Eye relief is the distance from your eyes to the outer surface of the eyepiece lens of your binoculars where all the light rays of the exit pupil come to a focus and form the image. Your eyes should be at least 10 mm away from the eyepiece for you to have the entire field of view. Otherwise, you will lose on the exit pupil and not get a brighter image.

For those who wear glasses, it is often recommended you have longer eye relief than those who don’t because the glasses add extra distance. This is to enable you get the best comfort while viewing. Eye relief of 16 mm or more is ideal for eyeglass wearers.

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.