The built-in finder on my IIIa and IIIf were fine when I did not need to wear glasses. They are difficult to use, for me anyway, with tri-focals. I can't get my eye close enough to the eye-piece to see the full frame. My solution is to use the Leica BrightLine finder for my 50mm Elmar and Summitar. I recently purchased a CV 35mm finder to use with my Jupiter 12, and that works fine as well. The lines in the CV finder are a little heavier than those in the Leica finder, but I am getting accustomed to them.
Jim N.