The eyepieces are where the viewer puts his eyes when viewing.
These differ from the objective lenses in the following ways:
1. They are considerably smaller than the objective lenses, even though the housing seems exactly of the same diameter in the case of most compact (pocket) binoculars of the roof prism type.
2. Usually they have eyecups against which your eyes can rest. Usually these cups are retractable or can be folded away.
3. In the case of the porro prism binoculars, where the whole focus mechanism is external, they move in unison with one another as you adjust the focus knob/wheel (the objective ones are fixed).
4. Finally, the one on the right hand side, known as the “diopter”, is adjustable as well. (In the case of high quality “fixed focus” binoculars both are adjustable).
5. Contrary to what many people think, magnification and field of view are not achieved by the objective lenses, but by the eyepieces. That goes for eye relief as well.