Binoculars

Binocular is a precision optical device which is capable of capturing distant object images and displaying them in an enlarged vision. Elements of sharpness, contrast/brightness, magnification, resolution and other image quality parameters are rendered through field of view, field depth, focus range, correction, stereoscopic effect and other functional parameters. Optical and mechanical devices in the binocular work in coordination to render the quality parameters by optimizing the functional parameters.

History of Binoculars

Binocular reviews

1700

The earlier years of 1700 witnessed the initial forms of the monocular called the spy glasses. They had a limited field of view and mostly their usage was limited to opera houses. Hence they were also called by the name opera telescopes. The materials used for making the barrels of these spy glasses were mostly brass, ivory and silver. They were protected with a leather cover.

Early 1800s

  • Voigtlaender combined the two opera telescopes with a frame containing two bridges to form the earliest binocular.
  • Lemiere from France updated this design by adding a third bridge.
  • The subsequent updates to the device witnessed many changes involving addition of turning and focusing knobs, focal length adjustments, eye draws, nose room, eyepieces and other extended parts. The external barrel and the eyepiece tubes got separated from each other. The movement of eyepiece lens with respect to the fixed main lens made it possible to change the focal length and field of view for the first time.
  • Multiple forms of frame bridges were introduced to adjust the distance between the two eyepieces within an optimum range of values.
  • The concept of the first curved bridge was introduced in France. Designs and paintings were done on the binoculars to make them look elegant and royally appealing in nature. The core metal continued to be brass while the added decorative were made of solver, gold and ivory.
  • The next generation of binoculars was built with technical improvements in the magnification and field of view. The end of 1800s saw the development of twin telescopes. The design of the bridge was improvised to optimize the inter-pupilar spacing. Construction of the adjusting knobs was improvised to a great extent which made change of focal length easy. Objective lens’ aperture was optimized to values between 10-12mm. Magnification factor was enhanced to a range of 5X-20X. There were improvements in the mechanical design too. The length of the twin telescope was reduced to less than 160MM which could collapse to 120MM making it portable.

1854

This was the year of prism binocular. Image erecting prism was invented by Ignatio Porro in this year. This prism had some unique optical features like right angled construction, multiple internal reflections and image erecting feature. The introduction of Porro prism eliminated the need for image erecting lens and its related components. Now the binocular design was made simple with objective lens, Porro prism and an eyepiece with filed and eye lens. Hence the binocular was made more compact than the twin telescope.

1895

Improvements were made in the optical design of the Porro prism by Carl Zeiss. The prism was recess machined into an aluminum casting and prism cover to provide stability and enhanced focus. The mounting of the prism in close proximity to the eyepiece lens increased its diameter coverage from the objective lens. The distance between twin prisms was also optimized for utilizing the magnifying power of the objective lens to maximum extent for producing clear images.

1897

  • Roof surface was introduced into the prism design. The design of prism was changed into a pentagonal shape for compact image erection mechanism. Moeller introduced his own version of the prism which had six internal reflections for producing relatively sharper images.
  • Improved versions of the Moeller prism were introduced by Abbe-Koenig and Leman which were designed to straighten the path of light after the final internal reflection.

1900s

  • The early 1900s saw many improvements in the design and application of the modern binoculars. The first word war was responsible for the mass production of these devices in Europe and the USA.
  • Swarovski Porro service glass was introduced to make the prism for the German military forces in the year 1940. The magnification factor was increased to 6X30. This system was considered to be the standard for all the subsequent mass produced binoculars.
  • Major improvement in the magnification factor was witnessed in the year 1991 with the invention of pocket binoculars. SLC roof prism increased the sharpness and helped in achieving the much required balance between contrast and brightness factors. Swarovski products continued to dominate the binocular market till 1999 when hunting binocular was introduced for the first time.

The history of binoculars in the 1900s and 2000 periods is split into sections.

  • 1949-1985 was dominated by the Porro prism with the production of draw tubes in 1957 and antireflection coating into the prism in 1976.
  • 1985-2013 was dominated by roof prism. Major developments in 1989 (pocket binocular), 1999 (EL binocular), 2010 (SLC-HD binocular) and 2013 (compact line binocular).

