I usually shoot my Nikon D300 at ISO 200 (the default). However, in certain circumstances, I have noticed noise in darker out-of-focus areas (i.e. in the bokeh). You can completely remove it in Lightroom by just dialing in a luminance noise reduction of 5. However, this prompted me to check out the noise as a function of ISO in a realistic image using default RAW conversion in Lightroom instead of some silly test chart. The NEF was shot at 14-bits and using only lossless compression. I used a 50mm f1.8 lens at f2.8. Here is the awesome scene I shot:
Here is a 100% crop at ISO 100
Clearly there is far more noise in the bokeh. There is very little chance you will ever see that in a print and as I said, you can take care of it easily with just a tiny bit of luminance noise reduction, but it might be good to be aware of it.
Now for completeness:
This is where I reached the maximum shutter speed on the D300 (1/8000 s), so I cannot show you any more. The D300 does very well even at ISO 6400 as I reported before. At the low ISOs, Capture really doesn't do much better than Lightroom in noise reduction for those who are curious to know. However at high ISOs (>800), Capture does far better.
Interestingly, if you look at the filesizes of the jpeg exports I made from the 100% crops at high quality, you can see the exact same trend. Jpeg size, due to the way the compression operates, correlates with noise as the agorithm cannot distinguish between noise and real information. Here is the list:
ISO Size (KB)
Since the scene and lighting was exactly the same in each image, the only thing that is different is the amount of noise in the image. Amazing that the effect is that clear.
Next: Analysis of the filesize in terms of noise