"In photography and optics, vignetting (/vɪnˈjɛtɪŋ/; 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.
