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.)