Compactness and ruggedness are two major features to consider when looking for a pair of binoculars. Compactness relates to whether roof, porro, or other prisms are used in the binoculars, which affect the weight and size of the binoculars. Waterproofing, and rubber armor designs make the binoculars more durable to withstand the rigors of portable, outside use in all elements.
Binoculars rely on pairs of prisms to fold and erect the image. Doubled roof prisms are small and light enough that a pair of binoculars designed around them can have short, straight, and lightweight tubes. Thus, they are very compact and are the better choice when weight and size are key considerations. Porro prisms and other types of prisms are bulkier and, when paired, result in a tube design that bends out at a side angle. Porro prism binoculars have the classic bulged tube designs and are larger than roof prism pairs.
Waterproof binoculars go further and are designed to totally exclude water. They also may be nitrogen purged–filled with dry, inert gas–as an additional barrier to internal moisture, rendering them fogproof as well.
Rubber armoring provides a final level of external ruggedness and helps protect your binoculars from abrasion, damage, or loss of collimation if accidentally dropped. The rubber also makes them easier to grip in damp or wet weather conditions.