50 iSO is not the important factor unless the lower ISO allows the longer  exposure the noise is not much of a factor on Canon Camera sensors even less so on newer models... But even so the results wont be as good above 400 .. Some Cameras have had to use 200 ISO as standard in order to retain some quality at 400 Here you will be to some degree dependant on the technology of the sensor which varies from maker to maker.. Having a fixed focal length or range  helps.

  In answer to those who have asked me via email or IM  You can access your lower ISO on the 5D the settings go from 100-1600  with H for 3200 and L for 50 owners of other Canon cameras can check if you have the L  setting as well as H you have the full range

 

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt