Anyone who owns a telescope or has looked at advertisements for one has seen a cryptic number like "f/8" or "f/5" used in the description of the instrument. This number represents the telescope's focal ratio, an important specification that can tell you many things about the instrument.

If you multiply the focal ratio by the telescope's aperture, the result will be the telescope's focal length. The focal length is the length of the light path from the main mirror or lens of a telescope to prime focus (the location of the eyepiece, or a camera for astrophotos). For example, my telescope is a 114 millimeter (mm) f/7.9 reflector. That means the focal length of my telescope is 7.9 X 114 = 900mm. Additionally, the focal length of the telescope divided by focal length of the eyepiece you are using results in the magnification. That means for my telescope described above, I know I will need a 9mm eyepiece to obtain a magnification of 100x (900mm/9mm = 100).

A telescope with a low ("fast") focal ratio offers lower powers and a wider field of view at prime-focus for for a given eyepiece. When using the telescope for prime-focus astrophotography, smaller focal ratios mean shorter exposure times. Just the opposite is true for larger focal ratios ("slow" telescopes): higher magnifications result for a given eyepiece, but the field of view is more limited. And exposure times for pictures tend to be longer.

A common myth is that fast telescopes offer brighter images than slow ones. Image brightness is only influenced by the telescope's aperture, which controls the telescope's light gathering ability. Say you have a 6-inch f/10 telescope and a 6-inch f/5 telescope, each operating at about 100x. If you point them both at a distant galaxy, the image in each will be equally bright. That's because they are both 6-inch telescopes, gathering the same amount of light, and magnifying by the same amount. The difference is that a 15mm eyepiece is used in the f/10 to obtain 100x, while a 7.5mm eyepiece would be needed for the f/5. (For comparison, the 15mm eyepiece would provide 50x in the 6-inch f/5). This is a somewhat significant difference, because many observers find that eyepieces smaller than 7.5mm can be less comfortable to view through (you generally need to hold your eye closer to the lens as the eyepiece size decreases). This makes it less comfortable to view through a "fast" telescope at high power, especially if you wear eyeglasses. This effect can be relieved somewhat by using a Barlow lens with an eyepiece. A 2x Barlow lens will make the 15mm eyepiece act at 100x in the f/5 while maintaining the same comfortable eye relief. In addition, there are several lines of eyepieces currently available which combine high power with very comfortable eye relief, but they are very expensive.

Actually, the question isn't really which focal ratios are better, but rather what kind of observing do you want to do. Small focal ratio telescopes are definitely better for deep-sky observing because of the lower power and wider fields that only they can provide. (Yes, contrary to what you may think, lower power is better for faint, deep-sky objects! This is why better telescopes from reputable companies usually come with only low power eyepieces.) They are also better if you plan to try any astrophotography because of the shorter exposure times needed to obtain good pictures. Larger focal ratio telescopes are often considered better for planets and double stars, because it is easier to obtain the high powers necessary to view these objects with the "slow" scope. But as I mentioned above, with the use of a good Barlow lens, you can actually obtain the same high power with a fast scope. In general, an f/8 telescope can be considered average and effective for general observing.