Criterial for choosing binoculars

When we choose a binoculars, usually, these factors should be considered: Buying a binocular must makes tradeoffs. In order to get the brightest possible image in low light conditions you will need low magnification with a large "objective lens". But many of us don't want to sacrifice magnification. After all isn't this why we are buying a binocular? We want to see objects far away as though they were close! You may also want a "wide field of view" so you can see as much action as possible at a sporting event. So, you will find that you need to know how, when and where you are most likely to use your binocular. In the end you may decide to buy several different models to meet the needs of the different types of viewing situations you intend to enjoy.

1.     Magnification (Power)
Magnification (power) means the degree to which the object you are looking at is enlarged, or how much magnification the binocular has. Generally, the higher the magnification, binoculars will be more difficult to have a bright, steady and wide image. The level of power affects the brightness of an image, so the lower the power of a binocular, the brighter the image it delivers will be. Keep in mind; increasing power will reduce both field of view and eye relief.

2.     Objective lens size
Normally, this size indicate the diameter of objective lenses, given in millimeters, objective lens is the lenses at the bottom of the binocular. The objective lens determines the light gathering ability of the instrument, with the greater light gathering ability of a larger lens translating into greater detail and image clarity. This is especially useful in low light conditions and at night.

3.     Prisms types
There are two common types of prism systems, the porro prism and the roof prism, which both serve to invert the image. By design, roof prisms are more light in weight and more compact, and porro prisms are superior optical performance than roof prisms, if the roof prisms coated with an anti-phase shift coating, a phase corrected roof prism binoculars is equal to a porro prism binoculars in optical performance assuming all else is equal.

Usually, there are two basically common glass styles for porro prisms: BK7 and BAK4. The main difference between two styles is the glass density or refractive index, BK7 utilizes borosilicate glass and BAK4 uses barium crown glass. The BAK4 is the fine glass with higher density, which can eliminates internal light scattering and produces sharper images than BK7 glass, the BK7 loses some of the light that strikes near the edges of the prisms. There is a method can determine the type of porro prisms used, you can hold the binoculars away from your eyes at a bright light source to see the cone of light coming through, with BK7 you will note a square shape cutting off part of the cone, whereas with the BAK4 you generally see the complete round cone of light.

4.     Coatings types
The optical elements of the binocular are coated to reduce internal light loss and glare, which in turn ensures even light transmission, resulting in greater image sharpness and contrast. Choosing a binocular with good lens coatings will translate to greater satisfaction with the product you ultimately select.     The various terms below are general descriptions for comparison sake:
Coated Optics (C) ---- means only that one or more surfaces of one of more lenses have received an single layer anti-reflection coating.
Fully Coated (FC) ---- means that all air-to glass surfaces have been coated.
Multi-Coated (MC) ---- means that one or more surfaces of one of more lenses have been coated with multiple films, some surfaces could be single coated or some not coated at all. There are many different types multiple coating, such as ruby coating, green coating and broadband multi-coating, generally, the high transmission broadband multi-coating is best.
Fully Multi-Coated (FMC) ---- means that all air-to-glass surfaces have received multiple films
Lens coatings range in quality as follows: coated -- fully coated -- multicoated -- fully multicoated. Coated lenses are the lowest quality and basically will not result in a product that will satisfy you. Fully coated lenses are quite economical and can work well for you, depending on your needs. Multicoated or fully multicoated lenses are both very good choices. Fully multicoated lenses give the best light transmission and brightest images, and are therefore the most desirable. An uncoated glass surface can lose up to 5% of light transmission due to reflection and scattering. A single layer of anti-reflection coating can reduce loss to about 1.5%. Multiple layers of different anti-reflection coatings can further reduce loss to as low as 0.25%.  

5.     Near focus distance
Near focus is the closest distance to the observed object that the binoculars can be used while retaining a sharp focus.
Serious birders request a near focus of about 15 feet but 20-25 feet is usually acceptable. Some binoculars manufacturers adjust particular binoculars to have a closer near focus but generally the infinity focus is compromised in optical sharpness.
To measure mathematically, find the adjustable range of the diopter and divide the square of the power by the positive figure of the measured range. .

6.     Field of view

The size of the area that can be seen while looking through a pair of binoculars is referred to as the field of view. Field of view can be expressed as an angle, or a distance at 1000 meters or yards . The linear field of view refers to the area that can be observed at 1,000 meters or 1,000 yards, and is expressed in meter or feet. A larger field of view translates to a larger area seen through the binocular.
The Apparent field of view is the visible width of the field as seen through the binocular, and can be obtained by multiplying the real field of view by the magnification. Hence for a 10x50 with a 5º field of view, the apparent field would be 50º.

7.     

7.   Exit pupil
The exit pupil is the circle of light you see when holding a binocular or spotting scope about 6 inches away from your eyes and looking at the surface of the eye pieces. The larger the exit pupil, the brighter the image obtained will be. Having a large exit pupil is advantageous under low light conditions and at night
The "exit pupil" on a binocular can be used to correspond to our own eye pupil. Generally a 5mm "exit pupil" will provide good brightness over many conditions. This number will be between 5mm and 9mm. 9mm of dilation is the maximum amount for the human eye, and this number tends to decrease with age.

