There are two basic designs in binoculars: The traditional “porro prism binoculars” and the so called “roof prism” binoculars. The porro prism binoculars have an offset zig-zag design, with the eyepieces (where you put your eyes) and big objective lenses (at the front) not in the same line, the objective lenses being wider apart than the eyepieces. These type of binoculars are by design heavier and bulkier than the other type, the roof prism binoculars, and will never fit into your pocket.
However, by having the objective lenses closer together than the eyepieces the porro design can be made compact – the so-called “reverse porro”. These are excellent compact binoculars, and although they tend to be heavier than roof prism binoculars, some of them will fit into your pocket and will be light enough to pass as a pocket binocular. An excellent example of a reverse porro compact binocular is the Bushnell Legend 10×26 Compact Porro Waterproof Binocular, which might be small enough to fit into your pocket (dimensions: 6.2 x 5.6 x 3.3 inches), but is a bit too heavy (22 ounces) to be comfortable. However, the Pentax Papilio 6.5x21mm Super-Close Focus Binoculars (also available in 8.5×21), which is a reverse porro design, small enough to fit into your shirt or jacket pocket with dimensions of 4.5 x 4.3 x 2.2 inches, but at 16 ounces it might be a little too heavy (as a pocket binocular) for a some people. The 8.5×21 weighs the same, but is slightly bigger with dimensions of 6x6x3 inches and maybe just too big to fit into a shirt pocket.
On the other hand, some reverse porro binoculars can be just as light-weight as roof prisms binoculars. The Galileo 8×22 Mini-Compact Binoculars weigh as little as 8 ounces! But these are few and far between.
The other type of design is the roof prism binoculars, where the eyepieces are situated directly behind the objective lenses. Whether big (normal size) or compact, they are sleek and streamlined . This means that they are perfectly designed for not only being compact, but indeed being pocket size as well. They not only weigh less than the other type (“porro”), but are smaller by design as well; on top of this, they can fold even smaller when not used. My 8×25 pocket roof prism binocular is 3.94 inches (100 millimeters) wide when folded open to fit my eyes, but folds down to 2.76 inches (70 millimeters) when I put it in my pocket.
As a matter of fact, some roof prism pocket binoculars are so small, they are simply just too small for an adult person to handle with comfort, like the Audubon 4100 Mini Binoculars 7×18 (dimensions: 2 x 2.5 x 4.2 inches), or the Tasco Essentials 8×21 Binocular (dimensions: 4.2 x 3.2 x 2.2 inches ; 8.8 ounces) from Bushnell. So you have to look out for that as well. On the other hand, they could be