Ray, you are right about number 4. However, Metabones did make different speed boosters for different model cameras, and original generic MFT Speed Booster, the BMCC, BMPCC GH4/5 all had specific recommended SB, for example. But because they can not control variance in camera lens mounts or ca era manufacturing by the various camera makes, they use the published standards or a sample camera for setting the mount. This is why some Pocket Speed Boosters fit tight and sime loose, due to issues by BM to keep the mount consistent. BM also changed the original EF mount in the BMCC when people complained about some EF lenses not hitting inf. focus, which added to the issues with lenses and adapters on those cameras. Metabones is not free from errors, and the OPs SB needed replacement or adjustment to correct the issues it was having.
Thirdly, I don't see how shimming would affect a lens that is truly parfocal. If the lens focuses correctly at one zoom setting, it should do so at any zoom setting, that's what parfocal is about. Unless you're talking about shimming to set the lens at some average point where nothing is critically in focus, but close enough.
First a parfocal Zoom lens is designed to be parfocal with a given lens mount FFD dimension. The camera this parfocal lens is being used on must also have a lens mount FFD that matches the standard being used.
The PL mount is 52mm and Canon EF mount FFD is 44mm, for example. The PL. oh t can be shimmed in most cameras, whereas the Cankn EF mount is fixed. Now, if the Camera Mount is off by as little as 0.020 of an inch, a mechanically set parfocal cine Zoom or any parfocal Zoom (except for zooms that are controlled by the camera electronics) will not give parfocal performance as the “Back Flcus” is off, and the focus will change when you zoom, get this off my a 0.10 of an inch, and the lens will no longer hit inf. focus.
This is pure physics, nothing magical here. So while my copy of the Oly 14-35mm f/2.0 Zoom is giving parfocal performance on my AF100, it might not give parfocal performance on the new Pocket 4K. Is your 14-35 parfocal on your Pocket 4K?
This is why the Cine PL Mount was designed to be shimmed, so the FFD could be corrected and reset back to the 44mm stsndard, and correct for lens mount or camera mount wear, which will also throw off the FFD over time. Stainless steel is being used now to reduce wear issues on these mounts.
This also explains why some Sigma 18-35s will give very close to parfocal performance on some cameras and not on others. It all boils down to what the camera’s actual FFD is and the build quality of thenlens sample, which vary from lens to lens.
There are errors in very type of manufacturing, and expecting error free performance from one company and not another is very un realistic. Even robotic, CNC and other computer controlled manufacturing is not error free. Even the new Nikon Z cameras have bad copies shipped, and need adjustment or replacement.
Nothing is perfect, but we try to be as close as we can.
Cheers