Eyepieces with a short focal length (4 to 8 mm) provide high power, but, as a rule, the higher the power of the eyepiece, the shorter its eye relief. The shorter the eye relief, the closer your eye must be to the eyepiece, making it less comfortable to use.
An alternative is a Barlow lens inserted between an eye-piece and the telescope. A 2x Barlow, which doubles the power of the eyepiece, is the most common size. A 12 mm eyepiece with a 2x Barlow produces the same high magnification as a 6 mm eyepiece, but without giving up the comfortable eye relief of the longer 12 mm eyepiece.