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Astronomy binoculars can provide you with an extremely enjoyable observing experience, but only if your instrument is properly matched to your eye and observing
conditions. This quick guide covers some of the most common questions we get about using binoculars for astronomy, and should provide you with enough basic information to make an informed choice about which
binocular is the best match for your particular situation.
Frequently Asked Questions:
FAQ about choosing and using binoculars:
Why should I use binoculars for astronomy?
What is an “astronomy binocular”?
What is a “binocular telescope”?
Can I use your large-aperture binoculars for terrestrial observation & spotting?
How far can I see with your binoculars?
How large a binocular can I expect to handhold?
When do I need a tripod?
Why isn’t my cheap department store tripod good enough?
What about zoom binoculars?
What can I see in the night sky with binoculars?
Can I use my binoculars to observe the moon and planets?
FAQ about binocular specifications & terminology:
When you refer to a binocular as a “15x70”, what do you mean?
What is collimation, and why is it important?
Do you really need a professional collimator to collimate a binocular?
What is “light gathering power”?
What’s the difference between “roof prism” and “porro prism” binoculars?
What's the difference between BaK4 and BK7 glass prisms?
Do I need a “long eye relief” binocular?
What is the difference between “center focus” and “IF”, or individual focus binoculars?
Do I need a waterproof binocular?
Are your binoculars “shockproof”?
What is “rubber armoring,” and is it important?
What are the different types of binocular coatings?
What about “phase-coatings”?
What is “exit pupil”, and why is it important?
Specific FAQ about Garrett Optical binoculars:
Are the increased prices indicative of higher quality in your product line?
What are the major differences between your four lines of binocualars?
Are your triplet lens binoculars better than your doublet lens binoculars?
Binocular performance FAQ:
Why does my binocular show distortion near the edge of the field of view?
What is “false color”, and is it a problem?
Should I expect my binocular to offer the same image quality as my telescope?
Why should I use binoculars for astronomy?
For the beginning amateur astronomer, binoculars are one of the best and easiest ways to learn your way around the night sky. The wonders of the universe are captured through stereo, or two-eyed vision in a
way that cannot be reproduced with the single eyepiece of most telescopes. Also, binoculars have a much wider field of view than telescopes, allowing you to see a much large part of the sky at a time. This
simplifies finding objects and nicely “frames” the target you are observing, showing you its context in relation to the other astronomical objects surrounding it. For the experienced amateur astronomer, a
good binocular can prove an invaluable asset for finding objects too faint to be easily seen in the telescope finder. Binoculars are also an excellent educational tool, and a great way to while away the time
while your telescope cools down, not to mention the satisfaction they can give as stand-alone observing instruments. Binoculars are the ultimate grab ‘n go astro instruments, and larger aperture models can
open up a whole new way to view deep space objects.
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What is an “astronomy binocular”?
You can use virtually any binocular to enjoy the night sky, but some binoculars are better than others. Generally, a dedicated astronomy binocular has a magnification of at least 7x to 8x, and an aperture of
at least 40mm. Amateur astronomer’s passion for binocular astronomy has led to the development of large aperture binoculars that can rival the light gathering power of small telescopes. These instruments can
have lens diameters up to 100mm and magnifications as high as 30x – or even higher, with interchangeable eyepieces. Because of the bright, point-source nature of stellar objects, astronomy binoculars need to
have good optics and excellent coatings and high-quality prism glass to provide the best views. See below for more info on coatings and prisms.
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What is a “binocular telescope”?
A binocular telescope could be loosely defined as a large-aperture binocular that accepts standard 1.25” or 2.0” telescope eyepieces, must be mounted like a telescope, and can attain similar levels of
light-gathering power and magnification as a small telescope. Garrett Optical’s Signature GT80-45 and GT100-45 binocular telescopes are two such instruments.
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Can I use your large-aperture binoculars for terrestrial observation & spotting?
