What is the eye relief for spotting scopes?

Eye relief is the distance (in millimeters) between your eye and the spotting scope eyepiece that allows the full field of view to be comfortably observed. It measures the spacing from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. Your eye should be positioned here to see the full field of view of the eyepiece.

Eye relief should be at least 10 mm; 15 mm will provide the best comfort, and you may need more if you wear eyeglasses.

Put your eye so it's just behind the eyepiece to take advantage of its eye relief. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Getting too close will prevent you from blinking and may also cause a black ring to appear around the field of view.

Updated 12/18/13