Skip to content

Binoculars – A Primer

Disclaimer: Nothing contained in this article is original. Information has been sourced from the public domain. I do not claim any copyright or authorship. If any reader notices an error and brings it to my notice, I shall be happy to correct it. I have tried to explain the background, evolution, definitions, trends etc so that the reader can form an informed opinion while selecting/purchasing optics and while using/ maintaining them.
Arijit Banerjee

What are Binoculars?

To start off, Binoculars are an instrument to view distant objects. The object could appear nearer or larger. 

Human beings, have two front facing eyes which capture and project a single image. What we see is the result of signals sent from the eyes to the brain. Usually, the brain receives signals from both (bi) eyes (ocular) at the same time. The information contained in the signal from each eye is slightly different and with well-functioning binocular vision, the brain is able to use these differences to judge distances and coordinate eye movements. What this means is that binocular vision allows us to perceive depth of field and speed of a moving object.

A Binocular is a pair of Telescopes connected to a frame, thereby allowing viewing by both eyes.

History – at least a bit of it

So, what is a Telescope?

A Telescope is a tube with an objective lens in the front and an eye lens to look though, at the back. It magnifies the object viewed. The evolution of the telescope started with Galileo Galilei, the Italian astronomers’ design of instrument – the Galilean telescope, Sir Isaac Newton’s use of a large mirror to view celestial objects – Newtonian telescope, and then Johann Kepler’s design called the Keplerian Telescope. A modern Binocular has its roots in the Keplerian design of refractive optics.

A bit of trivia here. For an astronomer who is looking at Deep Sky Objects (DSOs) it matters little if the image he is viewing is upright or upside down. Constant focusing is also a non-issue – planets and stars don’t fly away. So it is actually easier to design an astronomical telescope than a binocular. The latter needs the image formed to be inverted so that what is seen is what the subject is like in orientation and needs a focuser, since terrestrial subjects tend to move. A modern binocular too shows an upright image. However, the construction is far more complex.

The first person to design anything that looked like a binocular was a Dutch spectacle maker Hans Lipperhay. He applied for a patent for a telescope to his government in 1608. He was told, ok this is nice, but can you make something that allows use of both eyes? He tried, and put together two telescopes on a frame and presented it for inspection. He was told, great work mate. This is very good. But we can’t give you a patent, others must have had this idea too! That is bureaucrats at their best.

The problem was that the two telescopes in wooden tubes was a large and ungainly contraption. It wasn’t until industrial manufacturing of brass became common that ‘telescoping’ tubes could be designed. So, focusing was a pain. And someone had to figure out a mechanism for making the stuff smaller. In some manner, the optical path of light inside the binocular had to be folded.

I need to mention something here. I shall be using the term ‘lens’ repeatedly. I shall use the word lens to keep things simple. In optical instruments, any lens is actually a complex of 2, 3 or more lenses – cemented together sometimes – doublets, triplets and also with some stand-alone singlets. So the objective lens is actually a combination of some convex and concave lenses. So is the eyepiece.

In the 1700s, telescopes started being available, but it took a while for a binocular to emerge. This is because a binocular needs:

  • Two telescopes (with parallel optical axes) for magnifying distant objects
  • Matched magnifications of the two telescopes
  • Erect images
  • Hand holdable; Individually focusable, preferably jointly focusable
  • Adjustable interpupillary separation (human face anatomy varies from person to person)

In the early 1700s, small Galilean telescopes, called spyglasses or prospect glasses, had become common.


In 1823, the Austrian, J. F. Voigtländer patented the combination of two achromatic spyglasses into a pair of opera glasses by using a frame with two bridges.

Then came a central focusing knob with a threaded screw and redesign of the bridge to allow more nose room. Ornate instruments for the rich, some were in mother of pearl, some bejeweled – just two low powered telescopes 2.5X or so, for use in operas, ballets etc.

Larger Galilean binoculars with a maximum magnifying power was about 5-6X were invented. By the mid-1800s, mechanical and optical technology allowed twin telescope (Keplerian and even mirror / Gregorian) binoculars to be produced. The large binoculars were 750 mm long. The small twin telescopes collapse for storage to under 120 mm. The magnifying powers range from about 5X to 20X.

