Tool paradise since 1999
The prism in binoculars ensures that the image is always
projected in an upright position. The prism also folds the light
path so that the overall length of the binoculars is shortened and
the binoculars are more compact in size. A Porro prism is somewhat
simpler to make than a roof prism. In
Porro prism binoculars, the front
objectives are not in line with the oculars, making
them larger in size than roof prism binoculars. The advantage with
Porro prism binoculars is that the impression of depth is somewhat
better than with roof prism binoculars. After all, we see depth
because each eye sees a slightly different image thanks to the interpupillary
distance. If this interpupillary distance increases, the
impression of depth also increases accordingly.
With Porro prism binoculars, focusing is done by moving the ocular. This causes 'changes in volume' to occur in the binoculars, allowing air to shift in or out during focusing. With roof prism binoculars, focusing is done by moving a lens (group) in the binoculars. This makes it far easier to make roof prism binoculars dust-free and watertight.
In spite of the fact that it is somewhat simpler to construct Porro prism binoculars than it is to construct roof prism binoculars, manufacturers are able to produce both these types of binoculars of comparable quality. Yet, in the higher segment, the majority of binoculars are constructed with roof prisms.