I recently purchased this extension tube, which Tamron expressly states is for use with the the 90mm f/2.5 lenses exclusively:
Now, I don't own any 90mm f/2.5 lens, and I bought this tube with my 35-80 mm f/2.8-3.8 Model 01A in mind, being triggered by the outlook og having at least one extension tube with A* functionality. So what's this about "exclusively for....
I found out, when I received the 18F: As the Tamron Costom Mount telles us, there are two coupling keys that should match the slots on the lens aperture ring:
"Some mounts have two coupling keys. If the custom mount you own has two coupling keys, be sure to engage the two keys into matching slots on the lens aperture ring."
And whereas the coupling keys on the P/K and P/KA custom mounts are free to move relative to each others, they are mounted on the same rotating ring on the 18F tube and will only match on a lens with a maximum aperture of f/2.5. My 01A lens has a maximum aperture setting of f/2.8 - not much of a difference, but it is there:
Well, nonetheless I am happy to report, that it still works as it should on my lens and together with the SP 01F 2X teleconverter I now have my desired "macro lens for Pentax-A":
giving me magnifications from about 1.2 : 1 at infinity setting:
(ruler back-lit by PC screen)
up to about 2.5 : 1 at closest focusing distance:
And I think the combo produces a pretty nice image quality too:
(Taking a closer look at my wife's roses)