My extension tube set (Soligor for Pentax AF + P/KA) was not working correctly on my DSLR, so I checked the tubes, and found an interesting difference. It has three tubes (12mm, 20mm, 36mm).

This is a photo of the 20mm tube:

enter image description here

and this a photo of the 12mm tube (the 36mm is similar to this one):

enter image description here

Note the difference of the third (from the left) pin. On the 20mm tube it is similar to the other pins, on the 12mm tube it is always recessed.

According to Bojidar Dimitrov's K-mount page this pin transfers the "A" aperture settings information:

On the body mount, the "*" contact is slightly recessed. On the lens mount, "*" is a small pin whose head can be raised or lowered depending on the position of the lens aperture ring. When a numeric f-stop is selected, the pin's head is lowered so that it does not make contact with body's "*" contact; when the "A" aperture setting is engaged, the pin's head is raised, and it makes electrical contact with the body's "*" contact.

This explains the strange behavior of the extension tube. If I use the 20mm tube the camera always thinks that the aperture ring of the lens ( SMC PENTAX-A 50mm f2 in my test ) is set to "A" setting. It also explains why my camera+12mm+20mm+lens setup was not working the same way as my camera+20mm+12mm+lens setup.

Is it some kind of bug (they mixed up the tubes)? Or maybe it's just an undocumented feature of the extension tube set to have different elements? Is there a camera+lens combination when the non-recessed pin of 20mm has advantages?

  • It at least looks like the reverse is true of the pin at the far right: recessed in the upper photo, but not in the lower. I'm not sure what's transmitted on that pin and/or what its being recessed does, but you may need to take both into account for it to make sense... Feb 5, 2011 at 16:35
  • Thanks for pointing out, I missed that difference. According to the K-mount page that pin is for K-F mount so hopefully it's not relevant here.
    – asalamon74
    Feb 5, 2011 at 16:45
  • I'm looking at tubes to buy for pentax - does this mean that they won't set aperture at all, or that they will in one config, but not another
    – rfusca
    Mar 26, 2014 at 1:11

2 Answers 2


I've sent an e-mail to Soligor (as suggested by Pearsonartphoto) and their answer is:

It is not a bug, it is a normal feature. Why our manufacturer did it in this way, I don't know.

  • 2
    Thanks for posting your response from Soligor. Given their problems with controlling their manufacturing processes and their apparent lack of concern about it, I think I'll look to another manufacturer if / when I go looking to buy a set.
    – Mike
    Mar 23, 2011 at 23:00

I've thought about this several times, and I'm going to recommend that you take the set of extension tubes back. At the very least, email the company that made them and ask if it's supposed to be that way. They do say that their products are guaranteed for 2 years. Their contact info is on the bottom of that page. I don't know where you bought it from, but if online, there should be a customer service that can help you, or just walk in the store.

  • It's more than two years old so I have no guarantee. Anyway I'll send them an e-mail and ask about their opinion.
    – asalamon74
    Feb 11, 2011 at 13:20

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