Blur and Sharpen

I took this picture of a spectacular mammatocumulus display using the film that happened to be in my camera, Fuji NPH 400. The grain was more than I expected, particularly in the relatively neutral clouds. Here you see the unretouched scan.

My challenge was to eliminate the grain in the clouds while preserving the detail in the trees. Here are the steps I used.

• Since the trees are relatively silhouetted against the sky, I created a mask from the coverage of the trees. I did this by increasing the contrast and adjusting brightness until I had a hard edged mask. Because of the varying brightness around the several areas of the trees, I performed this task separately for each group of trees.
• Through trial and error, I figured out that the mask was not good enough. I used an erode filter to expand the mask a bit, and then I applied a gaussian blur with radius 3 to the mask. This was to soften the transition between sharpened/blurred areas of the final image.
• I applied this mask to the unretouched image and applies a gaussian blur to the image, again with a radius of 3, through the mask.
• I inverted the mask and applied an unsharpen filter to the leaves of the trees.