X50 binocular, figuring that if she lost interest in astronomy it wouldnât be any great loss. Robert had been on a 20-year hiatus from active observingâcollege, jobs, and âreal lifeâ intervenedâbut was determined to get back into the hobby. At first, he planned to buy a premium Zeiss, Leitz, or Swarovski binocular, but when he saw how good Barbaraâs inexpensive Orion Scenix binocular was, he bought a Scenix for himself. Is the Scenix as good as premium models? No, but for one-fifth to one-tenth the price, itâs astonishingly good.
If your budget is very tight, nearly any inexpensive binocular is better than nothing. Avoid gimmicks, such as âinstant focus,â red coatings, and so on. Look for a binocular that uses BaK-4 prisms and is at least fully coated. A 7X35 model is acceptable, and in fact may be a better choice than a 7X50 or 10X50 model at the same price.
For standard binoculars in the $75 to $250 range, most models from Bausch & Lomb, Celestron, Minolta, Nikon, Olympus, Orion, Pentax, Pro Optic, and Swift are reasonable choices. At the low end of this range, we think the 7X50 and 10X50 Orion Scenix models are the stand-out choices. (Weâd avoid the 12X50 Scenix and particularly the 8X42 model.) At the upper end of this range, we think the Orion Vista binoculars in 7X50, 8X42, and 10X50 offer very high value, as do the similar but more expensive Celestron Ultima models.
Some models in this price range include built-in digital cameras, which we consider a drawback. Too much of the price goes to the digital camera and too little to the optics. Avoid any âgimmickâ binocular. It almost certainly has trash optics and will be none too durable.
For standard binoculars in the $250 to $500 range, there are many suitable candidates. At the lower end of the range are the excellent Celestron Ultima models. At the middle and upper parts of this range, you will find many suitable models from Alderblick, Celestron, Fujinon, Nikon, Pentax, and Steiner.
Canon image-stabilized binoculars also fall into this range. Although they are popular with some astronomers and of excellent optical quality, we consider them a poor choice for astronomy. Their apertures and exit pupils are small, and they are expensive. Also, many people find their electronic image stabilization makes for an odd viewing experience. All things considered, if youâre concerned about image stability, we think a standard binocular on a decent tripod is a better choice.
For standard binoculars in the stratospheric super-premium price range, nearly any Porro-prism model with provision for tripod mounting is an excellent choice. This is the realm of world-class optics from companies like Leitz, Swarovski, and Zeiss. Premium models from companies like Fujinon, Nikon, Pentax, and Steiner are also in this price class. Binoculars simply donât get any better than this.
If you are considering a mid-range or better binocular, buying a used model can save you money. Decide which brands, models, and price ranges suit you, and then check Astromart (http://www.astromart.com) to see whatâs available (the selection varies day to day and even hour to hour). Depending on age and condition, used mid-range and premium binoculars generally sell for 50% to 80% of what youâll pay for the same model new. Donât buy an inexpensive binocular used, though. Chances are good youâll just be buying someone elseâs problem.
Like standard binoculars, giant and super-giant binoculars are available in various sizes and price ranges. Although we do not recommend a giant binocular as your first (or only) binocular, they have their place. In fact, some observers eschew telescopes entirely and spend all of their observing time with a good tripod-mounted giant binocular.
We classify giant binoculars as inexpensive (<$250), midrange ($250 to $1,000), premium ($1,000 to $5,000), and super-premium (>$5,000). Some inexpensive giant binoculars are surprisingly good for their price. You wonât mistake them for premium units, but they are quite usable. They are generally sharp at the center of the field, but with noticeable softness in the outer 15% to 30% of the field. Inexpensive giant binoculars often show significant chromatic aberration (false color) on very bright objects such as Luna and bright stars, but they are not really intended for observing those types of objects. For doing what they do bestâscanning Milky Way star fields and viewing open star clustersâthey serve quite well. If you can afford better, youâll find that spending twice as much delivers noticeably better image quality and mechanicals. As with standard-size binoculars, there are premium and super-premium models available for those who can afford them (large Fujinon, Leitz, Takahashi, and Zeiss models sell for more than $10,000, sometimes much more).
If youâre on a tight budget, the Chinese-made Celestron SkyMaster series is a good choice in a giant binocular. Three models are available, 15X70, 20X80, and 25X100. All use BaK-4 Porro prisms and are multi-coated. Eye relief is acceptable, at 18mm in the 15X70 model, and 15mm in the two larger models. The 15X70 model sells for well under $100, and even the 25X100 model can sometimes be found on sale for under $250. All models have provision for tripod mounting, which is necessary for these large instruments. The 15X70 model is water resistant, and the two larger models are waterproof. Image quality is mediocre, particularly at the edges, but is surprisingly good for the price. We think most occasional users will be quite pleased with a Celestron Sky-Master binocular.
For inexpensive semi-giant binoculars, we think the stand-out choice is the Orion Mini-Giant series. These fully multicoated Japanese-made optics are available in 8X56, 9X63, 12X63, and 15X63 models, ranging in price from $159 to $219 and with fields of view from 5.8Â° in the 8X model to 3.6Â° in the 15X model. Eye relief across the line is excellent, from 17.5mm to 26mm. The smaller models are hand-holdable, although all models have tripod sockets. Image quality is, if not quite up to the level of the premium brands, more than acceptable to most people.
We canât make other recommendations because our experience with giant binoculars is very limited. However, we will say that Fujinon offers several giant binoculars that are extremely popular with amateur astronomers and receive uniformly excellent reviews. If we wanted to buy a premium or super-premium giant binocular, weâd look at Fujinon models first.
Get Astronomy Hacks now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.