The Binocular Blog - Part 6: Image Brightness

In the previous post we had looked at the Exit Pupil, one of the factors that influence the brightness of the image that falls on our retina. There are also, two more factors that come into play, both related to the construction of the binoculars.  

Any light that reflects off the object you view with the binoculars must pass through a series of lenses and prisms before it reaches the eyes. If you want to see that object in greater detail, more light must pass into your eye the image should be as bright as possible. For that, the binocular should fulfill two criteria in its construction.

1. It should have a larger objective lens, ideally more than 40 mm and if possible, for birders, 50 mm.
2. It should have high quality glass lenses with appropriate coatings.

OBJECTIVE LENS

This is the first element through which light enters the binocular on its way to your eye. Like in lenses for cameras, the wider the 'aperture', brighter the image. The objective lens size, as explained in a previous post, is the second number (after the 'x'), on the binocular. In a binocular with a 10x25 specification the objective lens diameter is 25 millimeters. Similarly in the binoculars with specifications of 8x42 or10x42 the objective lens diameter is 42 millimeter. 

Binoculars with varying objective lens diameters. (L to R: 25 mm, 42 mm, 50 mm)

In two binoculars of similar magnification, like the 10x25 and 10x42 the binocular with the objective diameter of 42 will produce a brighter image in low light situations. Since most birds and wildlife are more active in the early hours of the morning or late evening, it goes without saying that a brighter binocular will be more useful. 

The only downside is that binoculars with larger objective lenses are naturally bulkier and therefore heavier. Especially ones with objective lenses of 50mm or wider.

LENSES & PRISMS

All binoculars use a combination of glass lenses and prisms. The quality of the final image is dependent largely on the quality of the glass used.

Less expensive binoculars use Crown glass and the more expensive ones use Flint glass. If the quality of the glass is poor the image quality will be compromised by chromatic and spherical aberrations. Chromatic aberration refers to the colour fringing of the edges of the object viewed, typically a bluish or purplish  blurred edging. 

 

Chromatic aberration - significant. (Pic Courtesy: Olympus Asia)

Chromatic aberration - reduced with ED lenses. (Pic Courtesy: Olympus Asia)

Spherical aberration refers to the loss of sharpness towards the edges of the image and thus affects clarity. This is overcome by using aspherical lenses. The highest quality binoculars use HD (High density) or ED (extra-low dispersion) glass, called Fluoride glass as it contains calcium fluoride. 

Some binoculars use plastic lenses. Plastics will make the binocular lighter and less prone for lens breakages during rough usage. However, when it comes to light transmission glass lenses are always better. Since glass reflects light more than plastic, they should be coated.

Lens without coating. (Pic courtesy: Olympus Asia)

Lens with coating. (Pic courtesy: Olympus Asia)

The coated lenses also come in four types, depending on the number of anti-reflective coating on the lens surfaces.

1. Coated (C) – A single layer of anti-reflective coating on one or more lenses

2. Fully coated (FC) –  A single layer of coating on both sides of the objective and ocular lens groups and also on the prism.

3. Multi-coated (MC) – Multi-layer coating applied to one or more lens surfaces, and single layer coating on others.

4. Fully multi-coated (FMC) – Multi-layer coating applied to all the lens surfaces.

Details of  such coating are usually found on the box and it would be a good idea to read all those specifications printed on it. 

Bushnell Trophy (FMC)

Vortex Viper HD (FMC)

Zeiss Terra ED (MC)

 Some older binoculars have it printed on the instrument itself.

Super Zenith (FC)

Surprisingly the four  decade old Zenith is brighter and clearer than some of the newer, inexpensive binoculars. Therefore, there must a major compromise on lens quality and coatings to reduce the price.

Similarly, the prisms in the binoculars are of two types. The two commonly used prisms are the BK-7 (borosilicate crown glass) and BAK-4 (barium crown glass), of which the latter is better at light transmission. These prisms are also given coating called the phase-correction coatings. The BK-7 prisms are used in the lower magnification and less expensive binoculars, wheres the BAK-4 prisms will be found in more expensive ones as the image quality in the latter is far more refined than what will be seen through the former.

The type of prism us usually not mentioned on the box but it is easy to make out which one is inside your binocular. Hold the binoculars a few inches from your eyes and look at the exit pupil. If it is perfectly circular, it has BAK-4 prisms but if it has square shadowy edges it has BK-7 prisms.

BAK-4 prisms in the Zeiss Terra ED 8x42

BK-7 prisms in my old Zenith 7x50. The grayish edges are made out easily.

 The final quality of the image seen will be determined by  combination of factors that include all the parameters mentioned in the last four posts. (Parts 3 to 6). Better the quality of the image, the higher the quality of its components and therefore proportionately increasing cost!

In the next post we'll see how to view the image properly and clearly with your binoculars.

For those who are impatient to go through the entire blog series, I'm giving below direct links to all the posts. You can go directly to the page you are interested in, though I suggest you go through each post as all of them contain some information that will be useful when you consider purchasing a binocular for your self.

