I have a Zacuto loupe for DSLR-video (it focuses on the LCD screen, not the eyepiece), and it is focusable and works on the groundglass, but it's around 2x or 2.5x, which is a little less than I'd want for groundglass viewing. 4x is ideal, but sometimes I use the Toyo/Omega 3.3x because it's longer and works well with a folding hood (like on a Graphic or Linhof), and I use a 6x with the 2x3" Technika, just because the 2x3" negs will be enlarged more than a 4x5" or larger neg.