A cup for an eyepiece of an optical observation instrument, comprises a peripheral wall which includes an eyepiece-connecting section for connection of the cup to an eyepiece, a transition section which adjoins the eyepiece-connecting section, a light-protecting attachment which, by way of a sectional part of the transition section, is joined to a side of the transition section that is turned away from the eyepiece-connecting section; a sight hole which is laterally defined by the peripheral wall; the peripheral wall being movable into a first, non-folded end position for someone who does not wear spectacles for preclusion of any incidence of stray light upon an eyepiece lens, and into a second, folded end position for spectacle wearers; and into a third stable position of ventilation, located between the first and the second end position, for someone not wearing spectacles.