All About Binoculars

Everything you need to know about marine binoculars.

Reading our All About Binoculars page will help you get more value out of our marine binocular review pages. If you're versed in some of the specific terminology we use when describing functions and features it will be a benefit.

One area where some binocular makers tend to fudge a little, esepcially in less expensive sets, is in their eye relief numbers. You should see the text in each review regarding this specification how it effected performance.

Using a marine binocular, in this case the Steiner Commander XP to scan the horizon.

On more expensive marine binoculars our testing has shown the eye relief numbers supplied by the maker to be accurate.

You'll find our all about binoculars list of useful terminology below.

All About Binoculars Terminology

Armor – Plastic or rubber coating that protects the binocular from dents and dings, provides a sure grip, and makes them easy to clean. A must for binoculars used in the marine environment.

Binocular - A hand-held tool used to magnify distant objects by passing the image through two side-by-side lenses joined by a hinge. Creating an image for both eyes simultaneously provides a sense of depth perception. Binoculars are available in a great variety of sizes, magnifying powers, and features to suit any purpose or preference.

Coated Optics – Microscopically thin lens coatings designed to reduce light loss and glare while increasing both sharpness and contrast. Coatings can range from a single layer on one glass surface to multiple coatings on all air-to-glass surfaces, and depending on color can affect the color temperature of the light passing through the binoculars.

Diopter Adjustment – A moveable focusing ring on the eyepiece allowing the user to adjust the focus for any differences in vision between eyes. A negative diopter value signifies Myopia while a positive diopter value signifies Hyperopia. Binoculars with a main center focus should have a single diopter adjustment while binoculars. Others forgo center focus and use a diopter adjustment for each eye as the focusing mechanism. In high-quality binoculars, the diopter adjustment is clearly marked and accurate enough to be set to an eyeglass prescription.

Exit Pupil – The diameter, in millimeters, of the circle of light leaving the binocular eyepiece. A larger exit pupil provides a brighter image and makes the binoculars easier to use in low light and at night. The size of the exit pupil is calculated by dividing the diameter of the objective lens by the magnification. The majority binos we’ve tested have a 50-millimeter objective lens and a 7x power rating yielding a hefty 7-millimeter exit pupil. In the dark, the average adult will only have a pupil aperture of 5 to 6 millimeters. This means the binoculars are providing more light than most eyes can use, a good thing in the sometimes dark and stormy marine world.

Eye Relief – The distance, in millimeters, the binocular eyepiece lens can be held away from the eye with the user still able to see the full field of view. Longer eye relief generally provides more comfortable viewing and is essential for eyeglass wearers. Use our reviews to qualify this number.

Fog Proof – Sealed binoculars that are nitrogen purged or filled to prevent fogging due to rapid temperature or humidity changes.

Interpupillary Distance – The distance, in millimeters, between the centers of the pupils of the eyes. The average interpupillary distance in an adult is between 58 and 72 millimeters. The hinge between the lens tubes allows a binocular to be adjusted for various interpupillary distances.

Magnification – The power of the binoculars. Nearly all the binos we’ve tested are 7x, meaning they make an object appear 7 times closer than it would with the naked eye. This is considered the maximum usable magnification in the marine environment.

Objective Lens Size – The diameter of the front lens measured in millimeters. A larger objective lens allows more light to enter the binocular and provide a brighter image. A binocular is commonly referred to by its magnification and objective lens diameter. The second number is the lens size.

Prism System – There are two types, Porro and Roof. Both provide for a reduction in the overall binocular size. Traditional-looking binoculars with an offset between the objective lens and the eyepiece use the Porro prism. Sleek looking binoculars with the eyepiece and objective lens in alignment use the Roof prism. Porro prism binoculars are generally less expensive to manufacture, but are heavier and bulkier than comparable roof prism binos.

Range Reticle – A vertical index line that overlays the binocular image used to determine the distance of an object if the height is known.

Waterproof – O-ring sealed binoculars capable of being submerged in water to a depth of less than one meter with no water intrusion or damage.

Mil Spec – What does it mean?

Ads for certain marine binoculars may make claims like "built to military specs" or "meets Mil Spec" or simply states "Mil Spec". To make sure you know all about binoculars you'll need to know the implication here is that these binos are built to some specific military design or standard.

So is this true? Well, yes and no. The binoculars are certainly not built to a military design specification but they do meet certain parts of a military standard related to environmental testing. That standard is Mil-Std-810E, which defines over 20 parameters that products supplied to the military might need to meet depending on their use.

If you scanned or read this page you should now know all about binoculars and be able to make good use of our binocular review pages.