Eye cups - Binoculars
The eye cups on binoculars are the 'rings' that are situated around the ocular as it were. Eye cups can be made from silicon, which makes them flexible and easy to fold back, or from hard plastic. In most cases, the eye cups can be twisted inwards or outwards to adjust the binoculars to accommodate eyeglass wearers. Eyeglass wearers will twist the eye cups inwards to ensure that the distance between the binoculars and the eyes is not too great. In contrast, someone who does not wear glasses will twist the eye cups outwards to get a clear view of the image. Eye cups also function as a type of sun shade to protect the eyes from direct sunlight.
The photo above shows the left eye cup twisted outwards for use without glasses. The right eye cup is twisted inwards for use with glasses. Generally speaking, of course, both eye cups would be set in the same position. However, in this photo, the difference in the positions of the two eye cups is clearly shown!