I admit to a guilty pleasure: I like vignettes. In other words, I like a dark area around my subject, to make it stand out.
Which , when subtle enough (yes, I am laying it on a little thick here so you can see it clearly), looks better to me than this, the original:
You can achieve a vignette like this by using the “Post Crop Vignette” function in the Lightroom Develop module’s “Efects” pane. Or in Photoshop.
But you can also do it with your lens. And in some cases, like when I shoot news, this is the only way: when shooting any editorial picture, you may not change anything (other than cropping, white balance and exposure adjustments). Distorting is not allowed!
But if I shoot with a fast lens wide open, I can get the same. Here’s my 35mm lens at f/4:
And here it is at f/1.4 – wide open. See the effect?
And no editing was done here – so I can do this for news pictures when I like.
Of course the drawbacks are obvious: very narrow depth of field can be too narrow. But often, the vignetting effect is beneficial, which is one reason I sometimes shoot wide open.