Of course! Many of our customers use their Garrett Optical binoculars for terrestrial observation in addition to amateur astronomy, and some use their instruments exclusively for hunting or other spotting
applications. We generally recommend that customers wishing to use their binoculars for terrestrial work adhere to the same mounting recommendations made for amateur astronomers, and we recommend 20-25x or
less magnification for most terrestrial applications. Large-aperture binoculars are a great substitute for a spotting scope because of the comfort of two-eyed viewing. Their excellent image brightness also makes them ideal for low-light applications.
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How far can I see with your binoculars?
The question should not be about how far you can see with a given binocular – with only your naked eyes, you can see millions of astronomical objects that are literally millions of miles away. A binocular (or a telescope, for that matter) simply helps you to
see more of these objects, and brings them closer than you would see them using only the naked eye. For example, take our Gemini 25x100 WP binocular; this binocular uses 100mm objective lenses that gather
approximately 200 times as much light as a young person’s fully-dilated 7mm pupils. It will also bring any object observed 25 times closer than when that same object is observed using only the naked eye – in
other words, an object at a distance of ¼ mile (1320 ft.) would appear only 52.8 feet away. Of course, this is a greatly simplified example and there are many other factors besides objective lens size
(aperture) and magnification that affect binocular performance – browse this FAQ to find out more.
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When you refer to a binocular as a “15x70”, what do you mean?
The two numbers in a “15x70” binocular refer to the magnification and aperture (lens diameter) of the binocular. Magnification is the first number associated with the binocular “size” – e.g., it is the “15”
in a “15x70” binocular. What this means is that the binocular will show an object 15 times closer than it would appear at the same distance with the naked eye. The objective lens diameter, or aperture, is
the second number in the binocular size – e.g., the “70” in a “15x70” binocular means that the binocular has an aperture of 70mm.
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What is collimation, and why do you put so much emphasis on accurate collimation in your literature?
Many binoculars, especially giant binoculars, suffer from poor collimation. A binocular is essentially two telescopes connected together. Collimation is the term that is used to refer to the degree to which
the images in these two "telescopes" are merged. When mis-collimation is severe, it is easy to see - when you try to look through a binocular, you simply can't merge the two images. What most people don't
know is that a binocular can be significantly mis-collimated without the user being aware of it. Often, the person using the binocular will be able to merge the images, but they will suffer from headaches
and eye strain, which will reduce the enjoyment of the observing experience. Thus, accurate collimation is a serious issue for every binocular owner. A binocular may be out of collimation for several
reasons, but poor quality control on the manufacturer's end is probably the most common. That is why every Garrett Optical binocular is carefully tested for accurate collimation on a custom-built laser
testing bench. If necessary, collimation is performed by a highly experienced technician on a U.S. Navy Mark V collimator - one of the best tools in the business. We also
spend a lot of time carefully packing each binocular that we ship, to minimize the chance of collimation shifting during shipping.
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Do you really need a professional collimator to collimate a binocular?
The answer to this one should be obvious. Some binoculars can be user-adjusted to acheive what is called "conditional alignment," or merging of the two images for one user's interpupilary distance, and some
companies who do not have access to a collimator promote conditionally-aligned binoculars as “collimated”. However, this does not mean that the binocular is truly collimated throughout the entire
interpupilary range. In some situations, conditional alignment can work for individual users, but often this means that alignment at interpupilary settings other than that at which the binocular was adjusted
is compromised. If your binocular is out of warranty or was not very expensive to
begin with, conditional alignment may be worth your time. Often, when collimation shifts during shipment on a professionally-collimated binocular, only one axis will be off and a gentle tweak to one of the
prism screws will bring the unit back into collimation. This can save you the cost of shipping the binocular back to the vendor for repair, and you avoid the risk that the repaired unit will have the same
problem.
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What is “light gathering power”?
