[Wasp]

I have a set of DOI branded extension tubes. They were bought from an auction site about a year ago and arrive in pristine, new old stock condition. From the age of the box, they probably date back to the film era and are equipped with couplers to connect the diaphragm in the lens to the camera body. At the time it seemed like a good idea to have that. Reality turned out a little different.

The auto coupling does not seem to work well with digital bodies. Perhaps the modern electromagnetic diaphragm actuators don't have the kick that the mechanical ones on film bodies have. On some lenses such my K 200mm f/2.5 this leads to overexposure because the diaphragm does not always stop down all the way.

Today I found that my new (to me) Rokinon 85mm will not mount on the shortest 12mm tube. I did not want to force it and gave up. It mounts fine on the 20mm and 36mm ones. Pentax lenses have no problems mounting on any of the tubes.

All in all it seems that the auto coupling is just not worth the hassle. I am really keen on converting them to manual aperture operation. The question arises: how hard can it be? Before I break out the screwdrivers, I would like to hear if anyone has attempted this before. It would be a bonus if the modification can be reversed at a later stage, but these things are hardly collectible and I would rather get some use out of them than keep them original.

I could just buy a set of 'unwired' tubes, but I would rather blow invest the money on lenses, flashes, etc.

[victormeldrew]

There are many options for fully manual tubes ranging from very cheap to fairly expensive. I opted for the former and they work perfectly well, especially considering the price I paid (equivalent to maybe 10USD). Personally I would rather sell the auto tubes and buy some cheap manual ones than to hack the autos.

[photoptimist]

First, this is not a fault of digital bodies. The root cause is either in the lens or extension tubes (probably a bit of both). In the Pentax aperture system, the body does not push the aperture closed. Instead, the body's lever moves out of the way and lets the lens lever move to stop the lens down. If the lens spring is weak or there's lots of friction in the lens or extension tubes, then the lens won't close down very fast.

The quickest and worst way to get rid of auto coupling is to cut the aperture tabs on the extension tubes. That's an irreversible modification that reduces the resale value of the tubes.

Whether a deft bit of disassembly lets you reversibly remove the aperture tabs depends on exactly how DOI made the thing. It might be easy or it might involve a zillion tiny ball bearings cascading from the tube when you open it up.

Good luck!

[sculptor666]

get pentax branded tubes. sure they haven't changed since 1979, but they're the best... especially if you really start stacking them. no weird ghosting in the center of the image if you stack past 67mm, no aperture problems, no vignetting on ff. once in a while on ebay, you can find them cheap. the no contact auto tubes hold the aperture open to focus, and stop down when you press the shutter button... you have to use a lens with an aperture ring though. i have a set of kenko tubes with full contacts, but they're not as good... second best though, and the only ones with an A contact that i trust.

[boriscleto]

The problem with the Kenko tubes is that they're not cheap. I got lucky and found a set on shopgoodwill in a lot that also had a black KX and a PZ-70. So I didn't mind paying over $100...

[sculptor666]

got mine for $30... just have to be patient.