Screw mount and 90mm lens

[Steve Kobrin <kobrin2@xman.wharton.upenn.edu>]

I have recently acquired a IIIf RD which I enjoy using much more than I
would have ever imagined.  It is hard to load and certainly less convenient
to use than my M3, but for some reason I cannot quite put my finger on, I
find I pick it out of the drawer rather than the M.  Perhaps it is its size
or its "feel."  When I use the IIIf, the camera seems to disappear, it
intrudes only minimally between my eye and the "picture."

I do wonder whether it is practical to use a 90mm lens with a IIIf.  I have
both a 35mm and 50mm and find that with the latter, precise focusing when
depth of field is limited is difficult.  I took some available light
pictures of my wife in a window with my Summitar wide open and found I
could not "rock" back and forth as I do with my M3 to keep focus precise,
because doing so required switching my eye from the viewfinder to the
rangefinder window.  With a tight shot, it is hard to both frame the
picture accurately and focus precisely.  

As I tend to use a 90 for people and to isolate I wonder if it is really
useable on a IIIf.  The problem will be exacerbated with the 90 as it
requires use of an external viewfinder which is even further removed from
the rangefinder window.  

Any comments or suggestions would be appreciated.  Furthermore, if I do get
a 90, any suggestions?  Newer TM Elmars seem very hard to come by.  Marc,
is your book on non-Leica TM lenses out yet??  

Steve