In 1854, an Italian scientist, Ignazio Porro invented the Porro prism. This was right angled isosceles prism. This system works on the principle of Total Internal Reflection (TIR). In operation, light enters the large rectangular face of the prism, undergoes total internal reflection twice from the sloped faces, and exits again through the large rectangular face. Because the light exits and enters the glass only at normal incidence, the prism is not dispersive. In other words, light is not lost to scatter. It is however important to note that glass manufacturing was still in its infancy. Glass had too many imperfections and impurities – particles, bubbles. These translated into imperfect/ unclear images.

The association of Carl Zeiss with the glass maker Otto Schott resulted in the production of the high-quality prisms that were essential for successful Porro prism binoculars. Ernst Abbe provided the optical design of these binoculars. These high-performance modern binoculars were first sold in 1894. This was the Zeiss Feldstecher 6X15.

In 1897, erecting prism systems incorporating roof surfaces were introduced into binoculars. They were first used in a penta-prism configuration by the firm of Hensoldt in Wetzlar, Germany. (BTW, your DSLR uses a Pentaprism –again Total Internal Reflection – so what the lens sees is what the optical viewfinder shows).

It appears that the Italian, Giovanni Amici was the first to add a roof surface to a prism in the mid-1800s. The Amici or roof prism deviates light by 90º. The Abbe-Köenig prism appeared in the early 1900s. With this prism, image erection is obtained without a displacement of the optical axis. While still used today, the Abbe-Köenig prism is the precursor of the Pechan-roof prism (also known as a Schmidt Pechan prism – 1964) in modern roof-prism binoculars.

By now you must be wondering why I am talking about prisms so much. It is the prism type, design and quality that defines binocular quality in many ways. We shall dwell more on prisms, later.

Binocular Basics

Every binocular has a basic specification set. It could be 8X42 or 10X50 or 7×21. The 8, 10, 7 followed by the letter X indicates magnification. Magnification is a value that indicates how large objects appear when looking through the binoculars. 10X means the subject under observation will be 10 times bigger. When using a pair of 10x binoculars, an object 100 meters away will appear to be the same size as when viewed by the naked eye from 10 meters away. For the photographer, assuming a 50mm lens to be 1X, an 8X binocular is equivalent to 400mm focal length.

The second number is that 42, 50, 21 …

This is the diameter of the objective (front) lens in mm. Larger this aperture, more the light gathering capacity of the binoculars. This is a good place to start. You can expect a brighter image, if the objective lens is bigger. A bigger objective lens also means a heavier binocular. A lower magnification comes with a steadier image. A lower magnification allows a bigger depth of field so less requirement to focus. A lower magnification also lends to a wider field of view. A larger magnification is tougher to hold steady, but will allow better study of details. So personal preference, fitness, purpose play a role in choosing a magnification and size of objective lenses.

Conventionally, binoculars can be categorized as:

  • Below 25mm: Compact/ Pocket type binoculars
  • 30mm – 49mm: Standard binoculars
  • Over 50mm: For astronomical observation or marine / other niche applications

Now here is an interesting fact. All other things like quality of manufacturing, quality of components etc being constant, in good light there is little to choose between 8X25 and 8X42 binoculars. It is the first 15 odd minutes from sunrise and the last 15 minutes of daylight or on a very overcast day when the difference in objective size will come into play. In low light the light gathering capacity of a larger objective lens becomes useful.

Field of View: Real field of view is the angle of the visible field, seen without moving the binoculars, measured from the central point of the objective lens. The larger the value is, the wider the view field available. For example, binoculars with a wider field of view are advantageous for locating fast-moving wild birds within the view field. 

There was time, decades back, when creative eyepiece design allowed binoculars to have fields as wide as 11 degrees. Then came a consumer demand of precisely corrected center of fields and narrow fields of vision – 6 degrees to 7 degrees was the standard (The Japanese industry standard is still 6.5 degrees to qualify as wide-field). Of late, wide fields are in fashion again. 9 degrees is the best performance today from Swarovski in their NNL Pure binoculars, closely followed by 8.8 degrees from In the Zeiss SF series.