8.     Eye relief
The eye relief is the distance between the surface of the eyepiece lens and the exit pupil.   Most binoculars have a standard eye relief distance of from 9mm to 12mm. This is long enough for just about everybody unless you wear eyeglasses and you don't feel like taking your glasses off to see through the binocular!
Long eye relief is primarily a characteristic that specifically benefits people who wear eyeglasses. But it is also great for everyone else because you do not have to press hard against the eyecups to see the whole field of view. Some binoculars with long eye relief do not have a "wide angle" field of view. This is because when designing optical systems it is difficult to provide both long eye relief and wide angle in the same system.  
Most of our binoculars come with rubber fold down eyecups, you can bold down the eyecups for the eyeglass wearer and fold up the eyecups for the people who do not wear eyeglasses. Now, we have a new style twist-up eyecups, which can allow instant, full field of view adjustment for both eyeglass and non-eyeglass wearer, and you even can twist the eyecups up and down freely for a comfortable location.

9.     Interpupillary distance (IPD)
IPD stands for 'interpupillary distance' or the distance between the pupils of your eyes. As this distance is different for each person, the binocular can be adjusted to fit by opening or closing the hinge. Many binoculars include an IPD scale in millimetres, on the hinge mechanism. IPD is set correctly by first opening the binoculars right out, then observing a distant object whilst folding them shut until a perfect circle is formed, you can hold the objective lens barrels of the binoculars and move them back and for the until you can see one circle of view.

10.  Brightness
Brightness is a glasses’ ability to gather and transmit enough of the available light to give a sufficiently bright image for good definition. Brightness also helps in differentiating colors of objects.
Brightness is one criterion to be considered when purchasing binoculars, but is not the most important factor. Given in order of importance to the overall brightness of a binocular, the following factors are worth investigating: objective lens diameter, magnification, the type and quality of the objective lens glass, type of lens coatings and type of prisms used. In general, large objective lenses, low magnification and fully multicoated lenses are most desirable.

11.  Resolution
Resolution is a measurement of the binoculars’s ability to distinguish fine detail (sharpness). Better resolution also delivers more intense color.
One formula for determining the theoretical resolution of various size binoculars is to divide the objective lens diameter (in mm) into 116. Keep in mind that this resolution is theoretical, actual resolution is determined by the quality of the optical components, the type and quality of the optical coatings, atmospheric conditions, optical and mechanical alignment, collimation and the visual acuity of the individual. Generally, a larger objective lens will deliver more detail to the eye than a smaller objective lens, regardless of the magnification of the binocular. A

12.  Contrast
Contrast is the degree to which both dim and bright objects in the image can be differentiated from each other and from the background of the image.
Higher contrast helps to see fainter objects or subtle detail that is important for deep space astronomy or for serious birders. Contrast is affected by the optical coatings, collimation, air turbulence, and objective lens, prism and eyepiece quality.

13.  Focusing systems
Usually, there are three focusing systems:
Center Focus: most of binoculars are of this type. Usually the right eyepiece has an individual diopter ring pieces, this diopter ring can compensate for any vision difference between your right eye and left eye.
You can focus the central focus binoculars as following steps:
A.      Close your right eye and look through the left side of the binoculars. Focus by turning the center focus wheel until the image is clear .
B.       Close your left eye and look only through the right side of the binoculars and turn the right eyepieces adapter ring until the image is clear .
C.       Now, your binoculars should be adjusted for your eyes, focusing for both far and near objects can be made by simply turning the center focus wheel.

Individual Focus: the right eyepieces and left both have an individual diopter ring pieces, this focusing system can provide precise focusing adjustments for image sharpness, and is more reliable than center focusing.
You can focus the individual focus binoculars as following steps: you must close one eye at a time and rotate another side of the eyepiece diopter ring until the image is sharp, then repeat for another side.

Fix Focus: This focusing system don't have the traditional focus wheel or eyepiece focus adjustment. Fix focus binoculars are great for action sports viewing or viewing situations where you don't want to loose time having to adjust your binocular, The focus is simply pre-set at infinity at the factory for clear vision with no adjustment necessary from about 15m to infinity. Fixed focus requires a deep field of view to allow pre-setting and for this reason is usually best used on lower magnification binoculars such as the 7x. As the focus is pre-set, normal vision is assumed, so spectacle wearers need to keep their spectacles on whilst using these binoculars.

14.  Focusing methods
External focus: When turning the central focusing wheel, you can see the eyepieces or objective lens move forth and back.
Internal focus: When you turning the central focusing wheel, you can not see the move of eyepieces or objective lens, as the lenses inside the binoculars move forth and back.
We think the internal focus is the better method than external focus.

15.  Construction
Construction is very important performance to any binoculars. The security of the barrel alignment and proper internal mounting and alignment of the optics are crucial to producing a binocular that's mechanically reliable, smooth functioning and long lasting.

16.  Collimation
This refers to the alignment of the two sides of a binocular. Collimation is the alignment of the optical elements to the mechanical axis. For perfect stereoscopic vision, both sides should be parallel such that one circle is seen when looking through them.  

17.  Tripod adapter
Most of binoculars (especially high magnification binoculars) have 1/4x20 threads on the bottom of central hinge in order to attach a tripod adapter, this tripod adapter can help you to mount the binoculars on a photographic/video tripod for more stable viewing.