Re your focusing problems, what are your cameras? I ask because as I calculate it 18" in front of the lens is ~ 4.5 focal lengths, so on your 8x10 extension must be ~ 1.25 focal lengths, i.e., 160 mm, of which ~ 25 mm will be due to the shutter.
Originally Posted by ntenny
I have smaller cameras than you, but even my 4x5 Cambo and the CC-401 I sold early this year have minimum extensions much shorter than 135 mm. Given the shutter's thickness, you'll need minimum extension of around 95 mm to make infinity with a 120.
I haven't tried either of my f/14 Perigraphes (60, 90) closeup. I haven't felt the need to use them for closeup since I have other more luminous lenses that do very well closeup.