Objective lens size is the factor that determines how much light can potentially be collected by the binocular, and thus how deep you can see in the sky itself. Note that how much light actually gets to the
observers eye is dependent on several factors, such as prism, size, baffling, and coating quality. In general, though, given similar coatings, you can use aperture to compare how much light will be gathered
by two different binoculars. Because a binocular’s lenses are circular in shape, the area of the lens is proportional to the square of the lens diameter. In other words, a 100mm objective will gather four times
as much light as a 50mm objective, even though it is only twice as far across.
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How large a binocular can I expect to handhold?
Most people have no trouble hand-holding a 10-power binocular; however, some prefer the steadier view provided by a seven- or eight-power glass. 11- and 12-power binoculars are near the practical limit for
hand-held observation without any external support.
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When do I need a tripod?
As a general rule-of-thumb, you should consider mounting any binocular over 10-power on a tripod at least some of the time. 15-power binoculars should almost always be sturdily mounted, unless you can brace
your arms (for instance, when reclining in a lawn chair). For 20x or more, plan on mounting the binocular all the time.
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Why isn’t my cheap department store tripod good enough?
Chances are a that a standard light-duty camera tripod won’t handle a giant binocular, unless your binocular is very light. The reason is that binoculars put eccentric loads on their mounts (click here for more on eccentric loads). Think about it –
a camera tripod’s load capacity is determined when it’s aimed horizontally – astronomy requires the tripod to provide the same amount of stability all the way up to the zenith. Every time the binocular
changes position, the load distribution on the tripod changes as well. Typical photo tripods lock in a given position and are not designed for the continuous panning required by astronomers and terrestrial
observers. For this reason, we have put together a larger selection of video tripods, fluid pan heads, and parallelogram binocular mounts for you to browse in our “Mounts and Tripods” section.
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What about zoom binoculars?
You will probably notice that Garrett Optical does not carry any zoom binoculars - this is because we consider zoom binoculars to be inferior in virtually every area of performance to fixed-magnification
binoculars. The design of zoom binoculars causes them to have an extremely narrow field of view, even at low magnifications, creating a "tunnel vision" effect. Also, the zoom mechanism itself is quite
fragile, and once it is broken the binoculars are useless. Finally, most zoom binoculars are not useable at the upper end of their magnification range because of a combination of mediocre optical quality and
too much power. Because of these and many other factors, we recommend avoiding all zoom binoculars.
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What’s the difference between “roof prism” and “porro prism” binoculars?
Probably the first thing you will notice about a binocular is its body style. That body style is a result of the type of prisms employed in the optical system. There are two primary prism designs used in
modern binoculars – porro prisms and roof prisms. Porro prism designs are the most common, but they are more bulky than their roof prism counterparts. Even though they are slightly larger, porro prism
binoculars can actually be easier to hold than roof prism designs for individuals with large hands. Roof prism binoculars feature a sleeker profile than porro prism binoculars, but because of their design
the roof prisms must be extremely accurately aligned to produce good performance. Because of this, a given roof prism binocular tends to be significantly more expensive than comparably-performing porro prism
glasses. Also, in order to match the performance of the better porro prism binoculars, roof prism designs require expensive phase coatings (see the FAQ on coatings below).
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What's the difference between BaK4 and BK7 glass prisms?
Yes! Prism glass type is one of the first specifications you should check when purchasing a new binocular. Less-expensive BK7 glass is popular in budget binoculars, but is unsuitable for most uses because it
causes dimming at the edge of the field of view. The result is a poor viewing experience. Only high-index barium crown (BaK4) glass prisms will provide acceptable performance. For example, a fully coated
binocular utilizing BaK4 prisms can be a decent performer, but the same binocular with BK7 prisms would probably be unsatisfactory. All Garrett Optical binoculars utilize only the highest-quality BaK4 glass
for their prisms.
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Do I need a “long eye relief” binocular?
