How to select an eyepiece ?
The brightness and the limit magnitude (IV) Using a same aperture and eyepiece, a f/10 scope will allow to get twice the magnification of a f/5 scope but with the great disadvantage to yield half the field of view and a dimmer image that the one of the faster scope. Comparing two optical systems, we can define the relative brigthness as the square of the exit pupil. In others words brightness is expressed by the exit pupil. We easily understand that like aperture, as the exit pupil increases the relative brightness increases in a 2 power law making the image quickly brighter. The fact there is an obstruction in catadioptric and newtonian telescopes up to 40% by diameter will cause loss of light, resulting in lost of image brightness. The same problem occurs with binoculars using poor quality prisms and eyepieces.
