Instead of lugging around a piece of cardboard with a cutout to help me frame shots as I did in my 4x5 days, I've just found out there are several iPhone apps that allow you to determine the correct focal length for the composition you choose depending on the lens and camera. I don't do cinematography, and use only full frame 35mm DSLRs, so bells and whistles are not needed. The apps that look most interesting are made by Artemis, Cadrage, and Alpa (or a developer associated with Alpa). They are not cheap, and probably have far more features than I need. There are also a few in the App store that are inexpensive, but it's hard to tell if they're any good. I'm intending to use an iPhone X and my widest lens is 21mm, full frame. One or two apps I've looked at seem to work only down to 24mm.
Any advice would be most appreciated!
Dan