ISO 50 is a complicated subject... and really depends on the nature of the sensor in your particular camera.
The following is something I posted over at Nikon Rumors in response to seeing someone say that ISO 50 was underexposing and then correcting:
(EDIT: I just want to be clear that his was in a discussion about landscape photography, where an aperture has been chosen for DoF and sharpness and the shutter speed is ranging to get the exposure right)
"Firstly, it's not "underexposing then correcting in software"... that's more of what happens with things like "Active-DLighting" and "Highlight Tone Priority" (on the Canon side) or using very high ISO. If anything, ISO less than 100 _overexposes_ and then pulls back in software. If you fix aperture and you expose at ISO 100 and get 1 second exposure... Then you drop ISO to 50 and take the same shot the shutter time will be 2 seconds. If the sensor can't be "less sensitive" than ISO 100 then you are overcooking the sensor (ie, overexposing). The camera can then correct in software to give you the correct looking exposure (possibly at the expense of highlights that might have been blown out and can't be recovered). Because of this there can be a small drop in dynamic range (you might have lost some highlights).
A few thing about this:
1. Physics means that an ISO 50 shot will definitely include the effects of more photons striking the sensor. That will reduce "random shot noise" which can give you a visible reduction in noise. I have done tests with my D600 and there is less noise at ISO 50,
2. At the worst case of a sensor that really can't go below ISO 100 at all, this process is equivalent to overexposing by one stop and pulling back your RAW file in post. Essentially it is equivalent to Exposing To The Right (ETTR http://en.wikipedia.org/wiki/Exposing_to_the_right
)... Which, in scenes of lower dynamic range can give you a tangible benefit of having less noise in the shadow areas. So once again we might be shaving off noise.
3. How much highlight you lose is very sensor dependent. The D600 has a lot of headroom in this area and if you look at DXOMark ( http://www.dxomark.com/index.php/Cameras/Camera-Sensor-Database/Nikon/D600
) it doesn't show a loss in Dynamic Range when going to ISO 50.... just that it doesn't gain much either.
In summary: be careful about spreading dogma like this. Every sensor / camera is different. In particular, I have done my own testing on the D600 and have found the tradeoff for lower noise to dynamic range to be more than acceptable. In extreme dynamic range scenes I may choose 100 to just be safe... but in all of my shooting with the D600 so far ISO 50 has produced better (less noise with the same / similar dynamic range) photos.
Everyone: don't just take my word for it (or anyone else on the internet) there is no reason not to do these types of tests with your own gear. One of the first things I do when I receive any new piece of gear is test it's limits. Not to be dissappointed but to know where they are so I can make correct decisions about tradeoffs in the field..."