Here’s an effect we forget sometimes. When a lens is wide open, it vignettes.
My 50mm lens at f/1.2:
And here is that same lens a stop and a third closed down, at f/2.0:
Can you see the difference? The first picture, wide open, shows significantly more vignetting.
Now I like vignetting – a lot, in portraits. But shooting portraits with a lens wide open is rather dangerous, since depth of field is very shallow and may not be sufficient. So I add vignetting in Lightroom – Post Crop Vignetting is one of the best controls in Lightroom for when you are shooting portraits.
And when you are not shooting portraits, avoid vignetting like this – so in those cases, avoid shooting with your fast lens wide open.