My new f/1.2 lens reminds me to point out that vignetting is a good thing.
Let me explain.
Vignetting means “making the outsides of a picture dark”.
A lens, especially a fast lens, used wide open (at the lowest f-number) does this rather a lot. Like in this example. Here is the 50mm lens at f/1.2:
And here at f/2, “stopped down” just over a stop:
As you see, the wide open image gives you a lot more vignetting: the corners are dark. This is mentioned as a drawback for lenses that do this.
But hold on! Often, vignetting is a desirable thing. Especially in portraits. Like many photographers, I often add vignetting using the “effects” tool on Lightroom. A little vignetting is hard to detect but makes the picture noticeably better. Vignetting, in this way, makes the subject in the centre looks like he is lit by a spotlight:
I could do this in Photoshop, true – but I like to say “I did it all in the camera”.
And that is why I believe that for a portrait lens, a little vignetting when the lens is wide open is not a bad thing. And I ignore it when lens reviews moan about vignetting wide open.