BINOCULAR TERMINOLOGY

What do the numbers mean on binoculars and spotting scopes?

This is the most common question about binoculars and it is a good one. You will see combinations of numbers like 7x35, 8x42, 10x50 or 7-15x35 and many more. The numbers to the left of the "x" always refer to how much magnification the binocular has. The number to the right of the "x" indicate how big the lens is at the bottom of the binocular (this is called the objective lens).

So, 7x35 means this binocular magnifies objects so they appear 7 times closer. This binocular also has a 35mm diameter "objective lens". A 10x50 binocular magnifies objects so they appear 10 times closer and has an "objective lens" 50mm in diameter. The 7-15x35 binocular is a zoom model because you can change the magnification from 7 times to 15 times. This binocular has a 35mm "objective lens".

Should I buy a binocular with the highest magnification possible?

Not necessarily. Here are a few things to consider when selecting a binocular to determine what magnification will be best for you.

Generally, the higher the magnification the more difficult it will be to have a bright, steady and wide image. Why is this? Try thinking of magnification in terms of using your portable video camera.

What happens when you "zoom" in on objects (using the highest magnification)?

You will find that several things happen:

• The image is "jumpy" (it is difficult to hold the camera steady)
• You only see a very small "field of view" or viewing area
• The image is not as sharp and clear
• The image is not as bright

Generally speaking when you increase magnification you may increase the possibility of all the above results. Most binoculars sold today range from 7x to 10x. Zoom models offer a range of magnifications from 7x to 20x or even higher.

Why is brightness important and how do I determine how bright a binocular is?

The brighter the image when looking through a binocular or spotting scope the better chance you will have of seeing a sharp and detailed image of objects viewed.

Brightness if affected by:

• How big the "exit pupil" is of the binocular. (this is an easy thing to determine and is explained below).
• The types and number of "coatings" applied to the optical system.

What is an "Exit Pupil"?

The exit pupil is the circle of light you see when holding a binocular or spotting scope about 6 inches away from your eyes and looking at the surface of the eye pieces. This little bright circle is the amount of light reaching your eyes.

The size of the exit pupil is easy to calculate. You simply divide the magnification of the binocular into the diameter of the "objective lens" or bottom lens of the binocular or spotting scope. So, a 7x35 binocular has an exit pupil diameter of 5mm (35 divided by 7). A 10x50 binocular also has an exit pupil of 5mm (50 divided by 10).

Why is "Exit Pupil" important?

You can use the diameter of the exit pupil calculation to determine how much relative light is available under different conditions. When we are in bright light our own eye pupil constricts to as small as 1mm in diameter. However, in very low light such as in the afternoon or early morning our eye pupil can expand to around 7mm. Our pupils open and close to allow enough light to see details and a sharp image.

The "exit pupil" on a binocular can be used to correspond to our own eye pupil. Generally a 5mm "exit pupil" will provide good brightness over many conditions. But in low light conditions it will not provide enough light to see as your own eye pupil may want. The result is a darker image and you may not see as much detail.

What are lens coatings?

Coatings are applied to the glass elements to reduce the amount of light with is lost due to reflections. Generally binoculars and spotting scopes are coated with a single layer of coating applied to each glass surface. More expensive models have multi-coatings applied with further reduces the amount of light lost to reflections.

What is "wide angle"?

Binoculars typically come in either standard or wide angle designs. Wide angle designs provide a wider field of view than standard designs. Field of view is simply the area which you can see at a certain distance. The image viewed is usually measured in the number of feet you can see across at a distance of 1000 yards. A binocular showing a field of view of 430' at 1000 yds. means you can see a horizontal distance of 430 feet when looking out to a distance of 1000 yards.

What is "long eye relief" and why is it important?

Eye relief is the distance from the surface of the eye piece lens where you can see the full field of view of the image you are looking at. Most binoculars have a standard eye relief distance of from 9mm to 12mm. This is long enough for just about everybody unless you wear eye glasses and you don't feel like taking your glasses off to see through the binocular!

Long eye relief is primarily a characteristic which specifically benefits people who wear eye glasses. But it is also great for everyone else because it is great to use them when wearing sun glasses or eye glasses! Some binoculars with long eye relief do not have a "wide angle" field of view. This is because when designing optical systems it is difficult to provide both long eye relief and wide angle in the same system.

What is a "fixed focus" binocular?

These binoculars are great because they don't have the traditional focus wheel or eyepiece focus adjustment. They are great for action sports viewing or viewing situations where you don't want to loose time having to adjust your binocular. These binoculars are designed so that you can see objects from about 70 feet away and beyond. You just pick them up and look!  The long eye relief model is great if you wear glasses!

What is the difference between a "porro prism" binocular and a "roof prism" binocular?

Porro prism binoculars have the "offset" design. The eye pieces are not the same distance apart at the objective lenses. Porro prism binoculars offer great viewing performance, are more "traditional" and are generally less expensive to manufacture.

"Roof prism" binoculars are distinguished by their "in-line" appearance. The eye pieces and objective lenses share the same axis. These binoculars also offer great viewing, are less bulky, and generally more expensive to manufacture.

Both prism designs can provide great viewing and it is generally just personal preference that determines which one is best for you.

What is different about compact binoculars?

Compact binoculars are developed to provide a small, lighter weight binocular which can be more easily carried. The advantages of compact binoculars is their small size, convenient use and they are great when you want to transport them with you when hiking or when larger binoculars are just not convenient to use.

But compact binoculars generally do not offer the brightness that larger binoculars do. Remember the discussion above about "exit pupil"? Compact binoculars generally do not have large exit pupils which means in low light conditions you will not see a bright or sharp image. For example, a 10x25 compact binocular has an exit pupil of only 2.5mm. During the day this is not a problem. But for early morning or late afternoon viewing they will not provide as much light as a larger standard design binocular.

How do I decide which binocular is best for me?

Buying a binocular involves "trade-offs". In order to get the brightest possible image in low light conditions you will need low magnification with a large "objective lens". But many of us don't want to sacrifice magnification. After all isn't this why we are buying a binocular? We want to see objects far away as though they were close! You may also want a "wide field of view" so you can see as much action as possible at a sporting event. So, you will find that you need to know how, when and where you are most likely to use your binocular. In the end you may decide to buy several different models to meet the needs of the different types of viewing situations you intend to enjoy.