Binoculars are a bit more involved than telescopes and a lot more can go wrong with them. Here are some examples of work that has been done to the ones that come in for repair, servicing and restoration.
Binoculars need to be collimated correctly. A collimator is shown with a pair of Nikon 10X50's on the mount having the collimation adjusted. This method is known as "tail of arc" which is the only method to ensure that the hinge and both sides are all parallel and that the binocular can be used at all pupillary distances. The pupillary distance scale appears on the centre hinge closest to the eyepiece and is marked off in millimetres. The scale usually has a range of between 58mm and 74mm. Everyone has a different pupillary distance and if the hinge and sides are not parallel then only at one pupillary distance will there be a single image, at all other settings there will be two.
I had a German WW2 Marine Flak binocular come in for restoration. The condition was far from optimal! The paint was flaking off and parts were missing or not working. A few pictures here show the original condition and the final condition before they were returned to the owners.