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Binoculars & Optics Guide

Whether you are a professional birder, an experienced hunter, or simply a casual observer, understanding how your optics work is an important step to getting the most from your outdoor adventures. After reading this guide, we hope that you will not only have the necessary knowledge to purchase the best binocular for your intended use and budget, but that you will have the necessary knowledge to operate your binoculars where it matters the most – in the field.

Please note that while this guide attempts to be concise, optical technologies and their uses can quickly become complex and seemingly overwhelming. Do not hesitate to contact us with questions that you may have, or with your own tips and tricks to get the most from your optic of choice. Enjoy!

The Basics

While different manufacturers and retailers may use various terms to describe their binocular’s features, there are a handful of terms that can be used to describe all binoculars. These terms are explained below. Depending on where, when, and what you want to use your binoculars for, some of these terms will be more important to your decision than others.

Magnification – This refers to how many times larger an object will appear when viewed through the optic compared to the size of the object when viewed with the naked eye, and is the first number used to describe a binocular (ie an 8 x 42 binocular has a magnification of 8 times). If you are using an 8 x 42 binocular to view an object that is 100 feet away, the object will appear to be the same size as if it were 12.5 feet away from you when viewed without magnification (100 ft divided by 8x = 12.5 ft).

It may be intuitive to think that a higher magnification is better, but this is not always the case. Optics with high magnifications (above 10x) are great for viewing objects across relatively open spaces with unobstructed views (an elk from the top of a ridge), but generally have a narrower field of view and benefit from a very steady hand or tripod. Optics with lower magnifications (6x to 8x) are better for viewing objects which are already relatively close by (a bird perched on a feeder in your backyard or a branch in the woods) due to their wide field of view and decreased shaking.

Objective Lens Size –The objective lens size refers to the size of the front lens of the optic (in millimeters), and is the second number used to describe a binocular (ie an 8 x 42 binocular has an objective lens size of 42 mm). Though optics with a larger objective lens will capture more light than optics with a smaller objective lens, the difference compared to the smaller lens is not always apparent and may mean that you’re carrying around a heavier optic than necessary. Many people detect little to no difference between a 26 mm and 56 mm objective lens in bright, sunny conditions, but opt for the larger objective lens if they plan to use their optic in low-light conditions such as a shaded forest or at dawn/dusk.

Exit Pupil Size – The size of the diameter of light that passes through the optic and hits your pupil, expressed in millimeters. The exit pupil size is related to both the magnification and objective lens size of the optic, and is calculated by dividing the objective lens size by the magnification. A 10 x 42 binocular will therefore have an exit pupil size of 4.2 mm (42 mm divided by 10x magnification = 4.2 mm). While bigger is generally better when it comes to an optic’s exit pupil size, an individual’s own pupil may limit the amount of light that is able to reach their retina.

An individual will not be able to take advantage of any exit pupil size that is greater than the maximum size that their pupil can dilate to. If you purchase a binocular with an exit pupil size of 7.0 mm, but your pupil only dilates to 5.0 mm, you may be paying for a benefit that you actually cannot use. The moral: choose the binocular with the largest exit pupil size that your eye is able to use.

Prism Type (Porro vs Roof Prism)

Porro Prism – The objective lenses on this style of binocular are spaced wider apart than the eyepieces, leading to a wide field of view and a 3D image with a rich depth of field.

Reverse Porro Prism – This type of binocular is similar to the Porro Prism style, but is more compact since the objective lenses are spaced closer together than the eyepieces.

Roof Prism – While roof prisms are named because of their roof-like appearance, the objective lenses on this type of binocular are in line with the eyepieces, creating a very intuitive and streamlined design.

Optical Glass – Different manufacturers use different names, but there are generally two types of glass used in binoculars and viewing optics: standard glass and Extra-low dispersion glass. While brightness, sharpness, and color will be ‘ok’ when viewed through standard glass, extra-low dispersion glass will provide the best resolution, contrast, and color.

Anti-reflective Lens Coating – Your binocular or optic works when light enters the lens, moves through the prisms, and enters your eye. Coating the lenses and prisms in an optic reduces the amount of light that is reflected out of the optic, resulting in more light making its way to your eye and a better image. Manufacturers apply coats of metallic compounds to their lenses and prisms to reduce reflection.

Fully Multi-Coated (FMC) – The highest level of coating. All of the lens and prism surfaces exposed to air are coated with multiple layers of anti-reflective coating. Fully Coated – All of the lens and prism surfaces exposed to air are coated with anti-reflective coating.

Multi-Coated – At least one or more lens/prism surfaces that is exposed to air is coated with multiple layers of anti-reflective coating.

Coated – One or more of the lens/prism surfaces that is exposed to air is coated with at least one anti-reflective coat.

Interpupillary Distance (IPD) – The distance between the centers of an individual’s left and right pupils which correlates to the IPD range of a binocular. If your IPD is outside of a binocular’s IPD specifications, you will see shading over part of the image while viewing. While the IPD range of binocular is a non-issue for most users, it may be something to consider for younger children.

Eye Relief – The eye relief refers to the distance from the lens that you look into and where the image is viewed correctly (ie where the image is in focus and the full field of view is seen). Individuals with glasses may need to pay more attention to the eye relief of an optic compared to individuals who do not require eyeglasses. In general, a larger eye relief benefits eyeglass users.

Close Focus – Close focus refers to the minimum distance that the optic has the ability to focus to. Everything else constant, a smaller close focus is better, especially for viewing birds, butterflies, and other wildlife.

Field of View – The field of view refers to the distance between the left and right edge of the image and is normally expressed in feet at 1,000 yards (ie 425 ft at 1,000 yards), or in angular degrees (ie 8.0°).

Putting it all Together

We know that this is a lot of information to take in initially, and you still may not completely understand every complexity or nuance relating to picking the right binocular for your needs. That’s OK! Refer to this guide while comparing different binoculars so that you get the best image and value possible.