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Why choose Binoculars?
Well this could be quite a lengthy answer, but we will try to keep it short.
Binoculars are one of the handiest and most widely used of all optical instruments. Virtually anyone who spends time outdoors owns, (or should own), a pair of binoculars. They are almost a necessity for the astronomer, hunter, fisherman, boater, sports fan, and experienced traveler.
A binocular is a very useful optical aid that can be used to enhance and bring out detail in distant objects, be it birds, nature, sports or astronomy.
Amongst many things binoculars are small, light, portable, efficient, quite robust and relatively inexpensive.
Binoculars will generally provide you with unsurpassed detailed wide fields of view that cannot be matched by either the unaided eye or spotting scope/telescope. This is because binoculars capture and present the image to both eyes, this can show up to 40% more detail than using a single eye on its own.
The wide field of view makes binoculars not only ideal for capturing moving objects such as at sports events or birds in flight, but when combined with the light gathering power and magnification you can see that they are also ideal for nature studies, astronomy, surveillance, boating…..and the list goes on.
What do those numbers mean? 7x50, 10x50?
This describes the power versus the diameter of the front lens. Technically, for example, we can say that a 10x50 has a magnification of 10 times (10x) and an objective lens aperture of 50mm. The front lens is called the objective lens and the diameter is known as the aperture.
A binocular that has 10x magnification (power) means that an object will appear ten times closer than it actually is. For example a bird 100m away when viewed through a 10x binocular will appear as if it was only 10m away.
There are practical limitations to the amount of magnification that can be used. More power is not always better. A balance of power, aperture and field of view needs to be found. Generally though 10x magnification for a 50mm hand held binocular is about as powerful as is recommended, otherwise the view may start to become shaky.
A binoculars' aperture is one of the most significant factors that determines how bright the view through the binoculars will be, this is known as the light gathering power.
We can calculate the comparative light gathering power of same magnification binoculars by squaring the diameter of the objective lenses. For example a 7x50 has twice as much light gathering power as a 7x35. (Calculation) 50 squared =2500 compared to 35 squared =1225.
Resolution (the ability of a binocular to show you small details) is also governed by lens diameter and is directly proportional to the diameter. In other words the bigger the lens, the better the detail.
From this we can see that bigger lenses provide brighter views with more detail. Again there are practical considerations in the size of the binocular you choose versus the job it will be used for. For example you would not choose a large 11x70 binocular to take with you on a day hike simply because it is powerful and gathers a lot of light. You would be more inclined to choose something like a 10x42. Likewise you would not choose a 7x35 if you wanted to view the night sky, a 60mm (12x60) or 70mm (11x70 or 15x70) binocular would be a good choice.
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Exit Pupil
A binoculars' exit pupil is the circle of light you see in the eyepiece when you hold up the binocular at arm's length and look at a light source. In general, the bigger the exit pupil, the brighter the image.
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To calculate a binoculars' exit pupil size, you divide the diameter of its objective lens (in mm) by its magnification. A 10 x 50 binocular has an exit pupil of 5mm (50mm / 10 = 5).
For efficient use of a binoculars' light-gathering ability, match its exit pupil diameter to your eye's largest dilated pupil diameter. A binocular is brightest when the pupil of your eye is the same size as the binocular exit pupil, this is when all of the binoculars' light is captured by your eyes.The eyes pupil diameter can vary considerably with age and light conditions. A young persons eye can dilate to about 7mm in very low light to less than 4mm in daylight.
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On average, you lose about 1/2mm of maximum dilated pupil size per decade between the ages of 20 and 50, declining to about a 1/4mm loss per decade thereafter - from 7mm or so in your teens, to perhaps 5.5mm by the time you reach 40, to 5mm at 50, etc. Your pupil size will depend on heredity and your general health.
Does the diameter of the exit pupil matter so much? It doesn't as long as there is enough ambient light so that the pupils of your eyes are smaller than the exit pupils of your binoculars.