Field of view at 1,000 meters (or 1000 yards):

Field of view at 1,000 meters is the width of the visible area at a distance of 1,000 meters, which can be seen without moving the binoculars. This performance factor is usually mentioned on the binocular itself.

Resolution or Resolving Power: The capacity to distinguish between two adjacent points or parallel lines. Resolving power is mathematically relevant to any lens but used more in microscopy and satellite imaging. Higher resolution translates into better sharpness.

Exit Pupil: Often paid least attention to, this is a vital performance parameter while choosing binoculars.

The exit pupil is the bright circle that can be seen in the center of each eyepiece when you hold the binoculars about 30cm away from your eyes with the objective lenses pointed toward a bright light. The larger the diameter is, the brighter the viewing field is, which is an important consideration when using binoculars in dark situations and for astronomical observation. The size and shape of this disc is crucial to the instrument’s performance, because the observer’s eye can see light only if it passes through this tiny aperture.

Exit pupil = The effective diameter of the objective lens ÷ Magnification

With 8×42 binoculars, the formula is 42 ÷ 8 = 5.3.
Therefore, the diameter of the exit pupil is 5.3mm.
This figure indicates the brightness of the image in view.

What is the relationship between bright/low-light conditions and the exit pupil of binoculars?

The pupil diameter of human eye changes depending on the ambient light conditions. The capacity of the human eye to constrict and dilate also reduces with age. Even if not linear, the popularly accepted measurements are as follows:

There shall however be exceptions based on health, disease injury etc.

Some illustrations – In low-light conditions (comparing 8×20 and 7×50 binoculars)

8×20 binoculars

With 8×20 binoculars

  • Pupil diameter of human eye: 7mm
  • Exit pupil of binoculars: 20÷8=2.5mm
  • Because the 2.5mm exit pupil of binoculars is smaller than the 7mm human pupil, you will perceive images as dark.


7×50 binoculars

With 7×50 binoculars

  • Pupil diameter of human eye: 7mm
  • Exit pupil of binoculars: 50÷7=7.1mm
  • Because the human pupil is about the same size as the binoculars’ exit pupil, you will perceive images as bright as when seen with the naked eyes.

In bright conditions (comparing 8×20 and 7×50 binoculars)

With 8×20 binoculars

  • Pupil diameter of human eye: 2-3mm
  • Exit pupil of binoculars: 20÷8=2.5mm
  • Because the human pupil is about the same size as the binoculars’; exit pupil, you will perceive images as bright as when seen with the naked eyes.

With 7×50 binoculars

  • Pupil diameter of human eye: 2-3mm
  • Exit pupil of binoculars: 50÷7=7.1mm
  • Because the binoculars’ exit pupil is larger than the human pupil, you will perceive images as bright as when seen with the naked eyes.

When the pupil constricts in bright light – the brightness of the image when viewed through either binocular shall be the same.

Midsize binoculars deliver smaller exit pupils, but that may not make any difference. If the pupil of the eye is contracted to a smaller diameter than the exit pupil of the binocular, the light falling outside the area of the pupil cannot be seen or used by the eye. So, for most people, in bright daylight, a mid-sized binocular (say 8X32) will seem just as bright as a full-sized binocular (say 8X42). For an older person, there might not be much difference even at dusk.

When things are moving around, however, it can be useful for the exit pupil to be a little larger than the pupil of your eye. If you’re bouncing about in a boat or being buffeted by wind, a binocular with a larger exit pupil helps you keep the binocular positioned where you can see the image. A larger exit pupil can also benefit a person whose hands are unsteady.

The best way to determine whether a mid-sized binocular will give you an optimum birding experience is to try it out in person. Try it in daylight. Try it at dusk and in the darkest conditions you expect to be using it. Remember that it takes at least a half-hour for the eye to become fully darkness adjusted. You can’t just switch out the lights and immediately compare the brightness of two binoculars.

