Thanks very much. Bill and Bart, for your insights on AF vs rail, and the effect of modern internal focusing lenses --which have, effectively, a combination focus/zoom ring -- on stacked image perspective. Bill, your examples are great.
I remember some time in the early 90s when I got the idea of doing what's now called focus stacking. I put a Nikon 120mm macro lens on a 4x5 Arca-Swiss monorail camera, and made three exposures focusing by moving the front standard. I noticed that the magnification changed. Then I made three exposures focusing by moving the back standard. Different perspective. Then I moved both the same amount, effectively doing what the rail does, and I had a third set of perspectives. I scanned all nine negs with an Optronic ColorGetter (that's how long ago it was), and tried to write some software to merge the images. It became clear early on that I'd need to take the perspective into account. That turned a problem that I could wrap my head around into one that was going to take a lot of work, and I dropped the project. I was supposed to be working on device-independent color, so this was definitely a diversion. IBM had a journal that they used to publish ideas that they didn't want to patent, but didn't want anybody else to be able to patent. I wrote up a submission, and forgot about it. I don't know if it was ever published.
What goes around, comes around, and now people have apparently solved the geometry problem on which I broke my pick. And I'm a customer.
I think I'm going to start with the rail, since the two lenses I want to use the most on this project, the Zeiss 100mm and the Coastal Optics 60mm, are both manual focusing lenses. I'll play with the Nikon 60mm and 105mm if I want to try some focus-mediated stacking. It would seem that each internal-focus lens will have its own geometry/perspective, depending on the way the designer traded off moving the lens and changing its focal length as it is "focused".
Again, thanks for all your help.