The F11 key and the 
icon in the toolbar give you an easy way of generating vignettes of your original image. A vignette has the original image clearly visible in the center of the the image, fading towards the filtered image at the outer edge.
Hitting F11 repeatedly, you will get a sequence of vignettes, rather like F7 but with the mixing always fixed at elliptical-outer-in, giving you that old style photo type oval look.
In fact the Vignette effect is F7 with mix parameters set to Elliptical Out-In, Mix Sharpness set to 80% and Original Image set to 48. If you, after pushing F11, you find an image you like, but would like to alter the vignette settings simply use the mix dialog.
