Binoculars.org

If your out searching for a set of binoculars, the first thing you will need to learn is what the numbers represent in the binocular name - for example 8×42 or 10×25.  What do these numbers tell you?  It is important to have an understanding of these numbers and what they mean.  Not having an understanding of this can mean choosing a binocular that is not good for your desired use.  Its not hard to have an understanding of this in just a few minutes to give you what you need to make the right purchase for your intended use.

Magnification x Objective Lens

When looking at the numbers 8×42 for example, we can break this into two parts - the number before the “x” and the number after the “x”.  The “x” is not representing mathimatical multiplication (8×42 is not 336 in this case), but is rather said “by”, for example 8×42 would be said as “8 by 42“.  The “x” is used to represent magnification or “times”.  Let’s talk a little more about that… 

Your first number 8 in 8x42 is the magnificationpower.  In this case, 8x or 8 times the naked eye, or 8 power.  Your naked eye see’s 1x magnification (meaning no magnification) so 8x would make everything look 8 times closer to you than if you did not look in the binoculars. 

Lesson Learned: The first number is your magnification - this is the “power” or “magnification strength” of the binocular. 

Your second number is your Objective Lens.  In our example of 8×42, the 42 represents the diameter of the objective lens in millimeters.  The objective lens is the outside lens, the lens that is pointing toward your subject. 

Lesson Learned - The second number is the binoculars objective lens size in millimeters

Now you know the basic meanings of 8×42 or 10×25.  Now lets see how these 2 numbers determine so much about the binocular and how it can help you decide what model is best for you.

What does the binoculars objective lens do?

Lets use an example of two binoculars.  One of them is a 10×25 and the other is a 10×50.  Both binoculars share the same 10x magnification - so looking out of each binocular would magnify your subject the same 10 power.  So what will make these binoculars so different?  Its the objective lens that is not the same in this case.  So what does that do for the binocular?

The 10×25 and 10×50 binoculars will be very different looking.  One will be small and compact, and the other will be full-sized.  Your binoculars objective lens will determine the physical size of your binocular.  The larger the objective lens, the larger the binocular.

Lesson Learned - Binoculars objective lens size determines its physical size

So what will be different looking out of them?  Lets stick to the same example of 10×25 and 10×50 and see why these binoculars will produce two different images.  If a binoculars job is to magnify objects, the objective lenses job is to gather light.  The more light the binoculars can gather, the brighter the images will be.  The larger the objective lens size, the more light it can gather.  So in our example, the image in the 10×50 binocular will be brighter than the image in the 10×25.   You may be asking yourself - ”heck, I want bright images so I will only look at binoculars that have large objective lenses”.  Remember, objective lens determines physical size, so what if you want a compact binocular to take in your pocket to the football game?  If physical size or compactness is important, you will have no choice to sacrifice on image brightness.

Lesson Learned - Your objective lens gathers light.

How magnification and objective lens work together

Your binoculars magnification and objective lens determine another important attribute in binoculars, called your Exit Pupil.  Exit Pupil is a measurement of the “hole” in millimeters where light travels through the eyepiece and into your eye for viewing.  Now that you have an understanding of magnification and objective lens, Exit Pupil is pretty easy to determine.  It is simply your objective lens number divided by your magnification number.  For example, a 10×50 binocular has an exit pupil of 5mm.  All 10×50 binoculars have an exit pupil of 5mm.  Just like all 10×25 binoculars have an exit pupil of 2.5mm.  The larger the exit pupil, the more light that is let into your eye and the brighter the images will be - however this isn’t always true - more about exit pupil:

The Exit Pupil of a binocular also is important with how it relates to the pupil size of your eye.  Your eye’s pupil is controlled by the iris which acts like a variable objective lens for your retina and will allow your pupil to change in size from 2mm to 8mm depending on the amount of available light.  Just like your eyes take time to adjust to the darkness, its because your pupil will adjust to the available light to help you with your vision.  On a bright sunny day, your pupil is usually daylight adapted to 2mm-3mm so any binocular with a larger exit pupil will not have any benefit from the extra light that is brought into your eye at that time.  However, if the object you are viewing is in low-light conditions, say in brush or shadow, the larger exit pupil binocular will have a benefit.

Lesson Learned - Exit Pupil has effect on image brightness

Summary:

Now that you understand what 8×42 or 10×50 means you are dangerous enough to get out there and select a binocular.  When selecting a binocular its important to consider two things.  What do I want to use it for, and what am I willing to carry with me.  What good is a binocular for travel if its so large that you don’t want to take it with you.  Also, what good is a compact binocular when you plan to study birds and want to get all the light gathering details you can get.  In general most binoculars with objective lens sizes 26mm and under are considered “compact”.  Binoculars with 30mm-36mm objective lens sizes are considered “mid-size” and binoculars with 40mm-50mm objective lenses are “full size”. 

There are plenty of other attributes to know about binoculars, such as Optical Quality, Field of View, Prism Type, Eye Relief, Close Focus Distance and more - all of which are covered in other articles on this site.