1.       Introduction
2.       Part – 1: Anatomy of a Binocular
3.       Part – 2: Purpose and Portability
4.       Part – 3: Magnification, Objective diameter & FOV
5.       Part – 4: Eye Relief
6.       Part – 5: Exit pupil & Interpupillary distance
7.       Part – 6: Image brightness
8.       Part – 7: Focusing fundamentals
9.       Part – 8: Weather sealing
10.   Part – 9: Purchase decision
11.   Part – 10: Care & Storage
12.  Afterthoughts and accessories

The Binocular Blog - Part 1: The Anatomy of Binocular

Link to the previous post is here

Before making a decision about which binocular to buy you should familiarize yourself about the structure and function of the various components of the binocular. The binocular is essentially two monocular telescopes mounted together by a hinged bridge that usually carries a focusing ring. It is one of the most frequently purchased optical instruments anywhere in the world. While it looks like two simple tubes from outside, a lot goes into designing one. There is one for everyone and for every situation.

Anatomy of a Binocular (Pic courtesy: Olympus Asia)

Most early binoculars were simple telescopes with a convex lens in the front and a concave lens at the rear to erect the inverted image. They are the Galilean binoculars and were simple instruments  with limited magnification. Even now they are available in the form of opera glasses and toy binoculars. Modern binoculars use Keplerian optics, with convex lenses in both the front and rear of the telescope to achieve higher magnification. However, the resulting image is inverted and has to be made upright by some intervening prisms. (See the illustration above)

The lens that faces the object you are viewing, on the front of the instrument, is the objective lens and is the first component light encounters on its pathway to your eyes. The light then travels through the barrel, gets bounced off the prism surfaces and then exits through the eyepiece or ocular lens, into the viewer's eyes. The eyepiece lens group is also responsible for the magnification achieved by the binoculars. In all modern binoculars the objective and eyepiece lenses  are not made of a single lens but   combination of convex and concave lenses in groups. 

Objective and eyepiece lenses. (Pic courtesy: Nikon)

There are two basic types of binocular designs.  The traditional Porro prism binoculars in which the objective lens and the eyepiece are not aligned in a line and has a 'M' shape. In the sleeker roof prism binoculars the objective lens and the eye piece are in a straight line and has a 'H' shape. 

Older binoculars had the prisms that were in a Z shaped configuration and set apart. These are called Porro prism binoculars and are the design we are familiar with. Due to their design difference the light is transmitted through the Porro prism binoculars in a zig-zag path shape.The result is that Porro prism binoculars are bulkier but comfortable to hold and produce a steady image. They are also easier to adjust and give a better depth perception.    

Light transmission in binoculars (Pic courtesy: binocularsinsight.com)

In contrast,the newer binoculars, called roof prism binoculars have the prisms set back-to-back and aligned in a straight line with the objective and eyepiece, so the light passes without deviation from the objective lens to the eye. Therefore prism binoculars are sleek but need steadier hands and are less adjustable because of the closer setting of the oculars. People wide set eyes might find the pupillary distance adjustment not entirely to their satisfaction.

The focusing mechanism in both types are also different.The focusing wheel on Porro prism binoculars moves the eyepieces forward and backward to achieve optimum focus. This increases the length of the binocular and also makes it difficult to make it dust proof or waterproof.  

Porro prism binocular has external focusing by moving the eyepiece.

Whereas, in the roof prism binoculars the focusing is by shifting a group of lenses within the barrel so there is no change in the length of the instrument.These internal focus lenses are usually located between the objective lens and prisms and is moved by turning the focusing knob on the bridge.

 

Focusing ring on the roof prism binocular.

There are surprisingly a lot of different parts and accessories for an instrument so small. Technical descriptions are beyond me but any binocular user should know its different parts if you are planning to use one. Here are a series of photos showing the different parts of a binocular.

Once you familiarize yourself with the various parts and their names the rest of the blog that will be dealing with the functions will be easier to follow. The next blog deals how to select a binocular that suits your purpose. 

For those who are impatient to go through the entire blog series, I'm giving below direct links to all the posts. You can go directly to the page you are interested in, though I suggest you go through each post as all of them contain some information that will be useful when you consider purchasing a binocular for yourself.

1.       Introduction
2.       Part – 1: Anatomy of a Binocular
3.       Part – 2: Purpose and Portability
4.       Part – 3: Magnification, Objective diameter & FOV
5.       Part – 4: Eye Relief
6.       Part – 5: Exit pupil & Interpupillary distance
7.       Part – 6: Image brightness
8.       Part – 7: Focusing fundamentals
9.       Part – 8: Weather sealing
10.   Part – 9: Purchase decision
11.   Part – 10: Care & Storage
12.  Afterthoughts and accessories

(Apologies! Some parts of the blog has the font in a very small size and I'm quite unable to get it to  larger size despite several attempts. Can't seem to find a plausible reason for it.)