The focusing mechanism of a pair of binoculars, by means of which the focus is adjusted when looking at an object through the binoculars, is found between the two barrels of the or/and at the top. This can either be a wheel (dial) or a toggle knob.
How does focusing work? When the focusing wheel/knob is adjusted, the distance between the objective lenses and ocular lenses (in the eyepieces) is changed, until the image is “in focus”. In the porro prism type binoculars the focusing system is totally external: The eyepieces themselves, connected to one another by means of a “bridge”, move forwards or backwards as the focusing wheel/knob is adjusted.
In the case of roof prism binoculars only the focusing wheel/knob is external – the rest of the focusing system in internal. By adjusting the focusing wheel a focus lens inside each barrel is moved forwards or backwards until the image is “in focus”. The eyepieces are fixed and do not move as in the case of the porro prism design.
The speed of focus is important: How quickly can the focus be adjusted. This is important when viewing a moving object. Some focusing wheels have to be turned a full 360 degrees for focus from infinity to close by, which is inconvenient. The better ones cover the full spectrum in about 140 degrees.
Some binoculars, Auto focus binoculars, have no central focusing knob/wheel; in their case the focus has been fixed at the factory.