Eric, Returning from a trip to a canyon in southern Utah my friend informed me that all of his digital pictures were marred by a fingerprint smudge on the lens that created a dull haze to a good 75% of all images right in the center. I took copies of the images to see if I could use GIMP to improve them. Out of the recesses of my spotty memory I recalled reading something on the web about removing haze. I connected the dots and realized atmospheric haze was very similar to the fingerprint haze in these photos. I only read a few image editing sites, your key among them, so I found your article right away. With a bit of trial and error I found some good settings for unsharp mask that dramatically improved the quality of the images. Seriously amazing.
On a few images I even applied the unsharp mask to the entire image then selected the oblong fingerprint smudge area, highly feathered the edges (150 pixels) then repeated the unsharp mask on this area (sometimes increasing the amount) to avoid over sharpening the unsmudged corners of the image.
While the images are still soft, they have been salvaged from almost useless to descent quality by use of this one technique (OK, I did the usual color level adjustment too).
A picture is worth a thousand words so I have attached before and after images.
Thank you for taking the time to share your knowledge of image editing, especially your GIMP tutorials.