The eye relief is an important concept in the use of equipments like a telescope, microscope, or binoculars. By definition, eye relief is the distance from the eye piece at which the eye can be placed to the entrance pupil of the eye. Instruments with short eye relief will require the observer to press their eyes closer to the eye piece to see a clear image. Eye relief illustrated by # 3 in image.

Exit pupil larger than the pupil will waste the light, but allows more movement without a vignette. On the other hand, an exit relief smaller than the pupil causes a vignette image. Without the proper eye relief, you cannot enjoy your binoculars or eye piece properly.

Eye relief is very important for those who eyeglasses, too. Having a longer eye relief means a broader field of view. It is recommended for eyeglass wearers to have at least an eye relief rating from 14 mm to 23 mm.

Eye relief also plays an important role to shooters, as a vital safety consideration. Too short relief and the skin between the optic and the eyebrow will get cut because of recoil. This is called ?idiot cut?. Having an exit pupil larger than your pupil will allow you a clear view without vignetting.