Commercial Production of Binoculars

Commercial production was started as early as 1917. One of the known manufacturers was under the brand name of Nippon. The company manufactured many versions of the product under the names MIKRON, ATOOM, BRIGHT, NOVAR, ORION etc from. The brand name was changed to NIKKO in 1921 and the products under this name were made from 1922 to 1948. The name was changed again to Nikon in the year 1959. The other known companies for binocular manufacturing have been Kogaku, Nich-Doku, Tokyo-optical and Fuji till today from Asia.

How Binoculars Work

The functionality of binoculars can be explained by the way in which light from the target object enters the main lens, passes through the barrel, gets refracted through the prism and enters the field and eyepiece lens to form a magnified image of the object. Here the functionality of each part plays important role in shaping the quality of the final image as seen by the viewer.

Objective Lens

The main task of this lens is to capture the light and transmit to the internal parts of the binocular. The assembly is generally made of multiple lenses which have unique configuration. There are two standard types of configuration, namely achromatic and apochromatic.

Achromatic Type

  • This is called the standard type of configuration. This contains an image plane in the form of a curved surface. It performs the task of image plane correction.
  • This is an important function because the light entering the lens will be split into 7 constituent colors which can result in improper image formation.
  • The achromatic lens is responsible for the convergence of multiple color wavelengths into a single focus. This lens is made of extra low dispersion type of glass for giving color correction property.

Apochromatic Type

  • This type of configuration is used by high end binoculars. The basic functionality of the Apochromatic is said to be the same as achromatic. But it is made of non-exotic type of glass which can correct the errors of chromatic errors by more than 80%. It is also said to eliminate the other negative factors like color fringing and loss of resolution in the light coming from distant objects. This is said to be one of the main quality elements to get clear and sharp image of distant objects.
  • This type of lens is said to be index matched to correct all the errors from the complete spectrum of light passing through it. This is responsible for increasing the precision of the image.

Aberration (error) Corrections

At this stage the refracted light through the lens can have aberrations (errors). Some of the errors get corrected within the objective lens, while the others are rectified by the prism, field and eyepiece lenses. Some of the most commonly occurring errors in the objective lens are being listed here with the correction mechanism, so that the reader can understand the making of modern objective lens mechanism.

  • Coma is one such type of error which results in overlapping of light rays over each other. The resulting image clarity is reduced due to filed curvature. Optimized design of the objective lens can effectively reduce this value to a considerable extent. Most of the binocular opticians seem to do it by increasing the number of layers in the objective lens or by using glasses with extra low dispersion property.
  • Chromatic error occurs when the colors of light get focused onto different focal-points. Normally the red and blue point of focus will be further from the focal point of yellow and green colors. In such cases the viewer will not be able to see the yellow and green colors of the object which may appear as blank. This is said to be corrected with the help of apochromatic lens with high quality chromatic error correction feature.
  • Astigmatism is a type of error which results when the different wavelengths (colors) of the light gets focused on different points in the same horizontal line. This can be corrected by placing another type of optical object in front of the objective lens. This optical object is supposed to have the same astigmatic value of the objective lens, but in reverse direction. That means the overall value will be converted to zero. This task is said to be performed by the prism of the binocular.
  • Distortion is a type of error in which the multiple images of the object appears to be overlapping on each other. Distortion can be corrected by optimizing the cube field of the angle.

After going through these corrections the light rays enter the prism located within the binocular tubes.

Binocular Prism

The binocular prism is responsible for correcting the image orientation as the light passes through it with many refractions and reflections. The other function of the prism is to shorten the path of light by considerable extent in the horizontal direction to make it sharper. The binocular may use one of the two types of prism, namely the Porro prism or the roof prism.

Light passes through the prism from one end. The interiors of the prism are coated with silver or aluminum which reflects the light at multiple angles within its internal surfaces. Finally after the image inversion corrected it gets passed onto the field lens in the path. Image correction is the basic function which is common for both types of prisms namely the Porro and Roof. However these two vary in their design and output quality.

