Choosing My Binoculars

One of the things that came out of the JLR Naturalist Training Program (for me) was the need felt for a pair of binoculars. I had trouble with a pair some years before, and had experienced some serious vignetting (blackening of the edges) while trying to look through them. I figured that this had to do with my wearing glasses, but not knowing the first thing about binoculars, I didn’t realize this was an addressable issue, nor did I really apply my mind to it. I simply stayed away from using binoculars instead. (I was aware of the dioptric adjustment, but that has never worked for me, possibly due to my astigmatism or more likely, ignorance).

We were handed Olympus 8X40 binoculars during our field walks in the NTP and using them, I wondered how I managed all these years without a pair. Binoculars are a truly invaluable aid to the serious nature enthusiast.  I duly set out hunting for a pair to buy, as soon as the NTP was done. Beyond being able to spell the word correctly, I knew next to nothing about binoculars. Karthik explained a little, I learned a little more from Dr. R and other folks around, and subsequently read up still more online to build a rudimentary picture in my head. This post seeks to record some of this basic understanding.

We know that the binoculars’ primary specification is represented as two numbers separated by a cross. 8X40, 10X50 and suchlike. Let’s take the 8X40 for our illustration. The 8 represents the degree of magnification – what you look at is magnified 8 times. The 40 represents the size of the outer or larger lens (the objective lens), in millimeters. Why is this important? Larger the objective lens, more the light the binoculars captures. You can view that bird better in late evening light if you have a larger objective lens. Corresponds somewhat to the aperture measure in photography terms.

Now there is also a relation between the magnification and the size of the objective lens. If you bump up the magnification, you should logically have a commensurately larger objective lens size to be able to gather enough light. This relationship is measured by what is called the Exit Pupil. Size of the objective lens divided by the magnification. 40/8 in our example is 5. And this is represented in millimeters:  hence, an Exit Pupil of 5mm.

With this in mind, there are three other basic concepts that are worth addressing.

There is a parameter called Eye Relief, measured in millimeters, which broadly represents the max distance from which you can look into the eyepiece without experiencing vignetting. Why is this important? If you wear glasses, you will have to allow for the intervening distance from the eyepiece, and that is precisely the problem I had run into earlier. The binoculars need to have an eye relief measure large enough to accommodate your eyeglasses in the way.

There is also a self-evident measure of the minimum distance the binoculars can focus from.

Last is in terms of body design. Porro Prism and Roof Prism. Readers who are familiar with binoculars terminology will kindly refrain from cringing at my crudely dumbed-down interpretation of these two terms. I’ve been at this for all of two days and have the most basic degree of understanding possible. Porro Prism binoculars have the typical binocular shape familiar to us all, where the eyepiece and objective are not aligned along the same line. The eyepieces are in two little cylinders which are parallel to each other, and these two lie between the two larger cylinders which contain the objective lenses. Roof Prism binoculars have both the eyepiece and objective lens affixed at the two ends of a single cylinder, to put it very crudely. This only describes what they look like. I didn’t gain too much of an understanding around how each one corrects the inversion of the image, or what their pros and cons are. Once I figured out this understanding didn’t really matter as far as my immediate choice of purchase was concerned, I abandoned that trail. So much for my scientific temper.

Dr. R was kind enough to share a crystal-clear guide to choosing a pair, aimed at the beginner. This piece is by Sumit Sen, and is on the Kolkatabirds website. Here’s the URL:

From this guide, I pecked out the following normative checklist:


Desirable   Value

Magnification & Objective 8X40 or 10X50; indicates 8X40 good enough for beginners
Weight < 1 kg
Exit Pupil (Objective/Magnification) > 3 mm, < 7 mm
Eye Relief (from a glass-wearer’s perspective) 15 mm +, with fold-down eyecups
Close focus < or = 10 feet
Body design Porro Prism or Roof Prism, either is okay
Weather proofing Nitrogen purging is preferable, weatherproofing is nice-to-have
Coating Multi-coating is best, though a nice-to-have

Scouting around, I found the Nikon Action 8X40 EX (Porro Prism) retailing at a little over Rs. 11K on This model conforms to all the specs on the checklist with the single exception of close focus distance. ~16 feet as against the normative < 10 feet. I’ll probably go ahead and buy it anyway in a week or two.

Post Script: You knew the Caveat Emptor was coming. Readers will pardon any inadvertent errors in this post. Being quick and accurate on the uptake is not one of my strengths, and it is quite possible that I’m completely off on some of this stuff.

9 thoughts on “Choosing My Binoculars

  1. deponti says:

    Nice review of binocs 😀


  2. Balajee says:

    Very Informative post.
    So when are you buying ?

  3. Ravindra says:

    succinct review Badri! Good one !

  4. Rekha says:

    Very well written & informative, Badri…

  5. Ram Sharaph says:

    Nicely explained in a simple manner Badri – a good guide for beginners to familiarise themselves with the basics of binoculars. 8×40 seems to be good enough for most hobbyists. 10×50 is desirable, but is on the bulkier side. Olympus and Nikon both make good ones, but the droolworthy one is Carl Zeiss – the brightness of which I have not seen in any other.

    • Badri says:

      Thank you Ram. Agree completely about the Zeiss. Another constraint with the Nikon 10X50 vis a vis the 8X40 is the minimum focusing distance. While the Nikon 10X50 is bad enough at ~16 feet, the 10X50 is worse, at ~22 feet. The price differential between the two is not much. Incidenally, I did order the 10X50 finally, notwithstanding this constraint.

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