Focusing a very wide rangefinder-coupled lens is one of the advantages a rangefinder camera has over an SLR -- seeing the double image is a lot easier than trying to figure out by looking through the SLR lens whether you're in focus (made harder by the fact that really wide lenses are often relatively dim -- f4 or slower). But using the auxiliary viewfinder, with its inherent imprecision in framing, is a disadvantage of a rangefinder camera.