But don’t assume that a larger binocular will necessarily give you a brighter picture.

Eye Relief: Eye relief is the distance from the eyepiece to the point where you can see the whole picture. People who wear glasses often have trouble using binoculars, because their glasses hold the binocular’s eyepiece too far away from their eyes. It’s a problem with many binoculars, but particularly with smaller ones.

Over the years, things have improved. Manufacturers have increased the eye relief and have developed eye cups that have click stops so that the viewer can dial the precise amount of eye relief needed.

The eye relief situation is worse with 10x than with 8x binoculars, because higher magnification requires an eyepiece with a shorter focal length, and that results in shorter eye relief.

People who need glasses but do not have astigmatism can look through binoculars without their glasses, adjusting for their nearsightedness or farsightedness merely by focusing the binoculars. For them, one solution is to take their glasses off before raising the binoculars to their eyes. It’s a technique with drawbacks – one of which is missing the bird while fooling around with the glasses. Of course, if you wear contact lenses or don’t need glasses at all, the eye relief issue won’t matter much.

It makes a difference how a person’s glasses fit. Eyeglasses that fit close to the eyes offer less interference than glasses that ride far out from the eyes. Some people who wear glasses simply purchase a special pair of glasses for birding. Glasses with small lenses can fit snugly under the brows, almost touching the eyelashes, and allow the binoculars to rest close to the eyes. That may be enough to put the binoculars at the proper distance for the person to see the whole picture.

Bifocals can make viewing through binoculars tricky, as the part of the image seen through the bifocal section won’t be in focus at the same time as upper part of the image. Experienced binocular users position their binoculars to that they can look only through the main part of their glasses. Non-line or progressive glasses make it almost impossible to focus binoculars. If you’re getting special glasses for birding, be sure to get ‘line bifocal’ glasses. Or, it may also help to request that the bifocal be a small section at the bottom of the glasses, just enough for consulting the field guide.

It is challenging, but not impossible, to design mid-sized binoculars with long eye relief. It requires extra glass in the eyepiece. And there are such binoculars available.

So do look for a larger eye-relief while choosing bins. Place your eye comfortably and correctly in line with the optical axis. This will reduce chances of black-outs/ kidney beans. Try not to let the eye piece float in the air. Anchor the binoculars below the brow or on the nose bridge. Please adjust the interpupillary distance to suit your face so that you see one image through both yes. Those who view binoculars with spectacles on, start with the eye-pieces down. Make sure you can view the entire field – edge to edge – comfortably.


When we are taught physics in school, the approach is to simplify things. Diagrams showing a simple mirror (reflection), a slab of glass (refraction) or a prism (dispersion), portray a single ray of light.

In real life, there is no single ray of light but a bunch of them entering the binocular at various angles of incidence. And the lens in the front (or the lens at the ocular side) is not flat but curved. Suddenly things become complex.

Aberrations are in the nature of optical systems. A lens in theory is supposed to focus light at a point. But in reality it does not. Light focuses spread over an area. More the spread, more blurry the image.

Light is made up of various component colours, each with a distinct wavelength. The visible spectrum is made up of Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).

Chromatic Aberration: This happens when the various wavelengths of light focus at different points. The image formed is not only not sharp but will also have a green or purple edge – this is also called fringing. Try looking at electricity lines through the binoculars at mid-day The black wires against the white sky is likely to exhibit a purple (or and green) fringe. You can try looking at a crow or an egret and see how chromatic aberration looks like.

Comatic Aberration: Also called Coma. Here point sources of light that are located off axis appear like a comet with a tail. This is very important to astronomers while looking at stars.

Correction of aberration is a determined by glass quality, coatings on lenses, coatings on prisms (porro prisms need no anti-reflection coating) and mathematical design of lenses.

Distortions: Because of curvature of lens surfaces, the image viewed could appear wider in the middle – Barrel shaped – hence Barrel Distortion. Take a selfie from as close as you can with a cellphone front lens. Your face will appear more rounded/ fatter than it actually is – Barrel distortion (the cell phone camera is actually wide angle).

