Limiting magnitude indicates the faintest star a certain sized instrument will show, as indicated in the table below. Because of the many variables involved, the "faintest magnitude" indicated below is not an exact figure. According to astronomy author Philip Harrington, "Apart from aperture, other factors affecting this value include the quality of the telescope's optics, seeing conditions, light pollution, excessive magnification, apparent size of the target, and the observer's vision and experience."

Depending on circumstances, it's possible to better these estimates by a half magnitude or more. It’s often possible to see certain objects that are much fainter than the stated limiting magnitude of a telescope, while other supposedly brighter objects don't appear nearly as bright as one would expect. The magnitude is an integrated figure, as though the object were a point source, even though it actually might be a quite extended object. A faint, but small and concentrated object is almost always easier to view than a large extended object.

For more info, check Philip Harrington's web site: http://ourworld.compuserve.com/homepages/pharrington/