This channelhttp://www.youtube.com/user/ChristopherKblog/videos
Yup that's what I've read as well. I've not seen any issues with IQ on any images at 50 so far, everything is pin sharp straight from the RAW files. Perhaps ISO50 used to be bad in one generation of SLR but no longer is?
I wouldn't use ISO 50 unless you are sure of the DR of the scene. Even then I would just prefer a straight forward ISO 100 photo carefully applying ETTR to what ever limit you can get away with, checking the RGB histograms to make sure that none of the colour channels have clipped. best to bracket a few exposures near the ETTR limit so if you have slightly misjudged things you have a safety margin form a photo slightly less exposed.
One thing you have to be careful of is that the RGB histograms are created by the embedded jpg in the RAW image, and the embedded JPGS takes on the processing styles you have selected, e.g. saturation, contrast and especially white balance (these have no affect on the RAW though). Thus if you have a very warm White balance set then the RGB histogram may show the red channel is clipping, which is only true in the jpg, not your RAW.
There is a technique called UniWb that tries to mitigate this. For the most part I simply bracket a few exposures 1/3rd stop apart at the extreme of th ETTR exposure.
To be sure, there is no such thing as a true ISO 50 setting. Your camera is using ISO 100 so has all the noise properties of ISO 100. As Exception points out, the camera simply over exposes during capture and then reduces exposure to generate the final image even in RAW format. You just have no control over what is happening to the highlights. So stick to ISO 100 and expose to the right as much as you can before highlights clip. Back on your compute bring down the exposure to the correct level. You will get much better results than ISO 50.
In fact, I suggest you turn off these extended ISO ranges as they really don't ad anything you cannot do yourself better.
