The iPhone 7 Plus has two front-facing cameras: a 4 mm "wide-angle" lens on the (user's) right and a 6 mm "telephoto" lens on the left:
I want to use an external lens that clips over the internal 6mm camera, which has the unfortunate side-effect of covering the internal 4mm camera.
I can use the "2x" button in the Camera app to switch to zoomed mode, which either uses optical zoom with the 6mm camera or digital zoom with the 4mm camera depending on light conditions. Unfortunately, it seems to use the data from the 4mm camera to determine which to use. And if the 4mm camera is covered, it never switches to the 6mm camera at all.
Is there any way to force the phone to use the 6mm camera? I would prefer a solution that doesn't involve an app, since I won't be able to access it from the lock screen.