Interpupillary distance - Binoculars
The interpupillary distance is the distance between the pupils of both eyes. The average distance for adults is around 65 mm. The interpupillary distance of the binoculars can be adjusted by moving both halves of the binoculars outwards or inwards accordingly.
The interpupillary distance is correctly adjusted when you see a perfect circlethrough the binoculars. However, if you see two overlapping circles, the interpupillary distance is incorrect and still needs to be adjusted. If the interpupillary distance is not set correctly, you will find that it is far more tiring to use the binoculars and you may even experience eye strain.
Obviously, interpupillary distance is not an issue with a monocular scope since you only use one eye here!