FAQ about Binoculars

Frequently Asked Questions about binoculars, a technical note prepared by NIPON SCOPE & OPTICS


1. How to adjust binoculars?

2. What do the numbers mean in the binoculars title?

3. What is Porro Prism or Roof Prism system?

4. What is field of view?

5. What are lens coatings for?

6. What is eye relief?

7. What is centre focus?

8. Why are some binoculars brighter than others?

9. How to choose an ideal pair of binoculars?

10. How do I know if binoculars prisms are make of Bak-4 or Bk-7 or K9 glass?

11. How to identify or estimate production date of vintage binoculars?

1. How to adjust binoculars?

Our left eye and right eye can be different in their vision and focusing capability. Therefore, to use binoculars properly, we must compensate for such a difference, and centre-focusing binoculars have an adjustment mechanism to achieve this. You can find that almost all the binoculars listed in our store have one eyepiece (usually the one on the right-hand side) which is independently adjustable with a marked scale.

To adjust binoculars, first, use a lens cover or your hand to cover the right objective lens which is on the same side as the adjustable eyepiece (note: better to keep both eyes open to avoid distortion by squinting). Look through the binoculars and use the central focusing mechanism to focus on a distant object until it is sharp and clear.

Second, transfer the lens cover to the other lens on the left, again with both eyes open, but this time adjust the focus on the same object using the adjustable eyepiece only, until it is clear. Your binoculars are now properly focused for your use. Adjust the distance between the two eyepieces so that they in the centres of your eyes (i.e., when the two cycles merge into one). You can now use the central focusing mechanism to focus on objects at different distances, but you should keep the adjustable eyepiece at the same focus setting unless it is accidentally moved or the binoculars have been used by someone else.

2. What do the numbers mean in the binoculars title?

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 THE Nipon 8×35 binoculars, it has 8x magnification (i.e., an object will be enlarged 8 times; or an object located 80m away will look like 8m away through the binoculars), and its objective (front) lens is 35mm in diameter.

3. What is Porro Prism or Roof Prism system?

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.

4. What is 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 the binoculars have, the wider the FOV.

5. 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 most binocular 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 lens and prism.

6. 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.

7. 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.

8. 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 10×40 binoculars, its Exit Pupil is 40/10=4mm. For the Nipon 10×50 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.

9. How to choose 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 8×21, 10×25 or 12×32 compact binoculars, depending on which magnification level you wish to choose. Such a model has all the functions required for these purposes and they are light in weight. These products have a unique feature with new optical coating technology (fully multi-broadband coated green lenses) which greatly reduces light reflection over a wider spectrum and increases image sharpness.

8×21 Compact Binoculars

In addition to the applications as mentioned above, if you wish to obtain a brighter image over a wider observation range while keeping the binoculars reasonably small, you could choose a pair of binoculars with bigger objective lenses, such as 8×35, 10×40 or 10×42 compact binoculars. The larger the objective lenses, the more light they let in, thus the brighter the images. These binoculars have advantages of observing objects under low lighting levels over a greater distance. In addition, some of these binoculars are built with large eyepieces (15mm diameter or larger) to provide a very comfortable and clearer view.

NIPON 8×35 Binoculars

If you care more about the image clarity over a further observation distance and do not mind a slightly larger size of the binoculars, you may well go for a 7×50, 10×50 or a 12×50 binoculars model. These binoculars have larger objective lenses (50mm diameter) which allow more light into the eyepieces, making them ideal for seeing more object details and also for use in dim light conditions. The Nipon 7×50 and 10×50 binoculars are built with large 20mm eyepieces for a much more comfortable view. These binoculars are considered to have the best balance between magnification, image brightness and image stability.

NIPON 10×50 Binoculars

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 6-13×22 or 10-30×50 model. These binoculars have an adjustable magnification (power) ranging from 6x to 13x for the first example or 10x to 30x for the second model. Therefore, you can get a closer 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 higher power settings. This is because the front objective lens cannot enlarge to let in more light as the power is increased, so the view gets dimmer as the power is increased. In addition, given the same specification (i.e., size of objective lens and power level), the image clarity of a fixed power binocular is almost always better than that of a zoom binocular.

Overall, it is often not recommend to have a pair of hand-held binoculars with their magnification power greater than 30x. This is because that 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.

Larger binoculars with greater magnification power should be fitted to a tripod. The NIPON 20×80 giant observation binocular is a typical example with amazing performance.

NIPON 20×80 giant binoculars mounted on large tripod

If you want to view targets over a distance which is beyond the reach of binoculars, a spotting scope or a combined spotting scope/telescope is designed for such a purpose. Please refer to “Scopes FAQs” page for more information.

10. How do I know if binoculars prisms are made of Bak-4 or Bk-7 or K9 glass?

For the Porro prism binoculars, a simple way to tell if they have Bak-4 or Bk-7 glass is to hold the binoculars at about 20cm to look at the eyepieces. It is often written that the exit pupil of a Bk-7 or K9 prism is not perfectly round, while a Bak-4 prism is perfectly round, as shown in the picture below.

For binoculars, Bak-4 is generally preferable to Bk-7 or K9 as it has a higher refractive index and when used in a well-designed overall optical system, provides higher levels of image quality than would a prism assembly of the same type made with Bk-7 or K9 glass. It also facilitates the reduction of weight and dimensions in the construction of binoculars of the same size.

However, in reality, Bk-7 glass is reported to be more transparent than Bak-4 glass (reference). Some binoculars nowadays made of Bk-7 glass can also achieve good quality images and are suitable for most observation purposes. They are generally less expensive than those with Bak-4 glass. For most users, both design models (Bk-7 vs Bak-4) would look no difference in daylight conditions.

11. How to identify or estimate production dates of vintage binoculars?


Barr & Stround binoculars (1932-1971)

Barr & Stroud Binoculars (produced between 1950 and 1971)

ROSS binoculars – model guide

British binoculars from the 1950s and 1960s

The Wray Optical Company


Vintage British binoculars in store


Carl Zeiss Jena

Zeiss History

C.P. Goerz Berlin Trieder Binocle

Goerz Optics

Voigtlander (Note: this link is for Voigtlander lenses. Information to be updated for binoculars)

Vintage German binoculars.


Bausch & Lomb serial numbers

Bausch & Lomb date code (serial number scheme)

US WW2 military binoculars



Please send your comments or suggestions to: support@nipon-scope.com

Thank you!

Relevant links:

Binoculars FAQs

Binoculars FAQ – 56 Most Commonly Asked Questions Answered