THE EYEPIECE

He wondered why you do not put your eye at the focal point of a telescope mirror. That is where the image of a distance object is, and that is where you put the film in order to record the image. You do not put your eye at the image because it would be like viewing an object by putting your eyeball next to it. The object would be hopelessly out of focus. Instead you look through an eyepiece.
The eyepiece is a magnifying glass that allows your eye to comfortably view an image or small object up close. For a normal eye, the least eyestrain occurs when looking at a distant object where the light from the object enters the eye as parallel rays. It is then that the ciliary muscles in the eye are in a resting position. If the image or small object is placed at the focal plane of a lens, as shown in Figure (1), light emerges from the lens as parallel rays. You can put your eye right up to that lens, and view the object or image as comfortably as you would view a distant scene.

Figure 1: The eyepiece or magnifier. To look at small object, or to study the image produced by another lens or mirror, place the image or object at the focal plane of a lens, so that the light emerges as parallel rays that your eye can comfortably focus upon.