Astronomy
The Good, Bad, and the Ugly Telescopes You can find an eyepiece that will put an 8", F:11 telescope in its useful range of magnification. A 5.5 mm eyepiece would limit this telescope to a magnification of 406X.
For this reason, a standard minimum focal length for the eyepiece must be applied to this test.  The industry standard is 5.0 mm.
If a telescope needs an eyepiece greater than 5.0 mm so as not to exceed a magnification that is greater than twice its aperture (in millimeters), it is a bad telescope.
An 8" F:10 telescope will produce a magnification of exactly 406X when used with a 5.0 mm eyepiece. Any 8" telescope that has a focal ratio greater than F:10 will fail the test, while any 8" telescope with a focal ratio of F:10 or less is a good telescope. The smaller the focal ratio, the better.
Likewise a 16" F:10 telescope with a 5.0 mm eyepiece, will have a magnification of 812.8X, which just equals 2 X (16" x 25.4 mm/inch) or 812.8X