Let’s try a macro lens. From reviews, the high quality add-on lenses for iPhones are by the Moment brand, and their macro is labeled “10x.”
It’s a great lens, but the problem is it focuses only in a very narrow range. It comes with a translucent “hood” and seems optimized for the lens hood to be pressed against the subject. Moving back very much at all leaves us out of focus.
At that focus distance, with the hood pressed against the subject, the lens covers an area 1 1/4" long, a bit of a crop of your 35mm negative.
But, the image quality is amazing. Seriously, I’ve tested this lens against the fabulous APO Rodagon D 1:1 copy lens on a 24MPx full frame DSLR for camera-scans of 35mm film. The Moment-iPhone7Plus combo is close to the Rodagon-DSLR combo. I think that’s an amazing result. Stamp and coin collectors will want this lens.
For capturing a full 35mm negative, you’ll have to use two shots, merge them into one larger image, then covert.
With this lens, you get the full 12MPx of the iPhone 7 camera. Rough resolution of a converted image? Calculating 12MPx period. That’s enough for a 12x18" enlargement if you have a good negative.
Here’s the same test image, iPhone 7 Plus, Moment Macro lens, translucent hood pressed against the subject.