For general purpose use a good compromise or balance can be found for most users. An exit pupil diameter of between 4mm to about 6mm will provide a good balance for use under most conditions.
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Relative Brightness or (RBI Index)
Some manufacturers use Relative Brightness (RBI) as a measure of image brightness. It is computed by squaring the exit pupil. For example, 7x35 binoculars have a 5mm exit pupil (35/7=5). So their RBI is 25 (5x5=25).
A RBI of 25 or greater is considered good for use in dim light.
RBI is one of these measurements that is now largely redundant especially when the brightness of images seen is influenced more greatly by parameters such as the diameter of the objective lens. RBI can confuse unwary buyers into thinking that a smaller binocular appears to have better brightness specification than a larger binocular, but from what we have learned above we know this is not the case.
This RBI only has some relevance when comparing binoculars of the same aperture.
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Relative Light Efficiency (RLE)
Another figure used by some manufacturers is "relative light efficiency." This takes into account the varying light transmissions of coatings used on the lenses. Again this is another figure that should not overly influence your choice of binocular.
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Twilight Factor
Exit pupil, relative brightness and RLE comparisons are sometimes a useful guide, but they're not the best judges of how well a binocular will perform in low light conditions. As an example, 8 x 32mm and 20 x 80mm binoculars both have 4mm exit pupils which calculates to a relative brightness index of 16, but the 80mm binoculars will perform much better in low light conditions because they have a 625% larger light-gathering capacity and a higher "twilight factor."
The twilight factor is a much more useful specification to judge a binoculars' low light performance than its exit pupil size, etc., as it takes into account both light gathering and magnification.
The twilight factor allows you to compare different combinations of aperture and magnification to determine the one that best balances an increase in power against a decrease in brightness (or vice versa). The larger the twilight factor, the better a binocular is in low light.
Twilight factors of 17 and higher are best for twilight or night sky use.
To calculate a binoculars' twilight factor you simply multiply its objective lens diameter by its magnification and then finding the square root of that product. For example, an 8 x 32mm binocular has a twilight factor of 16, while a 20 x 80 has a twilight factor of 40. This explains and confirms the better low light performance of the 20 x 80 even though they have the same exit pupil diameters and relative brightness.
The twilight factor is a mathematical relationship and it does not take into account light transmission differences and overall quality of the binocular, all of which can have a large influence on the cost of the binoculars. In other words don't expect the same performance from a cheap binocular as you would from a more expensive binocular.
But, keeping these cautions in mind, the twilight factor still remains a more reliable guide to low light performance than exit pupil or relative brightness.
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Contrast
Refers to the degree to which both dim and bright objects in the binocular field of view can be differentiated from each other and from the background of the image. High contrast helps in observing fainter objects and in discerning subtle visual details. High quality optical coatings provide better contrast in an image. The other factors affecting contrast are: collimation, air turbulence, the quality of the objective lenses, prisms and oculars.
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Minimum Focus Distance or Near Focus
Another important factor is the minimum focus distance (How close you can get to an object and still see a sharp image of it in your binocular). This near focus capability is important for watching butterflies, the critical identification of birds in the field, or observing the intricate detail of insects or plants. A close focus distance of under 5m is ideal for many birding situations.
The actual close focus distance is dependant to some extent on your own eye characteristics, however the specification is made assuming 20/20 vision.
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Depth of Field
This is simply the distance in front of and behind the point of sharpest focus that an image still remains usefully focused in binoculars. A good depth of field minimizes the constant refocusing needed to keep objects sharp as they move about in the field of view. The higher a binoculars magnification, and the closer it is focused and the shallower its depth of field will become.
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Field of View (FOV)
The field of view is a measure of how wide a piece of the landscape you see through a binocular. It can be expressed either in degrees or in the linear diameter (metres) of the circular image visible in the binocular at a reference distance, usually 1000 metres.
