Correct aperture setting with Pentax and Ringflash

My dentist has just aquired a Pentax P30 with a 90mm Tamron Macro

Lens (plus rear converter) and a non-dedicated ring-flash. He asked

my to help him with the camera settings.

 

I took him through the basics but I was left wondering about the

correct aperture settings. The non-dedicated ring-flash indicated an

aperture setting of F11 was required (when set to auto with 400 ASA

film). However on closer inspection the Tamron 90mm lens has some

form of rear converter (also a matching tamron item) which I guess

is some form of extension tube or multiplier.

 

My questions are (1) what did Tamron market to go along with their

manual focus 90mm Macro lenses, is it a tube? if so what light loss

does it suffer? (2) If the table on the rear of the flash states

that F11 should be set, I assume I need to set a wider aperture to

compensate for the adapter, say F8 or similar.

 

After my initial tutorial, my face when numb and he proceeded to

drill my head, so time was limited. However I have got an

appointment in a couple of weeks when I said I would have all the

answers.

I suppose the Tamron was marketed with a tube to be used between ratios of 1:2 & 1:1, which mean 1-2 f-stops light loss. Good luck, hope your teth are good & boring enough for not-modelling...