Greeting! In this article you will know the binocular terminology in detail. We will try to describe the meaning of the number written over the binocular. Let’s be straight forward. The number pattern written over the binocular is like 10×25 where the 10x represents the power of magnification and the number 25 means the diameter of the objective lens. That’s pretty easy explanation.
Now if you want to know the detail all the numbers and meaning of the terms of binocular then you can go through the article.
What to look for when buying a binocular? Know all the terminology
If you want to know very details then you have to know the basic terms / terminology mentioned as follows.
- Adjustable eyepiece
- Aperture
- Armoring of binoculars
- Bak-4
- BK-7
- Basic design of binoculars
- Brightness of image in dim light
- Central focusing mechanism
- Central focusing wheel
- Central focusing knob
- Central focusing toggle
- Close focus
- Coating of lenses
- Collimation
- Depth of field
- Diopter
- Exit pupil
- Eye relief
- Eyepiece adjustment
- Field of view (FOV)
- Focusing mechanism
- Focus speed
- Fogproof
- Image resolution in dim light
- Interpupillary distance (IPD)
- Lens coating
- Magnification
- Objective lens
- Optical features of a binocular
- Porro prism binoculars
- Power of a binocular
- Prism design
- Prism glass
- Relative brightness index (RBI)
- Roof prism binoculars
- Speed of focus
- Twilight factor
- Vignetting
Diopter
One of the two eyepieces of a pair of binoculars is fixed and one is adjustable (usually the right one), called the diopter. By adjusting the diopter, the difference between your eyes can be accommodated. The way to do this, is as follows: Close your right eye (or put the lid on the objective lens of that side) and focus on an object some 10 meters away with your left eye, adjusting the central focusing mechanism. Now do the same with the left eye and focus on the same object, but now adjusting the diopter. Do not touch the central focusing mechanism at all while doing this. Now the binocular is calibrated for your eyes and from here only the central focus wheel/toggle has to be adjusted when focusing on an object. Some binoculars have locking mechanisms to prevent the diopter setting from being turned by accident once it had been set.
Some binoculars have an adjustable diopter for each eye – not only for the right eye. Invariably these binoculars are of the “auto focus” binoculars type. Their focus has been set at the factory and the viewer does not need to adjust the focus at all; the object is always in focus from about 20 feet to infinity. All the viewer has to do, is to calibrate the binoculars for his personal use by adjusting each diopter and locking in the settings right at the beginning.
Objective lens
Objective lens (also known as the “aperture” of the binoculars): A set of technical detail imprinted on any pair of binoculars could be something like this: 8×42, or 10×50 or 10×25. The second number (42, 50, 25) is the diameter of the objective lens (in millimeters), i.e. the two big lenses at the front of the two barrels. The bigger the objective lens, the more light is captured, resulting in a clearer and brighter image, which is important when used in fading light or when focusing on something in the shade of a tree. However, bigger lenses also means more weight, so this has to be kept in mind when considering to buy a pair.
Armoring of binoculars
Some manufacturers cover their instruments as far as possible in protective rubber or synthetic housing. These binoculars are known as “armored binoculars”. This protects the instrument from scratches and – very important – from bumps, which could harm the inner workings of the binocular. However, let there be no misunderstanding: No binocular can ever be “shockproof”, even though some manufacturers may claim so.
Prism glass
Type of glass the prisms are made of: Bak-4 is a high density glass, of better optical quality than BK-7 and by far the best for making binocular prisms. Fortunately nowadays even cheap binoculars have Bak-4 prisms, whereas that would not have been the case a decade or two ago. Really cheap binoculars all have BK-7 glass prisms.
Basic design of binoculars
The first thing that strikes you about binoculars, is the apparent difference in two basic types of designs. All binoculars either use “porro prisms” or “roof prisms”. The binoculars using porro prisms are the conventional design where eye pieces are off center to the barrels. In the case of the ones using roof prisms the eye pieces are in line with the objective lenses and have a sleek design. The advantages of porro prism binoculars are cost (less expensive) and the greater three-dimensional viewing effect, whereas roof prism binoculars are more compact and lighter, but are usually more costly due to the more complex manufacturing process.
Focusing mechanism
The focusing mechanism is located at the top between the two barrels of the binocular and can be either a wheel or a toggle. This mechanism is used to adapt the focus when viewing an object. Some wheels/toggles/knobs adjust easily and some not that easily. Some binoculars, known as “auto focus binoculars”, “self focusing binoculars”, “focus free binoculars” or “fixed focus binoculars” have no central focusing knob/wheel; in their case the focus has been fixed at the factory.
Coating of lenses
Not all the light arriving at the objective lenses at the front actually passes through – some is reflected. This happens with the light finally arriving at the viewer’s eyes at the other end of the binoculars as well – not all the light travels through the final two lenses either. On top of this the other 10+ glass surfaces inside the instrument contributes to scattering of light as well. Anti-reflective optical coatings reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast.
