Binocular Guide 3: An In-Depth Look

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Examples of porro prism and roof prism binoculars

Two Designs

Standard binoculars are made in two designs: porro prism and roof prism. While both designs have their benefits, roof prism models offer a more streamlined shape and are more easily held by smaller hands, whereas porro prism models generally offer a longer depth of field, a wider field of view and a more comfortable fit for larger hands.

Binoculars are constructed from materials such as polycarbonate/foam and die cast aluminum, making them both rugged and lightweight. Models designed for more extreme uses, such as wildlife viewing and outdoor activities (camping/hiking), often feature a protective rubber armor coating.

What Do The Numbers Mean?

To know which binoculars are the best choice for you, it is helpful to have a basic understanding of the terminology and various sets of numbers used to describe them.

For starters, all binoculars are identified by two numbers (for example, 7x35 or 10x50). The first number indicates the level of magnification or Power, while the second number is the diameter of the outer (objective) lens measured in millimeters.

The exit pupil of the binoculars is the size of the beam of light that leaves the eyepieces and is directly related to both of the above numbers. Exit pupil is determined simply by dividing the objective lens diameter by the magnification. Both examples above (35÷7 and 50÷10) would have exit pupils of 5, which is optimum for low–light conditions.

Field Of View

Another important attribute is the field of view, a measurement of the total horizontal view at a distance of 1,000 yards. magnification directly impacts field of view. Greater magnification brings the subject closer, thus reducing the amount of horizon seen. This attribute is especially important for birdwatching or viewing sporting events, as a wider field of view gives you an overall larger picture of the action.

If you wear eyeglasses or will be using binoculars while wearing sunglasses, you'll want to set your sights on eye relief. This attribute is the distance from your pupil to the surface of the eyepiece. Glasses wearers need a longer eye relief, from 12 to 20 millimeters or more.

The Optics

At the heart of your binoculars are the optics, the lenses and prisms that take in the light and magnify the image. Because glass reflects up to four percent of the light that passes through it, images can lose a significant amount of brilliance when they pass through a poor set of optics. Most optical prisms are made from borosilicate (BK–7) glass or barium crown (BaK–4) glass. BaK–4 is a higher quality glass that yields brighter and sharper images. To reduce light loss and retain image definition, the optical elements of binoculars are coated, usually with a thin layer of anti–reflective magnesium fluorite. Typically, the more coating on the lens and prisms surfaces, the brighter and clearer the image.

Examples of porro prism and roof prism binoculars

Lens Coating Choices

Lens Coatings

There are four major levels of coating to choose from. Coated is the cheapest option, providing a single layer of coating on at least one lens surface. Fully coated means at least one thin anti–reflective coating is provided on both sides of the objective lens system, both sides of the ocular lens system, and the long side of the prism. Multi–coated offers multiple coats of anti–reflective layers to most of the lens and prism surfaces. Fully multi–coated provides multiple layers of coating on all air–to–glass surfaces. How much difference can coating make? As an example, a fully coated 8x35 model can provide both sharper and brighter images than an 8x42 model with poor coatings.

Phase correction or phase coating is unique to roof prism binoculars. By design, roof prism binoculars split the light entering the barrels into two separate paths. After passing through the objective lens, the light waves are reflected off the surfaces of the roof prism and split into two out–of–phase beams. Light reflected from one roof surface is shifted from the light hitting the other roof surface, sometimes referred to as "out of phase" or "phase shift." Although the light waves are subsequently forced back together when they reach the viewer's eye, this phenomenon results in reduced contrast and image resolution. A manufacturer can apply a thin coating on the roof prism surfaces to force the light beams back into phase, thus improving image quality and contrast, creating a sharper view, especially noticeable when viewing fine detail.