I think this is a known issue with the Nikon cameras (from other threads I've seen) given the high sensor density. Actually it is an issue for all digital cameras in some circumstances (high ISO, long exposure etc.) The Nikon may have a lower threshold for when noise begins to appear.
Two things you can try are: A) expose to the right (don't under-expose but rather get as bright an image as possible without blowing the highlights, then correct in the raw converter), and B) look int some noise reduction software like NoiseNinja or NeatImage.
There is a way to completely eliminate noise: bracket a few shots over and under your ideal exposure, and blend them using a Photoshop script such as this one: http://www.farrarfocus.com/ffdd
This script merges the bracketed shots in such a way that it drops out the darkest areas of the underexposured (and therefore very noisy) shots, and replaces that with clean information from the overexposed shots. Correspondingly, it also uses the detailed highlight information from the underexposed shots to fill in the clipped information from the overexposed shots. You are left with a single file with one layer per exposure.
The result? Completely NOISE FREE images with full detail in both the shadows AND the highlights.
I've used the in-camera noise reduction on the D100. The results were ok, there was less noise. The D50 is probably better at NR. I got better results by turning off noise reduction and using NIK DFine. I imagine other noise reduction utilities would work too.
Kathryn's suggestion is a good one too if you're on a tripod and have the time for making several exposures.
The other suggestion I would have is to avoid the problem in the first place by using the lowest ISO number you can and to compose so that you don't have a lot of shadow areas.
With slide film, those shadow areas usually go black and you needed to recognize those situations and either use the black areas in your composition, or just not include them.
With digital, there's a bit more latitude and you get more detail in the shadow areas, but you're faced with a similar situation in that the dark areas will still be problematic because of the possibility of noise.
How can you tell if there will be a problem? Use your camera's spot meter (does the D50 have one?). You'll need to be in Manual Mode. Set your exposure in whatever manner you normally use. If you usually shoot with Matrix metering, just change settings until you get the exposure you want. Now switch to the spot meter and point it at a shadow area. If it drops to, say, -2 or lower, you'll most likely get noise.
Now that you know what might happen, you can make a more informed choice as to compositions and exposure. It might be that you want to crop out the shadows all together. Or maybe you could add some light through fill-flash or a reflector, make mulitple exposures, etc.
Anyway, the point is to use your meter to detect problem areas before they become a problem and to learn the limitations of your sensor (just like learning the limitations of slide film)
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This script merges the bracketed shots in such a way that it drops out the darkest areas of the underexposured (and therefore very noisy) shots, and replaces that with clean information from the overexposed shots. Correspondingly, it also uses the detailed highlight information from the underexposed shots to fill in the clipped information from the overexposed shots. You are left with a single file with one layer per exposure.
The result? Completely NOISE FREE images with full detail in both the shadows AND the highlights.
Cheers!
Kathryn's suggestion is a good one too if you're on a tripod and have the time for making several exposures.
The other suggestion I would have is to avoid the problem in the first place by using the lowest ISO number you can and to compose so that you don't have a lot of shadow areas.
With slide film, those shadow areas usually go black and you needed to recognize those situations and either use the black areas in your composition, or just not include them.
With digital, there's a bit more latitude and you get more detail in the shadow areas, but you're faced with a similar situation in that the dark areas will still be problematic because of the possibility of noise.
How can you tell if there will be a problem? Use your camera's spot meter (does the D50 have one?). You'll need to be in Manual Mode. Set your exposure in whatever manner you normally use. If you usually shoot with Matrix metering, just change settings until you get the exposure you want. Now switch to the spot meter and point it at a shadow area. If it drops to, say, -2 or lower, you'll most likely get noise.
Now that you know what might happen, you can make a more informed choice as to compositions and exposure. It might be that you want to crop out the shadows all together. Or maybe you could add some light through fill-flash or a reflector, make mulitple exposures, etc.
Anyway, the point is to use your meter to detect problem areas before they become a problem and to learn the limitations of your sensor (just like learning the limitations of slide film)