The iPhone, at least the ones I can test, cover an area larger than the 35mm negative at their minimum focusing distance. So, if used for FilmLab on 35mm films, you’ll lose some of the native resolution of your iPhone camera.
I had thought the iPhone 7 Plus with a 2x lens (56mm equiv) would be a real advantage, covering a smaller subject area than the 1x lens (28mm equiv). You touch the “2x” button and you zoom in 2x on your subject. That’s true and does a great job at distance, but there’s a wrinkle at shorter distances.
You hit the 2x button, and the iPhone Camera app automatically selects the lens that will produce the best image. The 56mm will only focus to about 1 foot, so for closeups like a 1" x 1.5" subject, the Camera app uses the 28mm lens and delivers a digital zoom. It still delivers a 12MPx file, but I believe I can see the artifacting of the digital zoom process.
That said, hit the 2x button and you get a good clean image of an area 1 3/4" long. Pretty good for our uses. Rough resolution of a converted image? Calculating 12MPx x 0.8^2 x 0.5^2 = about 2MPx. Plenty for web, probably enough for a 4x6" print.
An app with access to the iPhone 7 Plus camera can select which lens to use, so FilmLab could control the lens. But, given the 1’ close focus limitation, FilmLab won’t want to use the 56mm lens.
Here’s my test shot iPhone 7 Plus, 2x selected, Apple Camera app, at roughly the minimum close focus distance.