  • Porro Prism is the earliest form of prism to be sued in the binocular. This is actually an assembly of two prisms which have isosceles triangle shape. These two are said to be mounted on each other in such a manner that their hypotenuses face one another. The long axes of the two triangles are exactly opposite to each other. This results in the inversion and reversion of the image, finally ending up with the corrected image focus. Since the resulting shape of the combination of two prisms is said to be in “Z” form, this is used in larger sized binoculars. The objective lenses are separated at the front end of the binocular. However they are also offset with respect to the eyepiece lens. In addition the prism reduces the distance travelled by the light by folding it though refraction and multiple reflections. This is supposed to increase the focal length of the binocular with minimum possible physical distance between the objective lenses and the filed lens and eyepiece lens. When the light from the two objective lenses reach the twin eyepiece, it is made to undergo convergence at the eyepiece bridge and the image is displayed in eh couple of eyepieces as one.
  • Roof prism is said to be formed by mounting one semi-pentagon type of prism over one Schmidt type of prism. The combination of these two into one compound prism is said to result in the formation of a good quality image generator. Image conversion happens after the light passes through the multiple walls of the compound prism through refraction and reflection. The inner walls of the roof prism are coated with silver to increase the sharpness of the resulting image. The dimension of the prism is relatively smaller compared to that of Porro prism. Hence this can be used extensively in compact binoculars.

The roof type of prism has more tolerance for error corrections compare to the Porro type of prism. The light rays travel in a relatively straighter path through this prism compared to the Porro type. The number of reflections in this type is said to be 6 compared to 4 in the Porro type.

Coating

Coating with anti reflection materials of transparent nature is said to be done on the objective lenses. This helps in absorbing maximum volume of light that passes through the lens. The other types of coatings used in the binocular are for phase correction, mirror coating, dielectric coating,

  • Anti reflection coating is done with the help of transparent material like the magnesium chloride. Advancements in the field of optics have resulted in multilayer coating which can reduce the reflections to less than 0.5%. This results in improved quality of the image by retaining the natural color of the object in view.
  • Phase correction is a mechanism which is said to be limited to the roof type of prism only. As the light enters prism it gets split into two parts which again get reflected from the twin roofs of the prism. This is called as polarization which creates a separation between the two rays. The angle of separation is said to be the polarization angle. This is also called the phase shift. This process can also result in interference of light waves, leading to distortion of the image. This is said to be done with the help of dielectric material which is coated within the inner walls of the prism. As the twin light rays get reflected through this medium,. The angle of polarization and the phase shift values are reduced to great extent, resulting in the formation of a single sharp image.

After passing through the prism the light rays enter the ocular prism. Here the image gets magnified. This is the one which determines the parameters of the binocular like the eye relief and field of view. This lens is also responsible for correcting the orientation and increase in the brightness of the final image. Coating is also done on the ocular lens of the binocular. The light passes through these lenses and reaches the eyepiece.

Parameters for Image Quality Improvement

  • Magnification is a process by which the view of the image is made to undergo an escalation in its dimensions in order to produce clear visibility. The object appears to be closer than the actual distance. This factor depends on the diameter of the objective lens and the strength of the ocular lens. Since it may not be possible to increase the objective lens diameter after a certain maximum value, it is the power of ocular lens which makes all the difference to the magnification factor of the image.
  • Brightness of the image depends on the quality of exit pupil. This is the volume of light which enters the viewer’s eyes after leaving the eyepiece. This is said to be derived by dividing the objective lens diameter by the magnification factor. If the exit pupil of a binocular is higher, it means a person wearing eyeglasses can also see the image clearly because the brightness of the image is higher.
  • Field of view is another aspect which determines the quality of image seen by the observer. This specifies the linear width of the filed covered by the binocular as well as the angular coverage. This value is obtained when the linear width is divided by 52.35.
  • Type of focus determines the point from where the focus of the binocular is made to happen on the target object. There are two types of focus, namely the centralized focus and the individual focus. In case of individual focus each telescope in the binocular has its own focal point which gets ultimately focused on the point of convergence at the bridge point nearer to the exit pupil. On the other hand the centralized focus uses a wheel which can be rotated in clock and anti clock wise directions to increase/decrease the focal point distance from the exit pupil. The focal point changes can be effected by rotating adjusting the eyepiece. The rotation of the ocular lens and the eyepiece together can help in increasing /decreasing the focal length of the binocular.
  • Alignment factor is one of the critical parameters of the binocular for producing single image from the collimation of two parallel images from the two ocular lenses. This is done with the help of eccentric rings located in the vicinity of the eyepieces.

