I have a Minolta Auto Bellows 100mm lens (no focussing functionality, just aperture), that I am trying to use to duplicate slides with a A7R III and Minolta bellows I, focussing rail and slide duplicator attachment. My goal is to take a 1:1 photo of a 35mm slide, but I'm having trouble getting the lens to focus within the range of the bellows and focussing rail. I can get the lens to focus on a slide that's about 400mm from the film place, but that's well beyond the range of the bellows.
From what I understand, the farther the lens is from the focus plane of the camera, the shorter the focal distance. I thought I should be able to extend the bellows a bit, then use the focussing rail to get the slide in focus, but no combination of lengths seems to work to get the slide in focus. Can any give me a good explanation of how this is supposed to work, or if it can work at all with a 100mm lens?