The Vignette tool darkens the edges of an image with control over darkness, tint and falloff with on-screen adjustments for size and position.

Radius: Size of the area outside which the darkening operation is at full strength. By default, this encompasses the image boundary.

Aspect: Controls the aspect ratio of the vignette. At a value of 1.0, the vignette is circular. At 2.0, it is twice as wide as it is high. Use this parameter for precise control, or just grab the handles at the edges of the vignette to reshape it.

Spread: Sets the start of the falloff area and is defined by a dotted line in the image. Inside the spread there is no vignette effect.

Falloff: Weights the gradient toward the spread line or toward the edge. The default of .5 causes a darkening half way between the spread line and the radius.

Strength: Specifies the extent to which the effect is applied where 0 is no effect and 100 is full effect.

Exposure Compensation: Corrects for any resulting changes in overall exposure.

Color: The vignette by default just darkens the edge toward black but the color value can push the vignette toward any color.