Each degree is equal to roughly 17.5m at a distance of 1000 metres. For example a 10x42 binocular with a FOV of 6.1degrees will have a FOV of 107m at 1000m. This means that at a distance of 1000m your are seeing a span of 107m across the diameter of the view. To calculate using mathematics you find the tangent (tan) of the angle in degrees and multiply by 1000. Example : tangent of 6.1 = 0.107, now you multiply 0.107 by 1000. Presto there is your 107m at 1000m.
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A wide field of view makes it easier to quickly find a target as there is more area covered in your view and once located is easier to keep in the field. Binoculars with wider fields of view also present an image that is less prone to shake. Wide fields of view are sometimes achieved at the expense of clarity at the edge of the field, so don't go overboard on width of field when choosing a binocular unless you know it's sharp from edge to edge.
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Eye Relief
This is the minimum distance between a binoculars' eyepiece and your eye that allows you to see the entire field of view. Long eye relief is important if wear eyeglasses to observe, as your glasses can keep you from getting close enough to the eyepiece to see the full field.
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A comparison can be to imagine that the binoculars' eyepiece is a hole in a fence and you're watching a football game through the hole. If you get your eye close to the hole (or binocular), you see the complete football field. However, if you move your eye back from the hole, as eyeglasses effectively force you to do, you only see the centre of the field.
If you are near or farsighted, you can usually observe with your glasses off, as binoculars usually have enough focuser travel to accommodate for these eye conditions. Severe astigmatism or severe nearsightedness, for which glasses must be worn, requires a binocular with long to very long eye relief (15mm to 19mm or even longer), to allow you to wear your glasses and hopefully still see all of the field of view.
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A binocular with medium eye relief (10mm to 15mm) can be used with eyeglasses, but you'll lose some of the field of view. Excessively short eye relief can prevent your eyes from blending the two circular binocular images into one, giving the effect of looking down two tubes instead of through a window. Therefore, a binocular with eye relief below 8mm or 10mm is not recommended for those observers who must wear glasses.
Real life eye relief figures are often shorter than a manufacturer's specified figure. The manufacturers eye relief figures are measured from the eyepiece lens (which is usually recessed into the binocular body to protect it from being scratched). In worst case situations this can make a difference of up to a couple of mm to the total eye relief value.
Long eye relief binoculars are generally more comfortable to use in any case, even if you don't wear glasses - as short eye relief can allow your eyelashes to brush the binocular eyepieces annoyingly. This also deposits eyelash oils on the eyepieces that can damage their coatings if not cleaned off regularly.
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Inter pupillary Distance
This is the distance between the pupils of your eyes, measured from centre to centre. It is also the distance between a binoculars' exit pupils. If a binocular can't fold down or open up enough for its exit pupils to line up with the pupils of your eyes, a shadow will cut off part of the image you see in one eyepiece or the other.
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If you suspect that your eyes are more closely or widely spaced than average, make sure that your inter pupillary distance falls within the range given for the binocular in which you're interested. If you don't know your inter pupillary distance, look through a binocular that you know works for you and measure the distance between the centres of its exit pupils when it's set for your eye spacing.
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Binocular designations, what those words and letters mean
Most manufacturers describe their binoculars in their model designations. For example, "10 x 50 ZACF" binoculars are 10 power, 50mm objective lens, Armoured, Porro prism design with centre focus
The letter abbreviations are found below:
A or GA: Rubber armoured.
B: Porro prism binocular with a Bausch & Lomb-style one-piece body.
C: Compact roof prism binocular.
CF: Centre focus.
D: Roof prism binocular (abbreviation of "dach," the German word for "roof").
FP - This denotes that the binoculars are Fog Proof
FMC - Indicates that the lenses are fully multicoated. Coating a lens surface helps to reduce the amount of light that is reflected back or dissipated from the lens, thus ensuring bright images. Multi-coating further reduces the loss of brightness - providing the sharpest, clearest images.
L- This means that they have Long Eye Relief
H: H-body roof prism binocular.
IF: Individual focusing eyepieces.
