There is a rather common scenario where I'm shooting a horizontal subject from above with the image plane being parallel to the subject. I need to be able to adjust the camera-subject distance without disturbing the camera orientation (somewhat like the movement of a photo enlarger lens).
Ideally I'd have a latitude of 50cm (20") which is somewhat longer then the macro rails I've seen on sale offer
Also focal-length zoom is not an option here since the subject is not flat and a fixed normal FoV needs to be maintained
The camera in question is a 35mm DSLR.