Eyecups are an important feature to avoid stray external light. Eyecups can be made of rubber or of hard plastic. The advantage of hard, retractable eyecups is that they are very durable and are often variably adjustable with a locking function.
Rubber eyecups are soft and can be folded down when needed. However, over a longer period of time they can become brittle as the plasticizers within evaporate.