What is Exit Pupil and Eye Relief for Sport Optics?
September 12, 2018
The exit pupil is the width of the beam of light leaving the eyepiece, usually measured in millimeters. A large exit pupil is advantageous under low light conditions and at night because the larger the exit pupil, the brighter the image. For astronomical applications, the exit pupil of our Sport Optics should correspond with the amount of dilation of your eye's pupil after it is fully dark-adapted. This number will be between 5 mm and 9 mm (the maximum amount for the human eye). Maximum dilation tends to decrease with age. By age 50, the exit pupil may be close to 5mm. An exit pupil larger than your dilation just wastes light from the objective, since the outside of the beam falls on your iris and doesn't go into your eye. To calculate the exit pupil, simply divide the size of the objective lens by the magnification of the binocular or spotting scope. For example, the exit pupil of 7x42 binoculars is 6 mm.
Eye relief is the distance (in millimeters) between your eye and the binocular/ 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.