Eye relief (or “exit pupil distance”), is probably the most important single specification for individuals who must wear their eyeglasses while observing. Eye relief is the distance that the binocular image
is formed behind the eye lenses. In general, eyeglass wearers will require at least 14-15mm of eye relief to observe comfortably. Long eye relief (LER) binoculars feature 17-18mm of eye relief, which
produces an extremely comfortable observing experience for both eyeglass and non-eyeglass wearers. Most of Garrett Optical’s binoculars fall into the LER category.
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What is the difference between “center focus” and “IF”, or individual focus binoculars, and is one better than the other?
There are two different focusing mechanisms are commonly employed on binoculars – center focusers and individual focusing (IF) oculars. For most applications, either design is acceptable; however, in
situations where it is necessary to change focus rapidly, such as when birding, the center-focus design is clearly preferable. IF binoculars are often preferred for binocular astronomy, because once adjusted
they tend to hold focus well, and IF binoculars can have more diopter range for those who need corrective lenses but prefer to observe without their glasses. Some binoculars, such as our Gemini WP and
Signature giant binocular series use individual focusers for precise adjustments, while our Gemini LW and Classic series use the more common center-focus mechanism. Some binoculars, such as our Gemini LW
series, utilize specially-designed “slow center focusers” that allow the fine-tuning that is essential for astronomical observations. If you buy a center-focus binocular, be sure it includes a Right Diopter
Adjustment (RDA). An RDA allows you to compensate for the differences between your eyes and thus achieve maximum clarity. Individual focusing binoculars do not require an RDA, because each ocular adjusts
independently of the other.
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Do I need a waterproof binocular?
For binocular astronomy, waterproofing is not generally required. However, waterproofing can be desirable if you live in an area of high humidity, or if your binoculars will service double-duty as a birding,
hunting, or general observation glass. In damp or rainy conditions, binoculars that do not feature waterproofing are vulnerable to severe damage that can make them unusable. If moisture finds a way to get
inside the housing, fogging, mold, and damage to the optical coatings can result. Because of these risks, you should always choose a waterproof binocular such as Garrett Optical’s Classic, Signature, or
Gemini WP instruments if you will be observing in damp conditions. If a binocular is marketed as “fog-proof”, this normally means that the interior has been filled (or “charged”) with dry nitrogen gas to
prevent the build-up of internal condensation due to temperature changes (such as when taking the binocular outside from a warm car or house). Garrett Optical’s Classic, Gemini WP, and Signature lines of
binoculars are all filled with dry nitrogen at the factory, but because the size of the 80mm and larger binoculars means that the nitrogen will sometimes leak out over time, we do not make the “fog proof”
claim for these instruments.
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Are your binoculars “shockproof”?
Don’t be fooled by some manufacturers’ claim that their binoculars are “shockproof” or feature “shockproof prism housings.” This is simply not true – there is no such thing as a “shockproof” binocular. No
matter how well a binocular is constructed, it can always be jolted out of optical alignment. That’s why it’s important to treat all binoculars with care - like the precise optical instruments that they are.
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What is “rubber armoring,” and is it important?
Basically, this means that the outside of your binocular is covered with a durable rubber coating. Though it is never a good idea to expose your binocular to extreme conditions, rubber armor can help to
protect it from impacts, moisture and dirt. All of Garrett Optical’s binocular models (with the exception of our binocular telescopes) feature heavy-duty rubber armor.
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You seem to have several different pricing levels in your product line – are the increased prices indicative of higher quality?
You do get what you pay for. That being said, we have designed our binocular line around three different tiers of quality, to cater to varying customer needs. The first tier is the entry-level, affordable
Gemini LW binocular series. These units are intended for the more cost-conscious binocular buyer, or for someone who wants to experiment with binocular astronomy without making a major investment. The next
tier up is our Gemini WP series. These binoculars are intended for a more serious observer who wants a glass that will last a very long time, but isn’t yet ready to invest in a top-of-the-line astro
binocular. Our top tier is represented by the Signature Series binoculars and binocular telescopes – these instruments have the potential to last a lifetime, and approach the performance of the best out
there. Your personal budget and performance desires will determine which tier is right for you. Of course, some of our binoculars don’t fit in any of the tiers – the Classic series falls somewhere in between
the Gemini LW and Gemini WP, for instance. Also, the inexpensive Gemini LW series have features, such as light weight and portability, that may recommend them even to observers capable of affording much more
expensive instruments. Click here for a complete summary of our product lines.
