For those that don't know, what is vignette or vignetting?
"In photography and optics, vignetting (//; French: "vignette") is a reduction of an image's brightness or saturation at the periphery compared to the image center.
Vignetting is often an unintended and undesired effect caused by camera
settings or lens limitations. However, it is sometimes deliberately
introduced for creative effect, such as to draw attention to the center
of the frame." - Wikipedia
In general, a good lens should not vignette or at least show minimum darkening of the image corners.