The only way of obtaining the whole field of view is to have the right distance between the eye and eyepiece – referred to as the eye relief. The eye relief is not necessarily a decisive factor in choosing the right optical product, however it is a useful information if you wear glasses. The eye relief then should be at least 16 mm for comfortable viewing and getting a complete image. Binoculars with too short eye relief give people with glasses a tunnel vision and only show the middle part of an image. With riflescopes the optimal distance is 90 mm or even more for safety reasons (recoil).
Without glasses the distance when using binoculars should be minimum of 15 mm, although everyone has a preferred placement as long as they’re obtaining complete field of view with no blurry edges.
Most binoculars have movable eyecups, where you can adapt them to fit different face structures (if your eyes are further away from you nose etc.) and therefore maintain a steadier grip. If you wear glasses, the eyecups should be closed down so you can lean them onto your glasses.