I believe the extended ISO ranges are essentially using PP to push or pull the image by a stop. So, the camera takes a picture at ISO 200 and then pulls it down a stop. I imagine that this could have a negative impact on dynamic range, particularly in the highlights, although noise shouldn't be affected when you are extending downward.
Although I do believe that ISO 100 is native for both the K7 and K20. In fact, I'm pretty certain it is because Pentax's in-camera highlight correction lowers the ISO by a stop and then pushes it in order to make sure more of the high end is captured. This would explain why ISO 200 is the lowest option when highlight correction is turned on, and indicate that 100 is probably base sensitivity.
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