Assertion: eyepiece | /r/HasAspect | cup
Facets:
value: behind the eye lens
type: LOCATION
value: to aid the observer
type: PURPOSE
Source sentences:
text: However, if the eye relief is too large it can be uncomfortable to hold the eye in the correct position for an extended period of time, for which reason some eyepieces with long eye relief have cups behind the eye lens to aid the observer in maintaining the correct observing position.