Binoculars 101

In Gear 101 by PJ DelHommeLeave a Comment

Anyone shopping for binoculars quickly realizes they can spend $200 to $2000 for the same 7×42 pair. Honestly, what’s the difference aside from a few truck payments? You’d be surprised.

Exit Pupil & Eye Relief
Hunters need binoculars to work well at twilight, so pay attention to exit-pupil. It is the diameter of the shaft of light that actually reaches your pupil. In dim light, your pupil opens to around 7mm, so the closer your binoculars can get to that, the brighter and more detailed the image. A pair of 8×42 binos typically cast a 5mm exit pupil, which shrinks to 4mm in 10x42s. Eye relief is a number you want to maximize as well. The higher the number, the more comfortable binoculars will be to use for long periods, especially for eyeglass wearers. Shoot for at least 15mm of eye relief, 20mm or more for eyeglass wearers.

Roof Prism
Without any kind of prism inside your binos, the image would appear reversed and upside down. Today, nearly all hunting binoculars use roof prisms to get the image to your eye. Compared to older versions that used porro prisms, roof prisms allow for a much more stream-lined and compact binocular. And they are generally more rugged, being less prone to alignment and internal moisure issues.

Lense Coatings
Layering is all the rage with hunting clothes, and it’s integral to crisp, clear images as well. Binoculars have a series of lenses and each lense should have a coating or series of coating to reduce glare and optimize light transmission. This set of Lecia binos feature 42 layer of coating on top of fluorite-ion glass lenses. The fluorite helps to ensure low dispersion and help correct color aberration.

LD and ED Glass
Many models of binos, spotting scopes and rifle scopes will have LD or ED after the name. That refers to low or extra-low dispersion. In short, dispersion refers to what happens to light as it enters a prism; it goes from white light and disperses into all the colors of the rainbow. Think back to Pink Floyd’s album art from Dark Side of the Moon. When light disperses inside your binocular and gets distorted, chromatic or color aberration occurs. Your eye then sees fringes of color around the image. To correct this, companies employ ED and LD glass to minimize the effects of chromatic aberration.