In a binocular name, the first number indicates the magnification power and the second number is the diameter of the objective lens in mm. For example, for a pair of 10x40 binoculars, it has 10x magnification (i.e., an object will be enlarged 10 times; or an object 100m away will look like 10m away through the binoculars), and its objective (front) lens is 40mm in diameter.
What are Porro Prism and Roof Prism systems?
Since the objective lenses form images that are both upside down and reversed left for right, prisms are used to invert the primary image.
Porro prism and roof prism are two most frequently used systems for such a purpose. Porro prism binoculars are characterised by the eyepieces being offset from the objective lenses, while roof prism binoculars' objective lens and eyepiece are positioned in line for a more compact feature and reduced weight. Either type of prism system, properly manufactured, gives excellent optical performance.
What is the field of view?
Field of view (FOV) refers to the size of the area that can be viewed through the binoculars. This can be described either in degrees or in the width of the area visible (by ft. or m) at 1000 yards or 1000 metres. Given the same size of objective lens, the lower the magnification of the binoculars or monocular, the wider the FOV.
What are lens coatings for?
An uncoated optical glass lens or prism reflects about 10% of the light incident on one of its surfaces, thus allowing only about 90% of the light to pass through. Nowadays almost all binocular and scope lenses are coated with invisible coverings that work to improve the amount of light transmitted from the front lens to the eyepiece. Standard full coatings can reduce the level of light reflection to about 4% or lower; and more sophisticated multi-coatings can ensure 99% or more light transmission through the lense and prism. Multi-broadband anti-reflection lens coatings can even produce 99.5-99.9% light transmission.
What is eye relief?
This is the distance (in mm) the binoculars can be held away from the eye that still allows the user to see the entire full field of view as designed. Binocular users who wear eyeglasses for near- or far-sightedness can remove their glasses while observing clearly through the binoculars because the binoculars can fully correct for these eye defects. However, the users with astigmatism may need to wear their glasses in order to maintain sharp imaging through the binoculars. In this case a longer eye relief will become advantageous for these users, who cannot get as close to the eyepiece.
As a rough guide, an eye relief of 10-15mm should fit most users if this function becomes necessary. However, you may not need to look for this specification because most binocular models allow the eyecup to be either folded or twisted/pushed down to enable eyeglass wearers to enjoy an improved field of view.
What is centre focus?
Binoculars with centre focus system use one knob in the middle of the binoculars to move both lenses for fine focusing precision. This allows you to follow the action and switch from one object to another quickly. Binoculars with centre focus normally have one eyepiece (often the one on the right-hand side) which is independently adjustable to accommodate any difference between the two eyes.
Why are some binoculars brighter than others?
One of the most important factors affecting the image brightness is known as Exit Pupil, which is a bright circle visible when the eyepiece array is viewed about 10 inches away from the eyes. A larger Exit Pupil gives a brighter image. The value of Exit Pupil (mm) = Objective Lens Diameter / Magnification Power. So, for a pair of 10x40 binoculars, its Exit Pupil is 40/10=4mm. For 10x50 binoculars, the Exit Pupil is 5mm. The larger the objective lens, the bigger the Exit Pupil, and thus the brighter the image viewed. However, larger objective lens also means heavier and bigger the binoculars' body.
Binoculars for common use such as sightseeing and birding from near to medium range, an exit pupil size of 4-5mm is considered to be adequate for image brightness.
HOW TO SELECT AN IDEAL PAIR OF BINOCULARS?
Choosing a right pair of binoculars would very much depend on your main purpose of use. Here are some examples:
If you are looking for a handy pair of binoculars to carry around easily in your handbag or in the pocket, and to use it in numerous occasions such as stadium sports, indoor or outdoor concerts/plays, travel and birding, and you also want to keep the cost down, you may choose a compact model such as a pair of compact 8x21, 8x22, or 10x25 binoculars . Such a model has all the functions required for these purposes and its is very light in weight.
Alternatively, you can choose a 10x40 compact model which gives brighter images thanks to a larger objective lens, thus having advantages of observing objects under low lighting levels. In addition, its extra large eyepieces (15mm in diameter, even compatible with those 10x50 or 12x50 models) provide a comfortable view.
Another option is a pair of 8x42 binoculars with a better value of Exit Pupil of 5.25mm (i.e., 42/8=5.25). This product is slightly larger and heavier than the 10x40 model.
A top choice of this category is 10x42 binocular model which gives you the best image quality and tracking performance, and it is water/fog proof. The prices go up step by step for these models, but as you can understand, you will get what is paid for.
If you care more about the image brightness and do not mind a slightly larger size of the binoculars, you may well go for a 10x50 or a 12x50 model. These binoculars have larger objective lenses which allow more light into the eyepieces, making them ideal for seeing more object details and also for use in dim light conditions.
If you would like to have an added function to zoom in/out during the observation, you may also consider a pair of zoom binoculars such as a 6-13x22 binocular model. This item has an adjustable magnification power ranging from 6x to 13x, with 22mm objective lens. Therefore you can get a very close view of the target being observed. Please note that the zoom binoculars always perform better at the lower power level than they do at the highter power settings. This is because the front lens cannot enlarge to let in more light as the power is increased, so the view gets slightly dimmer as the power is increased.
Overall, it is often not recommend to have binoculars with their magnification power greater than 15-20x because, along with the magnified object, any movement or vibration will also be enlarged, including your own shakes and tremors. So the higher the power, the harder to hold the binoculars steadily. 10x or 12x power are adequate for most people for observations over a reasonable period of time, and zoomed function to higher powers is ideal for relatively shorter time observations.