Astronomy
The Good, Bad, and the Ugly Telescopes Now for the good telescopes:  Most of the better telescope companies such as Mead, or Celestron, make an 8" F:7 telescope, and some companies even make an 8", F:5 telescope. With the f: 5 model, you would need the smallest available eyepiece of 2.5 mm just to reach its maximum usefull magnification of 406X.
So what is the difference between an 8", F:10 telescope, and an 8", F:5 telescope? At the same magnification, they have the same resolution and brightness, but their focal lengths differ. The F:10 telescope has a focal length of 80", while the F:5 telescope has a focal length of 40". The smaller focal length telescope has a much smaller tube that makes the F:5 telescope easier to use.
What is the differeence between an 8", F:5 telescope using a 2.5 mm eyepiece, and a 16", F:5 telescope using a 5 mm eyepiece ?  They both have the same magnifying power of 406X, but the 16" telescope's image will be 16X brighter.  This extra brightness will enable the owner of the 16" telescope to view stars with an apparent magnitude as faint as 15.7, while the owner of the 8" telescope is limited to stars with an apparent magnitude as faint as 14.2.  Unfortunately, the 16" F:5 telescope has a focal length of 80" and is more difficult to handle than the 8", F:5 is.  In this case the extra light is worth it.