The Spot Exposure tool controls the light in a circular area of the image with onscreen adjustments for size and position.

Stops: With a 1.0 gamma, each positive stop doubles the amount of light, each negative stop halves it.

Center: The center of the blur region (and the least blurred area), expressed as percentage offset from the image center.

Radius: Size of the area outside which the blur operation is at full strength.

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: Threshold/softening region feathers the radius.

Falloff: Sets the median point between the Radius and Spread.

Color Balance: Tints the exposed regions.