A good quality binocular is said to have inbuilt balancing factors to control all the features to get high quality images for viewing. There are four main parameters which depend on each other for producing good quality images. They are the magnification factor, convergence factor, divergence factor and the step.

Apparent Field of View

The field of view which is visible in the binocular is called as the apparent field of view while the actual one as seen by the naked eyes is the actual field of view. There are three factors which can vary this value, namely the vertical misalignment, horizontal divergence and convergence. The alignment of the optical axes to get a single image can be done by reducing the three parameters to the maximum extent to which the existing technology allows.

In spite of the advancements in technology, the three parameters have not been eliminated so far, no matter how advanced the binocular is. Hence the most optimum way is to have a tolerance limit and restrict the error to that limit. For example the vertical misalignment can be allowed up to 15 Arcmin values, while the horizontal convergence and divergence tolerance are said to be 45 Arcmin and 20 Arcmin respectively.

Basic FAQs on How Binoculars work

What is the relation between magnification and brightness?

According to optics, the two factors of a binocular are inversely proportional to each other. So they need to be balanced in such a way that the final image quality is according to the expectation of the viewer. This could be achieved by optimizing the diameter of the objective lens, the exit pupil and the power of ocular lens.

How much coating is required for the Objective lens?

The value of coating on the objective lens and the ocular lens depend on the individual manufacturer. However the standard anti reflection coating on the incident surface can be categorized into single layer and multi layer coating. Single layer coating allows transmission of 98.5% of incident light, while multi layer coating allows 99.5% of incident light to be transmitted.

Why is aperture considered to be important?

Aperture is the light absorption capacity of the lens. This value determines the visibility of the target object under varying intensity of light conditions.

Which type of glass is better for the Prism?

The binocular prism is made from BAK4 or BK7 type of glass. The former is said to be good for exit pupil value. If the viewer holds the eyepiece away from the eyes by a few millimeters and focuses the objective lens towards a source of light, he will be able to see the exit pupil. As mentioned earlier it specifies the brightness factor. This is circular in shape for the BAK4 while it is square for the BK7. BAK4 type of prism is said to be good for bird watching and hunting, while BK7 is more suited for astronomical binoculars. This is said to be due to the long distance transmission capacity. The percentage of peripheral light loss is relatively lesser in BAK4 sine it is circular in nature.

BAK4 type of glass has a higher value of refractive index. The image quality and brightness in the standard range of 1000 yards is relatively higher for BAK4.

What is the relation between field of view and objective lens diameter?

Frankly speaking there seems to be no direct relation. According to optics, a binocular with relatively lower diameter of objective lens can also have higher field of view. Field of view is said to depend on the power and angular coverage of the ocular lens /eyepiece lens.

What are apparent and true fields?

True field is a factor which is obtained by dividing the apparent field by the magnification factor. True field is the view got by the viewer when seen through the eyepiece of the binocular. For measuring the apparent view one needs to look at the sky through the binocular. The angular diameter of the area seen through it is called the apparent view.

Top 5 Binocular Brands and Models

The year 2017 top 5 binoculars are said to be Celestron Nature, Vanguard Spirit, Leupold BX-1 McKenzie, Nikon Aculon and Bushnell Legend.

Celestron Nature

The top five models of Celestron Nature are the Nature DX 8X25, Nature DX 10X25, Nature DX 8X32 Nature DX 10X45 and Nature DX 8X42.

