Something else to consider; a reason why faster than needed lenses are sometimes wanted.


Typically lenses are most sharp when their aperture is stopped down a stop or two from it's maximum. For instancea very fast lens (e.g. f/1.2) will usually be sharper stopped down to a smaller aperture (e.g. f/2.8) than say another lens wide open to that same aperture (i.e. f/2.8 maximum aperture). So even though the lighting may only require a maximum aperture of f/5.6 you may not want to reach for a lens with a maximum aperture of f/5.6 if image quality is of utmost importance, but rather go for a lens with a maximum aperture of say f/2.8 or even f/4, then stop it down or even lower your ISO - or possibly both in the case of 2.8 or faster.


This is a bit of a generalization with plenty of exceptions to the rule I'm sure - there being many other factors affecting lens image quality - but for the most part if you don't have a comprehension of what those other factors are this is a decent rule of thumb.