Vaughn
Subscriber
I would much rather just use an approximately ISO 3 film in the 5x7.
I do not worry so much about diffraction as I contact prints and usually have DoF issues that I must deal with (I'd rather lose a tiny bit of sharpness to keep everything in focus).
I have used a nice slow film (Kodak Copy film), but at the cost of having to use an ortho film (and a cost to the pocket book...if I could even still get it!). The ortho film works very well in the redwoods, but it is more difficult to use with big skies where I actually need the slower film...a Catch-22.
I need to rig up a way to attach filters (such as ND) on the back of the barrel lenses so that I can easily use the lens cap-shutter method on the front. And/or look more into using a packard shutter (more weight to carry, but it might be worth it.)
The other answer is to stick with my 300mm in the Copal 3, but having a 210mm, 19" and 24" in barrels has gotten me spoiled!
Vaughn