These coating differ substantially in quality: “Coated” simply means a single layer anti-reflection coating on the two objective lenses; “Fully coated” means that all air-to-glass surfaces are coated, whereas the ideal coating, “fully multi-coated” means all air-to-glass surfaces have multiple layers of coats.
In the case of roof prism binoculars the lenses also have to be “phase-corrected” (PC) to ensure that the loss of image brightness and contrast is corrected.
Depth of field
The “depth of field” of a binocular refers to its ability to keep objects further towards the background and closer towards the foreground in focus without you having to adjust the focus. Stated in another way: Once you’ve focused on an object in the distance, how far behind this object (in other words, further away into the background) and how far in front of the object (in other words, closer to you, the viewer) is still in focus, without you having to adjust the focus. A special category of binoculars with super depth of field features is the so-called “auto focus” binoculars. They always stay in focus from 10 meter away to infinity.
Exit pupil
The exit pupil can be observed by holding the binoculars at arm’s length and looking throughout the eyepieces. The pencil shaped light you see is called the exit pupil. The diameter of the exit pupil defines how much light is transferred to your eye. The actual diameter of the exit pupil is computed by dividing the diameter of the front objective lens (in mm) by the magnification of the binocular. So, in the case of a 7×50 binoculars, the diameter of the exit pupil is 7.1. The diameter of the exit pupil is important in fading light. If it’s less than 7 mm, your binoculars will not allow enough light through to your eyes (in fading light, that is), unless you’re an elderly person, in which case 5 mm. will be enough.
Eye relief
For every pair of binoculars, there is an optimal distance between your eye and the eyepiece, which is called eye relief. If your eyes are too close to or too far from the eyepieces, you can’t see the whole picture: part of it is blacked out. This is called “vignetting“.You do not want to be as close to an elephant as I was (4/10/2010, Lower Sabie, Kruger National Park, South Africa) and not have the full field of view available!! If your eyeglasses prevent you from putting your eyes close enough and hit the “sweet spot” of the binocular, the picture shows what happens to your field of view So, if you happen to wear eyeglasses, you have to make sure the eye relief is at least 15 mm. Except in the case of very expensive binoculars, field of view and eye relief work against one another. If the manufacturer increases the one, it is at the cost of the other. If you do not wear glasses, you do not need good eye relief. If you do wear them, you have to be content with a compromise – unless you’re willing to dig deep into your pockets.
Eyepiece adjustment
The first thing everybody does when picking up a pair of binoculars, is to adjust the two barrels of the binoculars to fit the distance between his eyes. If you do not do this, the binocular is useless. The two barrels of the binoculars have to be adjusted to fit the distances between the pupils of your eyes, called interpupillary distance (IPD). These have to be adjusted until you have one single circular field of view.
Field of view
Another set of technical detail which is imprinted on the binoculars, could be something like this: 367 ft. at 1000 yds. This is the field of view (FOV), the width of the view at the particular distance. In this case 367 feet at 1000 yards. Could be designated in meters or in degree. Field-of-view is determined by magnification and eyepiece lenses. It is not true that binoculars with small objective lenses like pocket binoculars will inevitably have a narrow FOV. This can be addressed by the way the optics inside the eyepieces have been designed. Example: The Swarovski Optiks Pocket Binoculars 8×20 are very small pocket binoculars (dimensions: 3.98 x 1.5 x 2.3 inches) and have an aperture of only 20 mm and power of 8, but an excellent FOV of 345 feet @ 1,000 yards. Compare this to the FOV of 330 feet of the full-size NIKON 7294 Monarch ATB 8×42 Binocular, with 8×42 specifications.
FOV can also be expressed as degrees, e.g. 5.6 degrees, or 6.7 degrees.
Finally: FOV is handy, but not crucial, except in the case of young children, who still struggle to find and follow moving objects with a binocular.
Focusing mechanism
The focusing mechanism is located at the top between the two barrels of the binocular and can be either a wheel or a toggle. This mechanism is used to adapt the focus when viewing an object. Some wheels/toggles/knobs adjust easily and some not that easily. Some binoculars, known as “auto focus binoculars”, “self focusing binoculars”, “focus free binoculars” or “fixed focus binoculars” have no central focusing knob/wheel; in their case the focus has been fixed at the factory.
Focus speed
If you need the binoculars for watching wild animals, in particular birds, the speed of focus is important. While following a bird in flight, you need to change the focus all the time if the bird gets closer or further, and if your binoculars do not allow for quick adjustment, you will struggle all the way.
Obviously, this problem does not exist in the case of “auto focus” binoculars: They stay in focus all the time, from infinity to about 10 meters away. The bird can fly as fast as he can, straight at you, and it will stay in focus. Very convenient.