The other form of distortion is Pincushion Distortion where the image appears pinched in the middle. Look a large group photograph or a landscape photo taken with a very wide-angle lens.

Binocular manufacturers introduce a degree of filed flattening. Some more than others The Swarovsksi EL series with ‘Swarovision’ is reputed to have flat fields so that sharpness from edge to edge is same. However, while panning, such a binocular, in some users causes a ‘rolling ball’ or ‘globe effect’ that could lead to nausea and disorientation while fast panning.

Most binoculars have a certain amount of distortion consciously built into the design. At any particular point of focus, the center when sharp, the edges will be slightly defocused. By turning the central focuser, some of the edge can be brought into focus. If we imagine this view as a circle and the larger area in focus from the center – the better the binocular is.

Aberrations and distortions are equally important to camera lenses and other optics. A macro lens is one lens that is very sharp (so very well corrected for aberrations) and flat field so that the image as projected to the sensor is as parallel as possible to the sensor (and that is why macro lenses are slow to focus).

Prism Binoculars

Prism binoculars contain a reversing prism between the objective lens and the ocular (eyepiece) to reduce the length of the instrument and to reverse the projected image.


Binoculars of this type use a pair of Porro prisms in a Z-shaped configuration to erect the image. This results in binoculars that are wide, with objective lenses that are well separated and offset from the eyepieces, giving a better sensation of depth. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective. Porro prism binoculars were made in such a way to erect an image in a small space, thus binoculars using prisms started in this way.

Porro prism binoculars have been manufactured for a long time. If the constituent parts are well made and the glass is of good quality, porro prisms give a nice bright view. Porro prism binoculars may come with a central focusing arrangement or with individual focusing mechanism for each tube. The latter makes construction simpler – most marine or military use binoculars are porro prism. Making a weather sealed, nitrogen purged porro prism is possible – but the focusing will be slow an the binoculars expensive The Swarovski Habicht Porro prisms in 8×30, 7×40 and 10X40 are the finest porro prism binoculars available – and are expensive. In fact they have among the highest transmission of light at 96% and marvelous on axis sharpness an contrast (they use the best glass and coatings fro their lenses too). As on date they are sold only in Europe. The eye relief in these glasses might not suit a spectacle wearer though.

Roof Prism

In a roof prism binocular, the prism’s reflective surfaces resemble those of a rooftop, hence the name. The system used in binoculars consists of two prisms, with at least one prism having a roof edge. There are several different roof prism combinations that can be used, depending on the design and purpose, and desired features of roof prism binoculars.

The most common today is Schmid-Pechan’s prism system. It consists of a reflective surface with a metallic mirror coating. Roof prisms are reflecting prisms with two faces, separated by a narrow air gap, meeting each other. These two faces of this prism form a roof-like structure, that’s why the name roof prism is given to this prism shape.

When a light beam enters a Schmidt-Pechan’s prism it enters the first prism vertically and is reflected at the inner face near the air gap and reflected again on the underside of the prism after which it exits the prism via the air gap and vertically enters the second prism. The vertical entrance into the second prism reduces refraction and reflection losses at the entrance and exit surfaces. Due to the double reflection, only the beam path is deflected, the image orientation remains unchanged.

There are other roof prisms too. The Abbe- Koenig prism (e.g. Zeiss FL 8×42, 10X42 etc and Zeiss HT) are a slim prism that is in axis with the optical centres of objective and eyepieces and do not need any anti-reflective coating. The anti reflective coating ensures that least amount of light is reflected back. Better the coating better the transmission, brighter the image. In Schmidt-Pechan prisms, the reflective surface needs to be coated with a mirror coating. Cheapest option is to use no coating. Next is aluminum coating (the bathroom mirror type), Silver – more expensive and with chances of getting tarnished if not nitrogen purged; and the most expensive di-electric coating – which is expensive

However, all prisms need phase correction coatings so that aberrations are minimized and colours are true.

Roof Prisms In Binoculars: Roof prism binoculars with premium glass and precision prisms produce high-quality images. Roof prisms enable slim binoculars. However, the costs of producing roof prisms are higher than those for Porro prism binoculars.