I've noticed that very often when I take photos with my D50 and convert them to black and white, they get this weird grain, and I can't figure out what is causing it. Here is an example of what I'm talking about, though it might be kind of small. I can upload a larger version if that would be helpful. As you can see, it's mostly in the shadowy areas, like on the corner of the back hoe (excavator?) and in the clouds, particularly in the top left. These were taken in color as Fine quality Jpegs and then converted to BW via the channel mixer function in GIMP. It was a pretty gray and nasty day. Any ideas as to the cause and possible solutions?