Key Features

Main Parameters

 
Celestron Nature DX 8X25
Celestron Nature DX 10X25
Celestron Nature DX 8X32
Celestron Nature DX 10x42
Celestron Nature DX 8X42

Objective Diameter

25mm

25mm

32mm

25mm

42mm

Magnification

8X

10X

8X

10X

8X

Angular Field of View

7.2°

7.2°

7.4°

5.8°

Exit pupil

3.1mm

2.5mm

4mm

2.5mm

3.2mm

Eye Relief

14mm

14mm

17.5mm

14mm

14mm

Optical Coating

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

All the models are said to be installed with BAK prisms. They are recommended for sporting and bid watching purposes. The user could analyze them based on the exit pupil and eye relief parameters. There are many benefits of having high value of exit pupils. Some of the listed ones from the world of optics are said to be

  • The device is said to have better scope for hunting and bird watching.
  • The viewer is said to have the freedom to vary the positioning of the eyes with respect to the optical axis.
  • Ability to view objects clearly in fading lights gets enhanced.
  • The frame of the device is said to be molded from engineering grade polycarbonate which is said to be one of the toughest materials for heat and cold weather resistance.
  • The device is said to have an optimum focal ratio due to which the exit pupil might appear to be completely round shaped

Optimized values for the magnification are said to be the key features of the Nature DX models of binoculars. A closer look at the above table could make the user understand the close range of values for the magnification versus the other features which are said to be useful for bird watching. Reflection absorption is said to give closer views of birds over water bodies like lakes and rivers.

Vanguard Spirit

The top 5 models of the Vanguard Spirit are said to be Vanguard Spirit ED 1050, Vanguard Spirit ED 1042, Vanguard Spirit XF 10x42, vanguard spirit ED 8x42 and Vanguard Spirit XF 8420.

Key Features

Main Parameters

 
Vanguard Spirit ED 1050
Vanguard Spirit ED 1042
Vanguard Spirit XF 10x42
vanguard spirit ED 8x42
Vanguard Spirit XF 8420

Objective Diameter

50mm

42mm

42mm

42mm

42mm

Magnification

10X

10X

10X

8X

8X

Angular Field of View

Exit pupil

5mm

4.2mm

4.2mm

5.25mm

5.3mm

Eye Relief

19mm

16mm

16mm

19mm

19mm

Optical Coating

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

The Binocular models are said to be fitted with extra low dispersion glasses which is said to be recommended for bird watching as it gives a close range view for small size objects in long distances. The exit pupil for the Vanguard Spirit ED 8420 is said to be ideal for users with eyeglasses.

Leupold BX-1 McKenzie

The top 4 models of this product are said to be Leupold Optics BX-1 McKenzie 10x42mm, Leupold 119198 BX-1 McKenzie, Leupold 10x42 BX-1 McKenzie and Leupold BX-1 McKenzie 8x42mm Roof Prism.

Key Features

Main Parameters

 
 
Leupold Optics BX-1 McKenzie 10x42mm
Leupold 119198 BX-1 McKenzie
Leupold 10x42 BX-1 McKenzie
Leupold BX-1 McKenzie 8x42mm Roof Prism
 

Objective Diameter

42mm

42mm

42mm

42mm

 

Magnification

10X

10X

10X

8X

 

Angular Field of View

5.8°

5.8°

5.8°

6.01°

 

Exit pupil

4.2mm

4.2mm

4.2mm

5.25mm

 

Eye Relief

13.7mm

13.7mm

13.7mm

17.7mm

 

Optical Coating

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

 

Nikon Aculon

The three top models of Nikon Action are said to be the Nikon Aculon 7X35, Nikon Aculon 8X42 and Nikon Aculon 10X50

Key Features

Main Parameters

   
 
Nikon Aculon 7X35
Nikon Aculon 8X42
Nikon Aculon 10X50
   

Objective Diameter

35mm

40mm

50mm

   

Magnification

7X

8X

10X

   

Angular Field of View

9.3°

8.0°

6.0°

   

Exit pupil

5.0mm

5.3mm

4.2mm

   

Eye Relief

11.8mm

12.0mm

11.6mm

   

Optical Coating

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

   

 

Bushnell Legend

The five top models of Bushnell Legend are said to be the Bushnell Legend 10X42, Bushnell Legend 10X26, Bushnell Legend 8X42 and Bushnell Legend 12X50

Key Features

Main Parameters

 
 
Bushnell Legend 10X42
Bushnell Legend 10X26
Bushnell Legend 8X42
Bushnell Legend 12X50
 

Objective Diameter

42mm

26mm

42mm

50mm

 

Magnification

10X

10X

8X

12X

 

