Thanks for responding Bart...I currently use PS 2015 for stacking while shooting tethered using Canon Utility Software.
That'll work if the number of slices doesn't get too large. Otherwise, I'd prefer Helicon Remote for tethered shooting, because it allows to automate a lot of the tedious repetition (and avoid skipping slices), and they even have an Android App and an iOS version for tablets. The App can be used/evaluated for free if shooting small JPEGs (Demo license version), for Raws and large sizes one needs a paid license. This all works fine with AF lenses, just set the front and rear DOF focus position, and the App does the rest.
It allows camera control from the computer...never used a focusing rail although I understand the concept.
A focus rail makes sense if very many slices are needed (e.g. with macros, and 100 shots per stacked image), with very high accuracy (one or several microns per slice). It prevents missing a slice, and can run unattended if an automatic rail is used. Helicon Remote also accommodates the StackShot version. It also interfaces seamlessly with Helicon Focus stacking software, very useful if you do this stuff a lot.
It's the merging for large prints that is creating headaches....which is why I looking at the 5DS R.
Yes, that's why a 5DS / 5DS R should do very well. But Photoshop uses quite a lot of memory for stacking operations, so that may cause issues depending on your postprocessing hardware. That's another reason you'll want to get exactly the number of slices needed, and nothing more than that. The software can help with that.
Cheers,
Bart