I came across this simple hack the other day and quickly applied it to my Canonet. If the second image of the rangefinder is getting dim, it can be difficult to focus, especially in darker environments. If you can darken the main viewfinder, then the second image can compete and be visible. The trick is to darken just the centre of the main viewfinder with a piece of black tape (or even a black marker) so that the second image is easily visible. By using the black tape, the primary view is completely obstructed in that center area, making the focus more like a split image instead of a double image. Something semi-transparent would probably work more like a double image.