The Binocular Blog - Part 3: Making sense of the Numbers

Now that you have finally decided  to put some money on the table for your binoculars, it is time to understand some slightly technical aspects of it. Understanding some aspects of the instrument will help you make an informed decision and thus choose the right binocular for your purpose.

All binoculars, besides the brand and model names, will have some numbers inscribed on them. Deciphering these numbers are very important instead of randomly picking one pair for yourself. They are usually found etched on the barrel, punched on the body, on the focusing knob and just about anywhere the manufacturer fancies. 

On the barrel

 

On the rear barrel cover

On the focusing knob

 

On the eyepiece ring

Some will have only two  numbers and the third might be found in the only on the box or user manual. So make sure you read the small print on the box also.

On all binoculars, irrespective of brand, you will find two numbers without fail. A combination, on either side of an 'x', (7x50, 8x42, 12x50 and the like).

The first number is the magnification factor of the binoculars. It refers to the number the object viewed, gets magnified. The other way to look at it is how close the object appears. For example, an object at 500 meters viewed with a 10x binoculars will appear as if it is at 50 meters. You might get tempted to go for a higher magnification, because you want to see things bigger or closer. However, remember, the downside of high magnification is that if you have shaky hands, the shake is also magnified.

Therefore, chose the magnification that suits your need. For sport or wildlife viewing a 7x or 8x should suffice. If you are a keen birder an 8x will take care of most of your needs but for small birds a 10x will be a better option. Beyond this, 12x and more, will require steady hands or a tripod. Anything higher than 12x (astronomical binoculars) will also come with larger objective lenses and will be quite bulky and heavy for hand holding. So select a magnification depending on your usage.

The second number after the ‘x’ refers to the diameter of the objective lens in millimeters. Larger the diameter, bigger and heavier the binocular will be. The objective lens is the first glass element of the binocular, on which light hits as it enters to pass through to the viewer's eyes. 

Objective sizes

Larger the objective lens, the brighter the image will be. It also means you'll have a heavier binocular to carry around.

 

Some binoculars will also have a third number referring to the field of view (FOV) or angle of view. The field of view is the area of the object encompassed by the binocular image. Akin to the visual fields, it is measured in degrees or in feet and is the measurement of the width of the area visualized. The bigger the number the wider the field of view.

FOV in feet

Binoculars with lower magnification power or larger diameter objective lenses have a wider FOV. The linear field of view is given in feet at 1000 yards as in the binocular in the above photo. Some binoculars give FOV as an angle and the linear field of view can be calculated by multiplying the angle by 52.5. Which means, in the photo below, the binoculars with a lesser magnification, has a linear field of view of 372.5 feet at 1000 yards.

FOV in angle

Of the three, the field of view may not be a particularly important factor but for fast-moving animal or birds in flight, it will be easier to keep the target in sight with a wider field of view. In general, for stationary objects, the field of view is of hardly any concern. Therefore, these three numbers should necessarily be taken into consideration when you make your final decision.

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