The type of prism system determines the shape and often the size of binoculars. Although there are some variations, binoculars come in three basic shapes.
Illustration by J. Chavda
Wide-set In-line Narrow-set
Wide-set models have objective (front) lens offset outside the axes of their eyepieces by Porro prisms.
In-line binoculars feature a complicated prism system that permits straight barrels no wider than the objective lens.
Narrow-set binoculars have reverse Porro prisms, allowing the objective lens to be closer together than the eyepieces.
Compared to wide-set binoculars, in-line and narrow-set versions are less bulky, but they also have less relative brightness because their objective lens is smaller.
Which is best for you? That depends on the priority you put on compactness, weight, and brightness.
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