Monocular - Binoculars
A monocular but it is intended for use with one eye, as its name suggests. The main advantage of a monocular is that it is small and can easily be carried around in your pocket or bag, making it readily accessible at all times. The disadvantage is that you only use one eye for viewing and therefore you lack depth. There are monoculars that are just as good as the finest binoculars as far as quality is concerned.
There are very compact monoculars available, which logically have a small exit pupiland limited brightness. Obviously, this type of monocular is not really optimal for viewing. But the same thing applies to a monocular as it does to compact binoculars and that is that there is always a compromise. After all, it is far better to have a monocular in your pocket so that you will always have a viewer readily available rather than having a top-of-the-range pair of binoculars just lying in the cupboard because it is too cumbersome and heavy to take along with you!