Newbie question on light and F stops

While I understand what the numbers mean, I have one nagging question that keeps me awake at night. It comes into comparison of fast lenses for indoor sports. I've been looking at f1.8 and f2 and f2.8 lenses, all of which seem to be fast. However, my question is when I step these lenses down. If I step an f2 lens down to f2.8, does it let through the same amount of light as an f2.8 lens wide open? Or would the f2 lens still be faster at f2.8 than the f2.8 lens. My assumption would they would be the same as far as speed, but again I keep thinking I might be wrong???

No, you've got it right. When you stop the lens down, you block light. An f/1 lens stopped down to f/5.6 is no faster than a lens that is f/5.6 wide open.

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.