Fogged lenses can be a problem for those of us who like to use their binoculars in cold weather. If you’re not going to use your binoculars in cold weather, then nitrogen-filled binocs aren’t important to you.

Nitrogen-filled binoculars resist getting fogged up inside because they are sealed off from the atmosphere outside. Nitrogen gas provides a constant atmosphere in the sealed tubes of the tandem telescopes that make up a pair of binoculars.

Because each tube is sealed, the nitrogen gas won’t escape and moisture won’t get in. Your lenses won’t get foggy.

It is easier for the manufacturer to produce straight-line roof prism binoculars that are nitrogen-filled than the Porro prism design. Over half of the roof prism binoculars are nitrogen-filled, while barely one-fifth of the Porro prism binoculars are.

Nitrogen-filled binoculars are a mark of quality. Always go for quality when you can afford it.

The first shape is rather tubular, where the eyepieces sit practically in line with the objective lenses. The image below shows this shape of binoculars, which are known as roof prism binoculars.

The second kind of binoculars have more of a rounded look, where the objective lenses are set wider than the eyepieces. These binoculars are called porro prism binoculars. The drawing at the upper right of your screen is an example of porro prism binoculars.

Of course, each type of binocular has advantages and disadvantages.

When considering compact binoculars, the roof prism type is more common. That is because roof prism binoculars can be folded to a smaller size for compact carrying. Also, they can be made lighter and smaller than the porro prism types. Just perfect for a compact design!