Here's a puzzle. It may be answered when I decide how to mount this, but here goes...

A flange-mounted zoom lens with helicoid and focus distance only from 3.5-18 feet (studio lens)...no infinity. Flange registration distance is 100 mm (verbal info; I have to trust it for now).

I want to ANTICIPATE how much less flange distance I need to allow focus at infinity & include a removable spacer so that it can be used with the calibrated scale at the proper flange distance so it remains 'parfocal' (focus doesn't change with zoom change). When foreshortened to allow infinity focus, I expect it may become varifocal (refocussing required for zoom changes).

I would not expect there to be much difference in spacing.

Now, to go from 3.5 feet to a closer macro distance I do understand usually requires a significant repositioning/extension...like doubling for 1:1. Unless the math indicates a reasonably small spacer to improve the 3.5 foot minimum to something like 2 feet it might be worth doing, but not a tube.

It's going on a box for starters, then maybe a Speed Graphic to solve all the problems (shutter, extension, etc). For now, I don't want to the SG approach...the flange and mounting holes require a thick board or they will interfere with the front standard.

I can't imagine the difference in extension between 18 feet and 'infinity' being very much at all (50-150 mm zoom). It isn't on primes.

Does being a zoom throw wrenches into the works, or is physics physics and that's that?

Thanks