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What are the major differences between your "tiers" of binocualars?
In general, the Gemini WP series have comparable or only slightly better optics than the entry-level Gemini LW binoculars. However, the Gemini WP do employ much sturdier all-metal construction. The large-aperture
Signature Series have similar center resolution to the Gemini WP binoculars, but they do possess superior coatings, build quality, and edge correction.
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I notice that you carry some binoculars that use “triplet” objective lenses. Are triplet lenses better than doublet lenses?
Not necessarily – the additional element does give the optical designer an extra degree of freedom in correcting for field curvature, but producing a glass with an extra objective lens element usually won’t
produce as much a benefit as adding another element to the eyepiece. This is why the Signature 22x85 can have better edge correction than the Gemini 20x80 TWP. Contrary to popular belief, adding another
element does not improve the color correction of the objective in an achromatic optical system. To summarize, triplet objectives a preferable if all the other factors are equal, but given the choice between
a triplet binocular with a 4-element eyepiece and a doublet binocular utilizing a 5-element eyepiece, the latter would usually be preferrable.
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Why does my binocular show distortion near the edge of the field of view, and does this mean that the binocular is defective?
Most binoculars utilize extremely fast optical systems with focal ratios of f/4 to f/4.5 or even less. Even the best eyepieces will show some field curvature at these focal ratios, and the standard binocular
eyepiece is usually a simple, 4-element wide-angle design. This means that the center of the field of view often comes to best focus at a slightly different focus setting than the edges of the field of view.
This produces some flaring and distortion of star images near the edge. In practice, this is rarely a problem since the observer’s eye is naturally drawn to the center of the field of view, and it is natural
to move the binocular until the object being observed is in the center of the field of view – this is a completely different method of observing than is used with most telescopes. All binoculars experience
distortion near the edge of the field to some extent, but it is usually better suppressed in more expensive binoculars that utilize complicated eyepiece configurations.
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What can I see in the night sky with binoculars?
Virtually all the brighter deep space objects (open and globular star clusters, galaxies, and nebulae) and most solar system objects. Where binoculars really excel, though, is the showcase deep space objects
– they frame these objects in a way that few telescopes ever can for a true wide-field stereo viewing experience.
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Can I use my binoculars to observe the moon and planets?
Binoculars can offer outstanding low-power views of the moon, creating an impressive "three-dimensional" effect that isn't possible with the single eyepiece of a telescope. However, in larger apertures the moon's brightness can become unbearable, and for this reason many of our larger binoculars are threaded to accept standard netural density moon filters. Large aperture binoculars utilize extremely fast optical systems, so you can expect some minor flaring and chromatic aberration on extremely bright objects such as Venus and Jupiter (see the FAQ on “false color” below). Keep in mind that fixed-power astronomy binoculars were designed primarily for observing dim deep space objects, and their magnification is too low to see much planetary detail. If you are interested in primarily planetary observation and still want to enjoy the benefits of binocular observing, you’d be better off with one of our Binocular Telescopes. For casual observing, though, conventional binoculars of 15x-30x magnification can provide nice quick looks at the planets, as long as you recognize their limitations and don’t expect them to perform like an expensive telescope. The amount of planetary detail you will be able to see is dependent on your visual acuity and the magnification of your binoculars. For example, for most individuals the ring of Saturn becomes clearly separated from the disk at around 20x in moments of good seeing, but for some it may take 25x magnification, while others will be able to see clear separation at 15x.
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What is “false color”, and is it a problem?
