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