Angular Field of View

6.46°

6.46°

6.46°

6.46°

 

Exit pupil

3.60-5.30mm

3.60-5.30mm

3.60-5.30mm

3.60-5.30mm

 

Eye Relief

14.4mm

14mm

19mm

15mm

 

Optical Coating

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

Fully-Multi-coated

 

Main Uses of Binoculars

Hunting Binoculars

Hunting Binoculars

Hunting binoculars need to have high range of focal length, magnification, object sizing, field of view exit pupil and prism type. Individual focus binoculars are recommended for medium and long range binoculars while central focus types recommended for short range hunting purposes.

  • Magnification and the exit pupil of the hunting binoculars need to balance each other, if the hunter is to get the right image size of the object at long distances.
  • The focal length and field of view of the binocular are supposed to have more value of usage than the magnification.
  • The optimum diameter of the hunting binocular is said to range between 32mm and 42mm. Higher diameter could divert the focusing ability for hunting and lesser diameter could give limited view of the target object and its background.
  • Diameter and aperture of the objective lens are supposed to be responsible as the determining factors of aberration, dispersion and focal length of the system. Even the range of coverage and image sensing parameters can have a huge impact on the quality of image and the long and short distance of focusing. The variation of lens curvature with the same objective lens diameter is said to be made possible by the inherent manufacturing and fabricating processes involved with the lenses and prism of the binocular. The user needs to determine the exact attributes of the binocular after visual inspection of the product and sample testing.
  • One of the ways in which the quality parameters of the binocular could be improved is said to be through the reduction of aberrations which are part of the lenses and prism used in the binocular system. This is a fact which is accepted by the optic experts. The inherent aberrations can be reduced by following some simple procedures. Coma error could be corrected by increasing the number of layers in the objective lens or by using glasses with extra low dispersion property. Astigmatism error could be reduced by placing another type of optical object in front of the objective lens with exactly the opposite properties for astigmatism.

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Birding Binoculars

Birding Binoculars
  • Magnification is one of the features highly recommended for the birding binoculars. However this feature is said to be useful for birds which are supposed to be in the medium and long range of distances. For those birds which are in the short range of distances (of course out of naked eye view), the recommended binocular should have an optimum balance between the field of view, magnification, focal length, exit pupil and the ocular lens magnification.
  • Roof prisms are recommended for the birding binoculars as the parts which can optimize the field of view and the magnification factors. It is only by striking a balance between these factors that one can have optimum field of view for the birds which are at short, medium and long distance ranges.
  • The exit pupil of the binocular lens needs to be at optimum brightness and magnification to get a good view of the aquatic birds. This is to avoid the aberrations like astigmatism which might arise due to multiple reflections of the target object from the water body.
  • The recommended objective lens diameter is between 32mm and 42mm for the purpose of getting maximum light and avoiding heavy weight of the equipment.
  • Recommended exit pupil size is between 4mm and 6mm. This is said to be for the purpose of enhancing the field of view during twilight hours and dim light during cloudy weather conditions.

Best Binoculars for the Money

When users are looking for the best Binoculars for the money, it is obvious that they need more value out of the system than their investments. This could be categorized into two sections, namely the economy binoculars for the hobbyists and amateurs and high end binoculars for the professionals. Some such products and models could be categorized based on their level of usage.

Best Binoculars for Hobby Bird Watchers

Some of the recommended binoculars in this category are said to be Carl Zeiss Optical 8×42 Victory HT, Zeiss 8×32 T* FL Victory, Nikon MONARCH 7 10×42, Leica 8×42 Trinovid and Bushnell Legend Ultra HD Roof Prism.

Best Binoculars for Professional Bird Watchers

Some of the recommended binoculars in this category are said to be MONARCH HG 10x42, PROSTAFF 7S 10x42 Realtree XTRA Green, Action Extreme 8x40 ATB, EDG 7x42 and Action Extreme 16x50 ATB.

Best Binoculars for Professional Bird Hunters

Some of the recommended binoculars in this category are said to be Vortex Optics Diamondback 10×42, Carson 3D Series, Bushnell 138005 H2O and Bushnell Fusion 1-Mile ARC.