The term false color refers to the spurious color fringing seen when observing bright stellar and planetary objects with an achromatic optical system. Essentially, a conventional achromatic objective lens
design, such as is used in most binoculars, is unable to bring all the colors of the spectrum to focus at exactly the same point. This means that there will be some purple fringing around the brightest objects. Generally, chromatic abberation only becomes noticeable on the brightest stellar and solar-system targets. Usually, it is more pronounced the larger the binocular and the higher its magnification. Chromatic abberation is not usually an issue on deep space objects. Our Binocular Telescopes and some of our Signature Series binoculars have particularly good on-axis suppression of chromatic abberation.
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Should I expect large-aperture binoculars to offer the same image quality as my premium telescope?
Probably not, and certainly not if your telescope is an expensive apochromat. First of all, binoculars operate at much larger exit pupils than are typically used with most telescopes, which means that the natural abberations in your eyes will become more noticeable on bright objects. Also, let’s consider the economics involved in producing a large-aperture binocular - for a typical, $370 100mm binocular, you are getting (2) F/4.5 doublet
achromats, (2) prism clusters, and (2) wide-angle eyepieces, plus premium multi-coatings and all the mechanicals required to bring it all together and maintain collimation. We’re not aware of any $185 100mm
F/4.5 achromat packages than come with a high-quality diagonal, collimateable lens cell, and a wide field ocular, but if there were such a package, the performance expectations of any refractor aficionado
would be pretty low. The point is that the performance per dollar being offered by most imported binoculars is amazing when you consider the price in telescope terms - that's why large-aperture binoculars
are enjoying a surge in popularity – but this does not mean that you should expect the same level of performance on bright stars and planets as you would from a more expensive telescope. If you are looking for telescope-level performance with all the advantages of a conventional binocular, consider purchasing one of our Binocular Telescopes. That being said, when binoculars are used for deep space observation, their large aperture and wide field of view can offer an observing experience superior to that offered by almost any telescope.
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I hear a lot about different types of coatings in binoculars – is this just marketing hype, or are there really any significant differences between coating types?
Coatings are one of the most important factors in determining image brightness. Many budget binoculars have several uncoated surfaces – this is unacceptable, since each uncoated glass surface reflects about
4-5% of incoming light. In the worst binoculars, only about half of the light entering the instrument actually makes it to your eyes. The result is a dim, washed-out image and a poor observing experience.
Such binoculars are normally advertised as having “Coated Optics”, and are unacceptable for all but the least demanding uses. The simplest form of coating is a single thin layer of Magnesium Fluoride.
This is a good coating, but Magnesium Fluoride coatings lose about 1.5% of light per surface. That may not sound like much, but keep in mind that most binoculars have 14 to 20 optical surfaces that the light
must pass through before reaching your eye. That can translate to some serious light loss - in excess of 25% - for a magnesium fluoride-coated binocular. Binoculars with magnesium fluoride coatings on every
surface are called “Fully Coated” binoculars, and are acceptable for some undemanding uses, but generally not for astronomy. The next coating level is binoculars with “Multi-Coated” optics –
this generally means that at least the inside surface of the objective lenses and one or more surfaces of the eyepiece lenses feature coatings made of multiple very thin layers (hence the term
“multi-coated”). A properly applied multi-coating can appear any color but will normally look greenish-purple, and will transmit between 99% and 99.7% of the light passing through each surface. What this
boils down to is that, depending on the number of multi-coated surfaces, a “multi-coated” binocular can have various levels of improvement in light transmission and contrast over a fully-coated binocular. If
only a few surfaces are multi-coated, the improvement will be negligible, but if the objectives and eyepieces are multi-coated and only the prisms retain magnesium fluoride coatings, the improvement can be
significant. Unfortunately, there are no hard-and-fast rules for determining what constitutes a “multi-coated” binocular, so it’s possible for a manufacturer to sell a binocular that has multi-coatings on
only one or two surface, and no coatings on all the others. In this case, you’d be better off with a standard “fully coated” binocular. Beware of inexpensive binoculars where the multi-coating is too thickly
applied, as this can actually hurt performance over a Magnesium Fluoride-coated binocular. Also avoid so-called “Ruby-coated” binoculars like the plague. The best binoculars utilize multi-coatings on every
air-to-glass surface (including the long faces of the prisms) and are called “Fully Multi-Coated” or “FMC” binoculars. This type of optical coating can transmit 85-90% of the light entering the
binocular – which translates to significantly more light reaching your eye than with the lesser coating methods. Watch out for some manufacturers who claim “Fully Multi-Coated optics” but actually use
inferior Magnesium Fluoride coatings on the prisms – this is not a true FMC binocular. By there very nature, multi-coatings transmit light better over a larger area of the spectrum for better color fidelity.
