This may hit some beginners like a ton of bricks so brace yourself for a mild disappointment here: your lens isn’t wonderful at all f/stops. What? A Canon 50mm which opens to f/1.8 isn’t at its best when it’s wide open. And by ‘best’, meaning when a lens it at its sharpest. There’s something known as a lens’ ‘sweet spot’ .
Lens Sweet Spot – The aperture at which it’s at the sharpest and sometimes will even offer other things like better clarity.
It’s hard to give a blanket statement about the exact sweet spot for each lens because when you look up the chatter on the internet, there are such varying statements about, say, the sweet spot for the aforementioned Canon 50mm f/1.8. Some say it’s f/2.2 while others claim that the sweet spot is f/4.
A Good rule of thumb – a lens starts moving more towards perfection about two stops above its maximum aperture.
So where an f/1.8 can open all the way to f/1.8, you don’t actually want to shoot at f/1.8 if you want optimal sharpness and clarity. Try moving up a couple stops to f/2.2 or f/2.8 and compare the results you achieve.