What is Exit Pupil and Eye Relief?

Exit Pupil

The exit pupil is the width of the beam of light leaving the eyepiece, usually measured in millimeters (mm). The larger the exit pupil, the brighter the image will be under low-light conditions. Exit pupil size is calculated by dividing the objective lens size by the magnification power. For example, You want to know the exit pupil of an 8×42 binocular.  42mm / 8 = 5.25mm. The exit pupil of our Sports Optics should correspond with the amount of dilation of your eye’s pupil after it is fully dark-adapted. This number will be between 5mm and 7mm (the maximum amount for the human eye).

  •  For daytime viewing, a large exit pupil is not necessary but that does not mean that a binocular cannot be used for daytime viewing if it has a large exit pupil.
  • For marine application, a large exit pupil is favored as it is more forgiving and allows for more movement of the binocular while keeping the image in view.
  • For night viewing, a binocular with a 7mm exit pupil will provide the maximum amount of light to the eye.

Eye Relief

Eye relief is the distance in mm 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 10mm; 15mm will provide the best comfort, and you may need more if you wear eyeglasses.

You’ll lose the 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.

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