Weatherproofing
“Weatherproofing” includes both “waterproof” and “fogproof”. “Waterproof“ speaks for itself. The manufacturers have to make sure that no water/dampness enters the instrument at all, usually making use of “O”-rings. Sometimes the degree of “waterproof” is indicated by JIS-numbers (1-8), the higher numbers indicating the depth of immersion in water (e.g. “Waterproof up to 1 meter emersion for 1 minute”). If the instrument is not properly waterproofed, dampness could find their way into the instrument in cold, damp conditions. As the temperature drops sharply, the air inside the instrument is condensed, causing a lower than atmosphere pressure inside the instrument. Damp air from the outside is then drawn into the instrument, which condensates on the cold lens surfaces, rendering the binoculars useless. In the long run this eventually leads to the forming of fungi on the lenses as well. Waterproofing will also prevent dust and dirt from finding their way into the instrument.
“Fogproof”: The only useful way preventing the device from fogging up inside in cold conditions, is to have the instrument “nitrogen-filled”. This way even the slightest bit of fog is removed from the instrument, making sure that no condensation of fog on lens surfaces is possible at all. You will get some modern binoculars who have waterproof, fogproof and image stabilization features. You have to consider those issues when choosing your item.
Image brightness in dim light
Three factors determine the brightness of the image in dim light: The quality of optical coatings, a factor determined by the size of the exit pupil and finally a factor determined by the objective lens size and magnification.
Lens coatings: Not all the light arriving at the objective lenses at the front actually passes through – some is reflected. This happens with the light finally arriving at the viewer’s eyes at the other end of the binoculars as well – not all the light travels through the final two lenses either. On top of this the other 10+ glass surfaces inside the instrument contributes to scattering of light as well. This is where optical coatings come into their own: They reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast. In this way lens coatings make a huge contribution towards a better image, in particular in dim light.
Size of exit pupil: The size of the final lens through which the image has to go is crucial, in particular as compared to the size of the human pupil. The smaller the exit pupil, the less light passes through. In bright daylight this is no problem at all, but in dim light, when the human eye needs as much light as possible to see an image clearly, the situation changes altogether. The ability of a binocular to transmit light in terms of the exit pupil is expressed as its performance on the relative brightness index (RBI). It is computed by squaring the exit pupil. For example, 7×35 binoculars have a 5mm exit pupil (35/7=5). So their RBI is 25 (5×5=25). A RBI of 25 or greater is considered good for use in dim light.
Objective lens size and magnification: Some manufacturers claim that a combination of the magnification and objective lens size makes a greater contribution towards image brightness in dim light. A factor, known as the twilight factor, is calculated by taking the square root of the product of the aperture and magnification (aperture x magnification). According to this approach a binocular with 10×40 specifications (twilight factor: 20) will render a clearer image than a 7×35 (twilight factor: 15.4), even though it has a smaller exit pupil (4 mm) than the 7×35 (5 mm).
A final remark on brightness of image in dim light: The bigger the objective lenses, the more light is gathered and the better the chances of getting a brighter image. For this reason binoculars used for star gazing have really big objective lenses. For the same reason pocket binoculars with their small apertures will not be as effective in dim light as normal size binoculars.
Magnification / power
Magnification/power: A set of technical
detail imprinted on any pair of binoculars could be something like this: 8×42,
or 10×50 or 10×50.
The first number (8, 10, 12) is the magnification number, the “power” of the
binoculars. The object is magnified by 8, 10 or 12 times, which means that it
appears to be 8,10 or 12 times closer than seen with the naked eye. So, the
higher the magnification, the more detail the viewer is supposed to get.
However, there’s more to binoculars than simply looking for the more powerful
instrument, as will become clear down below.
Optical features of a binocular
All binoculars have the same set of optical features, whether small, normal size or big. These include lens coatings, close focus ability, field of view (FOV), focus speed, depth of view and ability to produce a bright image in dim light. These will be discussed below.
Basic design of binoculars
The first thing that strikes you about binoculars, is the apparent difference in two basic types of designs. All binoculars either use “porro prisms” or “roof prisms”. The ones using porro prisms are the traditional design where the eye pieces are off center to the barrels. In the case of the ones using roof prisms the eye pieces are in line with the objective lenses and have a sleek design. The advantages of porro prism binoculars are cost (less expensive) and the greater three-dimensional viewing effect, whereas roof prism binoculars are more compact and lighter, but are usually more expensive due to the more complicated manufacturing process.
Prism glass
Type of glass the prisms are made of: Bak-4 is a high density glass, of better optical quality than BK-7 and by far the best for making binocular prisms. Fortunately nowadays even cheap binoculars have Bak-4 prisms, whereas that would not have been the case a decade or two ago. Really cheap binoculars all have BK-7 glass prisms.