I'd like to sell my Sony RX100 VA that I use as a backup camera for my Sony RX10-IV (like-new thanks to the pandemic) and use my iPhone 12 Pro Max as my backup camera. I shoot a lot on ships and on Los Angeles beaches so an optical viewfinder that can frame the field of view on my iPhone is nearly essential. Does anyone have any experience with devices that can do this? A link and the cost of the setup would also be appreciated.
I don't know if this will fulfill your needs, but look at the IOS app Mark II Artist's Viewfinder. Can purchase in the App Store.