Punchcutters in the 20th century seems to have mostly used 2 kind of loupes for punchcutting: traditional jewellers eyepieces (you can see P. H. Rädisch and M. Carter using them at Enschede in several pictures) or a more sophisticated loupe on a stand like they use at the Imprimerie Nationale (C. Paput, Nelly Gable). Stan Nelson also has a beautiful one.
In his book Paput recommends "a magnifying glass on a stand, with a power of 3X and a diameter of 3,5 cm, which is reduced to 1,5 cm with a mask in order to eliminate distortion."
I tried several jewellers eyepiece but I've found them very uncomfortable and distortion prone, so I started to look of a magnifier similar to the one used by Paput, Gable and Nelson. This double lens magnifier by Bausch & Lomb has an excellent optic with very little distortion: http://www.bausch.com/our-products/vision-accessories/professional-magnifiers/double-lens-magnifier. It
was quite easy to attach it to the articulated arm of an Ikea desktop lamp. It works very well and can be place at any distance from the punch.
Also useful: Entry on Optics/Vision in David MacMillan's txt document:
Loupes. As preferred. I like a 5X loupe; others need less magnification.
Flip-down loupes that attach to your eyeglasses.
The best one is the "Ary" loupe marketed by the
Swiss watch materials firm Bergeon. Not cheap, but nice.
Available from suppliers such as
Otto Frei (watch division, www.ofrei.com)
Optivisor. The original brand is Donegal. The DX series has
glass lenses (better), the cheaper LX series has plastic lenses.
The number indicates the diopter, not the magnification.
Having bought any number of cheap knockoffs, I now only get
Donegal DX Optivisors.
Note: The "diopter" is a measurement in physical optics
(1/focal length, in meters) and so can always be stated precisely.
"Magnification" is a physiological measurement which depends on
what your own working distance is. Without measuring you, it can
only be stated approximately.
Here is a chart of *approximate* relations between diopters and
and a chart of approximate magnification and focal length for
Optivisor lens numbers (diopters):