Focusing with the Zeiss Macro lens: I have done more reading and experiments, and I have changed my mind from previous comments, which I have removed above.
Mauro is correct, turning the “zoom” doesn’t change the image size very much (but it does change a little). Instead, this mainly changes the range in which the iPhone AF can focus.
To focus on larger object, say 2" to 5" long, looking into the front of the lens, turn it clockwise. Zeiss says this allows focus from “five to eight cm away.” With my mount, the Zeiss label is now at the side of the camera, like this:
To focus on small objects, say less than 2", down to about 1" in length, looking into the front of the lens, turn it counter-clockwise.
Zeiss says this allow focus on objects from “three to five cm away.” With my mount, the Zeiss label is now pointing at the Apple logo, like this:
OK, then how to use the lens for capturing a 35mm negative area 1.5" in length? For this size object, the close focus setting on the zoom gives a much better resolution. I now suggest, for 35mm captures, use the close setting, looking into the lens turn it counter clockwise, and bring the camera right up to capture only the length of the negative image.
Here’s the resolution comparison, marks on paper, actual pixels, subject length 1.5":
This zoom control is incredibly confusing. My suggestion: Figure out for sure which setting gives closest focus by taking a photo of a 1" subject, filling the frame. Use that setting to capture 35mm shots.