M: Designates the binocular is a compact design with the objective lenses closer together than the eyepieces.
P: Phase-corrected prism coatings.
W, WA,or WW: Designates wide angle binoculars.
WP:This means that the binoculars are Weather Proof or Water Proof. You will need to look at the full specification of the model you are looking at to check which it is.
X: Designates the binocular magnification is high relative to the objective lens diameter.
Z: Designates Porro prism binocular with Zeiss-style two-piece Z-body.
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Binocular Coatings
Lens coatings play a very important role in the overall performance of binoculars. Each time light enters or leaves a glass element about 5% of the light is lost due to reflections. Binoculars may have up to 16 air-to-glass surfaces, and you will have light loss at every surface.
In the early days before optical coatings were available, less than half the light would actually reach your eyes, the rest of the light scattered around inside the binoculars causing even more unsatisfactory images. Larger objective lenses were used to compensate for this light loss, however the size and weight increased to almost non-practical levels.
In the 1930s it was discovered that when glass was coated with magnesium fluoride more light would be transmitted. This single layer coating technology was found to reduce light loss to about 1.5% per optical surface. In toady's Broadband Fully Multicoated lenses only about 0.25% or less of light is lost at each surface, and in the best binoculars, mind you they are very expensive, over 95% of the light is actually transmitted to your eyes.
The coating process depends on applying a perfectly uniform layer of coating material only a few nanometer's thick, if its too thick or too thin then it just doesn't work.
With roof prism binoculars another feature to look for is anti-phase-shifting coatings. Binoculars with this coating on the roof surfaces will deliver higher contrast images.
Types of coatings available
Coated Optics: One or more surfaces coated. This usually means a single layer of antireflection coating on some lens elements, usually the first and last, (the lenses you can see).
Fully Coated Optics: All air to glass surfaces coated, but any plastic lenses used may not be coated. Only a single layer of coating is usually applied to the surfaces.
Multi-Coated Optics: One or more surfaces are multi-layer coated. The multi layers are usually applied to the first and last lenses, while the others usually are single layer coated.
Fully Multi-Coated Optics: All air-to-glass surfaces are multi layer coated. This is the highest level of coating generally available and provides the best light transmittance and contrast. This process is the most expensive but worth the extra cost for the user who demands the extra quality.
What about (RC)- Ruby Coating?
Ruby coatings RC are actually intended to reduce glare in bright light. When using expensive good quality ruby coated binoculars the coating actually does what it was designed for, to reduce glare.
Unfortunately the market is completely flooded with cheap RC ruby coated binoculars. These are sold as the latest and greatest in optical fads. At the end of the day, the ruby coatings in many of the cheap binoculars are used to disguise the binoculars poor optical quality.
Ruby coated binoculars should not be used for astronomy as they actually reduce the amount of light that is transmitted to your eyes and also can change the colour of the image, resulting in objects with a green cast.
How do coatings work?
Coatings work by harnessing the wave nature of light in such a way as to cause destructive interference in the light waves reflecting off the surface. Its the same principle as two people attempting to sing a note. If they get it right the overall sound is louder (constructive interference), if they get it wrong then the sound could cancel out altogether (destructive interference).
In a similar manner if you can manipulate the light reflecting off two surfaces in such a way that the reflecting waves cancel each other out, most of the light energy that would have been reflected is forced to pass through. This is the basic idea behind lens coatings. Magnesium fluoride is the most common form of coating because of its ease of use and durability. The key factors for enhancing maximum light transmission is the refractive index of the coating used and the thickness of the coating.
Coated or fully coated lenses (single coating), work best for single wavelengths of light, some wavelengths are affected less, while others are not affected at all. With light being composed of many wavelengths (colour), you can quickly see the need for more broadband coatings that cover more wavelengths. This is why broadband multicoated lenses are used. Multiple coatings of different materials work effectively over a larger band of the colour spectrum.
Naturally for the user who demands the best light transmittance and contrast the broadband fully multicoated lenses will provide them with the best performance.
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