Eye relief refers to the distance (in millimetres) that the binocular can be held from the eye and still comfortably see the whole field of view.
This can be an important consideration when choosing binoculars, especially for spectacle wearers, or if you want to be able to use them while wearing sunglasses.
It can be anything from about 5 mm up to 23 mm.
The focusing on the binoculars means that most spectacle wearers can use them without wearing their glasses, but if you do need to keep your glasses on you will need an eye relief of at least 15 mm.
Soft Rubber Eyecups
If you are going to use the binoculars with your glasses on, then choose a model with soft rubber eyecups, which can be rolled back.
Binoculars with a long eye relief tend to have a smaller field of view, so you will have to decide which feature is more important.