Classification of Optical Systems

Binoculars have left and right side barrels linked to each other by an axle. Light enters the objective lenses and travels via prisms to the eyepieces. These two most commonly types of prism systems have resulted in two basic design types. 
1) Porro prism design ( named after Ignatio Porro,the Italian physicist who invented them in 1823 and first used them in binoculars), i.e. the classic design 
Porro Prism binoculars are characterized by the eyepieces being offset from the objective lenses. 
A distinctive shape characterised internally by the use of a combination of two right angle porro prisms and externally by the offset positioning of the eyepieces from the objective lenses. 
In optics jargon, this is called a compound lens which reduces distortion. The light then passes through the Porro prism (or roof prism in a more compact design). The prism serves two purposes: it produces erect images and folds the light path to reduce the physical size of the binoculars. Light then passes through eyepieces and enters your eye. One of the two eyepieces is usually adjustable to adaptor to each user. Both eyepieces are adjusted together by center focusing.
2) Roof prism design, i.e. the modern disign
Roof Prism binoculars feature lenses positioned in line for a more compact design. 
With the prisms positioned one over the other, the objective lenses and the eyepieces are in line. The result is a more compact design which allows compact binoculars to have full size power capabilities.