All Garrett Optical binoculars utilize Fully Broadband Multi-Coated optics, which we feel are the best available in their price range.
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What about “phase-coatings”?
There is another type of coating process used on some binoculars, called “phase-coating.” Because of their design, roof prism binoculars actually split incoming light into two separate beams as it passes
through their prisms. Unfortunately, after they are split, the two paths of light become “out of step” with each other. This in turn affects image quality. Phase coatings are applied to the roof prisms and
are designed to re-merge the two paths of light for maximum performance. Because of their sophistication, phase coatings are normally found only on relatively expensive roof prism binoculars. Phase coatings
are not applied to porro prism binoculars, because the porro prism design does not suffer from the same deficiency as roof prism binoculars.
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What is “exit pupil”, and why is it important?
Though the objective lens diameter determines how much light will be gathered by a given binocular, it only partially determines how bright an image you will see. Another factor besides aperture is in play
to produce a bright image: it’s called exit pupil. In general, a larger exit pupil equals a brighter image, providing you do not exceed your maximum dilated pupil size. An example is that if you choose a
binocular with a 7mm exit pupil but your eyes only dilate to 5mm, you are effectively wasting a good percentage of the aperture of your binocular. In order to achieve maximum binocular performance, exit
pupil size should be carefully considered and matched to your pupil size and observing conditions. Exit pupil is one of the most neglected and also the most important binocular specifications, and in most
cases it is easily determined by dividing the aperture by the magnification. In other words, a “10x50” binocular would have a 50/10=5mm exit pupil. As it turns out, a roughly 5mm exit pupil is a good size
for most people to choose. There are several reasons for this: one is that our pupils shrink in size as we age. Most people start out as children with a pupil size of at least 7mm, and that gets
progressively smaller as they age. Many 50-year olds have a pupil size of about 5mm when fully dilated, though there is certainly a lot of variance from individual to individual. Keep in mind that your pupil
will not dilate fully except in extremely dark conditions, so you will rarely get to take advantage of your maximum pupil size. In bright sunlight, most people experience a pupil size of around 2.5mm to
3.0mm. Because of this, large, 7mm exit pupil sizes have very specific applications. These include marine and wildlife observation at dusk, when maximum brightness is required. When making observations from
a rolling or pitching boat, a larger exit pupil can be preferable because it makes it easier to keep the binocular centered on your pupils. For astronomy, larger exit pupils are desirable because they can
provide brighter views of extended astronomical objects, such as nebulae and galaxies. However, when observing from light-polluted conditions, that same large exit pupil will maximize the brightness of a
light-polluted background sky. The result is all too often that the bright background sky washes out any gain in brightness due to the larger exit pupil. For this reason, a slightly smaller exit pupil of
about 5mm often makes more sense for amateur astronomers observing from light-polluted locations. To determine your dilated pupil size, either ask your ophthalmologist or optometrist to measure it for you or
try this simple test proposed by astronomer Ed Zarenski: standing in a dimly-lighted bathroom, position your face a few inches from a mirror. Using a dial caliper or, if a caliper is not available, a metric
ruler, estimate the size of your pupil reflection in the mirror.
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Thanks for taking the time to glance over our FAQ! If you have a question and it’s not answered here, please feel free to drop us an email. We will respond
promptly and answer your question as best we can.
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