Light Falloff/Vignetting

[tomczak]

I have scans that vignette. How do I un-vignette them easily? - the scanner and the camera are all gone...

[jsachs]

Without a reference image you can't compute an accurate light falloff function so some guesswork will be necessary using visual feedback.

You can create and apply a circular gradient to your image using the Gradient transformation (Oval gradient) and playing with the Color Line control to lighten the corners. To get a circular gradient that extends all the way to the corners of the image, shift drag the circle overlay's control points (this forces it to be circular and not oval) and enlarge the image window so it has wide enough borders to set the control points outside the image. Generally you will probably want the gradient to be light gray out to where the vignetting starts and transition to white at the corners. Then using the Filter operation the corners will be lightened.

[tomczak]

Have anyone used a little utility 'CornerFix'? While it supposed to only work on DNG files, it seems to correct for colour shifts in the corners as well. Did anyone find it useful?

https://sites.google.com/site/cornerfix/downloads

p.s. Jonathan's recipe works fine, but it's more complex a setup than PWP3.5 simple (but arguably more primitive) circular correction slider in Light Falloff (which is useful when one can no longer shoot the test image with the the lens/camera that is already gone). Is there a chance of reinstating it as an option?

[jsachs]

The old transformation assumed that light falloff was modeled by a cosine function which, in practice, turned out not to be a great approximation to the real world.

If you want to lighten corners without a reference image, you can use the gradient transformation with a circular gradient and play with the control points on the color line until you get a result you like.

Here are some hints to produce a good circular gradient:

1) Select Oval as gradient type but hold down the shift key when dragging to size the oval to force it to be circular and not elliptical. Make sure the center of the gradient is in the center of the image (use Window/Grid to identify the center).

2) Zoom the image out and enlarge the window beyond the size of the image to leave room to drag control points outside the image. This is required to get the largest circle to go through the corners of the image.

3) To darken the center, use the Filter operation and make the gradient light gray at the center and white at the edges - the darker the center gradient color, the darker the center.

4) To lighten the corners, use the Subtractive Filter operation and make the gradient black at the center fading to a dark gray at the edge. The lighter the edge gradient color, the more lightening.

5) Add additional control points to the gradient as necessary and play with their colors to fine tune the results. For best results, set the color line transition between control points to Sine.