Binocular Magnification Everything You Need To Know

Binocular Magnification: Everything You Need To Know

In this post, you’ll learn more about binocular magnification. You’ll discover how lenses, prisms and eyepieces work together as well other useful information to help you make the best choice for your optic needs.

What are Binoculars?

Binoculars are simply two telescopes side by side, one for each eye. They use glass, known as a lens, to capture light, and prisms to direct that light. The challenge is to direct the light so you get a sharp image that looks the same, just closer, than with the naked eye. That’s the simplified version that most of us are familiar with. Now, let’s dig deeper.

By the Numbers

Binoculars often have a set of numbers, like 7 X 50, either located on the large focus wheel between the barrels, or on the prism cover plate. In How to Get the Perfect Binoculars at the Perfect Price, you learned that these numbers are used to describe:

1) Power, or how many times the object is magnified, and

2) Diameter, in millimeters (mm) of the objective, or large, lens which is opposite of the eyepiece.

The objective lens diameter is the number listed after the “X”.  To get an idea of the objective glass size, you divide the number of millimeters (mm) by 25.4. That means a 42mm objective lens is slightly more than an inch and a half wide. A 50mm lens, then, is almost 2” across. And the bigger the objective, the heavier the binoculars will be. Because the Objective Lens is used to bring in light, larger lens size also means more light for the object you’re viewing.

Most binoculars today are classified as either “fixed” or “zoom”. They have one set of eyepieces, yet there is a growing segment of specialty binoculars with multiple eyepieces.  A brief description of these terms follows.


Binoculars labeled as 7 x 50 means that the observed object appears to be enlarged by seven times when compared to looking without binoculars. If you are looking across a field at a deer that is 800 yards away, by using 10X binoculars it will appear to be only 80 yards away. These are known as “fixed” magnification binoculars.


“Zoom” binoculars allow for changing the magnification. Let’s use 6-12 X 50 as our example. The numbers 6-12 mean that you can change the level of magnification from six to 12 times the apparent size. Zooming happens by moving a lever to increase or decrease magnification. The advantages/disadvantages of zoom binoculars is such a big topic that I will address it in a separate post.

Multiple Eyepieces

One type of specialty binoculars, generally designed for long-range observation, display numbers separated by a “/”.  These don’t zoom. They use multiple fixed-power eyepieces, meaning you switch the eyepiece by turning a turret. Common eyepiece sizes are 8, 12, 14, and 16mm, which have very high magnification and generally require a tripod for stabilization due to their weight.

Others, such as BT’s, (Binocular Telescopes) allow you to change the eyepieces. BT’s have a designation with “BT” followed by the size of the objective lens. Magnification is determined by the eyepiece focal length so magnification isn’t listed. The name tags of BT’s will have other letters to designate things such as “ED”(Extra Low Dispersion) for example,  BT-100XL-ED.  These types of binoculars are generally used for studying the night sky and are capable of magnification up to 90X.  The cost of such lenses can run up into thousands of dollars.

To understand why it doesn’t make sense for everyone to just run out and buy the binoculars with the biggest numbers, we’ll dig into magnification a little deeper.

Lens: Convex and Concave

Glass lenses are referred to as either convex, meaning that the middle is thicker than the sides, or concave, meaning that the middle is thinner. Think of concave as being “caved in” in the middle. Concave lenses make light rays scatter, while a convex lens will suck in the light. A convex lens, known as a converging lens, will make the image appear larger while bringing light rays together. That magnifying glass you used as a kid uses a convex lens.

Opera or theater glasses, along with some very cheap models, still use nothing more than a single convex lens with a single concave lens for the eyepiece. The design is known as “Galilean”. Other very simple binoculars only use two convex lenses.


The problem with capturing light is that the rays cross over. That means the image you’d see is flipped. Prisms turn the image. What is a prism?  It’s a big triangular piece of glass. Two are used because each prism can turn the image only 90 degrees. If the prisms are at 90-degree angles from each other, they are known as Porro prisms. When the prisms are arranged back to back, they are known as roof prisms. Roof prisms can ping-pong the light back and forth more than Porro prisms can. But wait! That doesn’t mean they are the best solution for all uses. Roof prisms don’t have the added depth and brightness you get from the original Porro prisms. And, they cost more.

Prisms take up space, and are a big contributor to the weight, that’s why Field glasses don’t use them. The images are flipped only by using lenses. They offer lighter, more compact, binoculars at a cost of lower-quality images.

How Do Lenses and Prisms Work Together Then?

Light slows down and bends when it hits a lens. That’s called “refraction”. That means the objective lens sends an upside-down, magnified light down the tube. The prisms rotate the light, bounce it back and forth a little, and send it through to the eyepiece to give you a magnified view. This trip that the light takes is called the “focal path”. And when the focal path is longer, magnification is greater. That’s the other great thing prisms do.

A few words about the Eyepiece

Earlier I mentioned that simple binoculars are made of just a couple of lenses. Today, many eyepieces are made up of three or more lens elements in two or more groups. There is often an eye lens and a field lens within the eyepiece. The eye lens is located closest to your eye, while the field lens is furthest from the eye.

In 1849 Carl Kellner invented the most common configuration used today. The arrangement is a plano-concave/double convex achromatic doublet (the flat part of the former faces the eye) and the field lens is a double-convex singlet. In 1975 the reverse Kellner was developed. It works better with small focal ratios, has a slightly wider field, and provides 50% more eye relief.

Effects of Magnification

Basic physics state that higher magnification will decrease image brightness assuming no changes to the objective diameter or to the optic quality. Put another way, you’re going to lose color and detail as you zoom in on an image.

You will also lose the region of sharp focus (depth of field) in the image as magnification goes up. Objects that are closer are affected more since the depth of field is greater at longer distances. Imagine watching an eagle flying over a lake just fifteen feet away. As long as it stays in the range of around 14 – 17 feet away, you still get a sharp image. You see another eagle that’s about 100 feet away. You get a sharp image while following it as it moves between 80 and 120 feet. But once the closer bird moves just a few feet further away you have to refocus. Constant changes like this will lead to eye fatigue.

You also lose your Field of View as you increase magnification. Field of View is defined as the width of an area found in your binoculars image. It’s either expressed as an angle (9̊), or as the width in feet of an image at 1000 yards. So, as an example, I’ve been able to spot deer in a wooded area with lower magnification (8X) and higher objective diameter, than my friend with 20X magnification because my field of view was better. It’s common for younger children and beginners to have trouble finding objects in smaller fields of view.

Final thoughts

There are many types of optics available today. New materials, designs, technology, and attachments, provide solutions such as anti-fogging, ED glass, stabilization with a VR (vibration reduction), and overall optical quality to create the right optics for you for your specific situation. Magnification is just one aspect to consider, keeping in mind that higher magnification generally means greater weight, more shakiness, a lower field of view, and lower light. Understanding how magnification works now will help you make a better choice.

Keep tuned here for more information to help you find the right products so you can enjoy your viewing experiences.