Eyepieces

When you look through a telescope or binocular you focus the distant scene by first pulling out then slowly retracting the tube, or by turning the appropriate screw. For visual observation you should always extend the tube beyond the point of good focus and then draw it in until the scene is seen to be sharp and well focused. This is to avoid the eye strain which comes from a prolonged accommodation of the eye to a close object. (The same is true when using a hand-lens. Bring the object in from a distance; do not start with it too close because, although it will be in focus well enough, the image will be close to the eye instead of at −∞ where it belongs.)

The telescope field of view is bounded by a circular stop which is usually sharply in focus. There may be an eyepiece adjustment to make it so. It is a stop placed at the prime focus, and it is there to prevent scattered light from the outer, unusable part of the field from entering the eye, thereby reducing the contrast of the scene. A second such stop, with the same purpose, is at the intermediate pupil of an erecting telescope. The two stops are optically conjugate.