Jim, good example.   On  Saturday I  wanted natural light shots of some birds in their rehab areas.  The zoom lens I could use is not fast enough and did not want ISO to be too high. As is, it took about 600 to 800 ISO equivalent for less lit shots at F/2.8 but these look just fine, even if a tad of luminance noise is visible on those with simple solid color backgrounds.  Overall, the photo session was quite successful. These particular images are for particular purposes and likely only a couple will be printed...and those I like so much I must print!

The F/2.8 was not a zoom but a macro lens, used as a portrait lens.  Maybe it is a portrait lens which I know will also do 1:1 macros....LOL.       Anyway, it is good to try a lens in many situations and see where it works well.  That is what I do and have a decent feel of what to use for a particular shooting situations, not to say I always am prepared for the unexepected which does get away. At least knowing the limits of a lens is good knowledge to keep in you photo pocket.