Monoculars

Monoculars for quick observation, small enough to slip into a pocket.

Monocular or binocular

A monocular weighs and bulks roughly half as much as an equivalent binocular and is used with one hand: ideal when observation is occasional and space matters.

The trade-off is eye strain: viewing with one eye tires faster and gives less depth perception. For prolonged observation, a binocular remains more comfortable.