Astronomy
Eyepieces One of the most confusing things to beginners is the bewildering array of eyepieces.  Why so many?  With a good telescope you will want at least three of them. The reason for this is that your tellescope can operate in four different "Ranges."
These ranges are 1) High Magnification. 2) Medium Magnification. 3) Low Power Magnification, and 4) Rich Field Magnification. High Magnification has exit pupils from 0.5 mm to 1 mm, Medium Magnification has exit pupils from 1 mm to 2 mm, Low Magnifications have exit pupils from 2 mm to 4 mm, and Rich Field Magnifications have exit pupils from 4 mm all the way up to the maximum of 8 mm.
To put your telescope in a desired range, just pick an exit pupil within the desired range, and multiply it by your telescope's F: Number. That will give you the focal length of the eyepiece to use.