The eyepiece is not the same as the exit pupil. The eyepiece holds the ocular lens just like the objective bell holds the objective lens. Most eyepieces have a fast focus adjustment to help correct the image seen through the scope and make it clearer for those of us with less than perfect eyesight.
The small circle of light that you see as you look through a scope at arms distance is called the exit pupil. The diameter in millimeters is the exit pupil size we discussed earlier with the formula objective lens diameter divided by the magnification equals the diameter of the exit pupil. So a 55mm objective lens with 5x power magnification will show you an 11mm exit pupil.
Now turn the variable scope power all the way up. Look through the eyepiece and you should notice that the exit pupil is a lot smaller. Try to imagine if you are now using that scope in a low light situation and have the power at maximum. That exit pupil will be dark and small, almost too small to use.
What is so important about the exit pupil? The larger the exit pupil in size the less critical the shooter’s head placement is. The distance from the shooters eye to the ocular lens with the exit pupil projected on it is called eye relief. The distance from your eye to the ocular lens in a low power setting allows for greater range of distance but the higher the magnification power the more critical your eye is to be exactly centered on the scope. At maximum magnification the exit pupil is smaller so your eye has to be more exactly centered on the scope to ensure a complete and clear image.
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