We discussed this briefly above.
It is the light beam which exits the eyepiece and enters the eye. This exit pupil (EP) should never be larger than 7mm. This value lets you calculate the minimum magnification that should be used with a telescope. If the EP is larger than 7mm, then light is being lost and wasted. In elderly people, the maximum opening of the eye’s pupil is somewhat smaller as it continuously decreases with increasing age. A 60 year old can achieve a pupil diameter of 4-5mm. You should therefore consider the choice minimum magnification best suited to your age.

The stronger an eyepiece magnifies (and smaller its focal length), the smaller the exit pupil will be. A 200/1000mm telescope gives an EP of about 7mm when used with a 35.7mm. A 10mm eyepiece will give an EP only 2mm in diameter.

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