Astronomy
Eyepieces To see how this works, say you want to operate your telescope in the middle of the Low Magnification range. The exit pupils in this range go from 2 mm to 4 mm with 3 mm at the mid point between them. Choose an exit pupil of 3 mm, and multiply that by your telescope's F: Number. If your scope has a focal ratio of 10, its F: Number is ten, so you should use an eyepiece with a focal length of 3 X 10 = 30 mm.
Why not just use a single eyepiece?  A single eyepiece simply can't do all that work. When you want to see details in the planets you need all the useful magnification you can get.  However, when you want to see an entire constellation, that high magnification cuts off too much of your field of view. A lower magnification is in order.
What kind of eyepiece is best?  There is no simple answere to this question. Much depends on your pocket book and your interests.