Addition to the Olympus 5050 noise test: what follows are image areas of the test scene 2 (all at sharpening 0) displayed at 100% (uncompressed, PNG):
 
 
 

ISO 100

ISO 200

ISO 400

The standard deviation (i.e. the noise) in the homogeneous image area to the left of the "Kr" is indeed lower in the ISO 400 image. However there is more visible noise in the image areas containing detail (in this case text) in the ISO 400 image than in the ISO 100 and 200 images. In other words the ISO 400 noise reduction filter apparently is only applied to image ares containing no detail, where noise is most visible.
 
 

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