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Teleconverter --> Extension Tube -- glass removal
Posted By: BretW, 12-12-2013, 06:46 PM

I will ultimately need the 100 WR Macro, but for now I have only the 18-135 kit lens for my K3. So I decided to get a used teleconverter (with the electronic contacts) off ebay and take the glass out. A used, but clean one from "Magnicon" cost me $20 shipped. I don't have a lens removal spanner, but I figured I would manage it, and if I destroyed the teleconverter, it would be a small loss.

I took the thing apart, carefully removing the bayonet mounts. Turned out, that part was unnecessary. All the glass was held in a central aluminum sleeve that was threaded in and then locked down with a threaded lock ring. It can all be accessed while leaving the rest intact. Live and learn.

Initially I thought I would just leave the aluminium sleeve out completely, so the opening in my new extension tube would be bigger, reducing any chance of vignetting. But in playing with the reassembled unit I noticed something. There are two moving levers in the mount, and if I move one of the levers by hand, the spring that puts tension on it cuts right into the space in the center of the converter. And if one looks closely at the outside of the aluminium sleeve it is clear that the spring rubs it under normal operation.

So, the lesson is, the sleeve needs to go back in order for the unit to be a viable extension tube. And one could easily not realize that if one never fiddled with that lever with the sleeve out, and the converter off the camera.

The last two shots show the difference in close focus without the extension tube, then with.

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12-14-2013, 03:24 PM   #2
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Wow, nice! Thanks for sharing the info!
12-14-2013, 04:48 PM   #3
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Sure, glad to share it, and to be in a community that values the pooling of this kind of information. Seems to be a hidden benefit of Pentaxianism.

I only wish I had thought to document the entire process. It was interesting.

Bret
12-29-2013, 08:00 AM   #4
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Hello--Great tutorial and tip! I'm trying to understand the "with electrical contacts" part. What era of lens is that teleconverter designed to function with? With the glass out of the converter, how are those electrical connections useful? Doesn't the lens require manual focusing now that you've completely altered its geometry? I guess what I'm asking is: a) is there a $20 teleconverter out there that functions fully with modern lenses; and b) once you've ripped the guts out of said teleconverter, are those functions (auto focus, metering) preserved?

Please forgive my ignorance; I'm just dipping my toe into macro. I've been considering getting a bellows set.

12-29-2013, 11:51 AM   #5
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No worries, those are good questions.

You are right that autofocus is lost with the converter/extension tube. That turns out to be just fine since the extension tube reduces depth-of-field dramatically, and so autofocus is more liability than asset. If you are shooting hand-held, course focus is accomplished with the manual ring, then fine focus by slightly moving the camera. Zoom also has surprising utility for focus, but never mind that for now.

What is preserved with the contacts is aperture control. If you go with tubes that have no contacts, then you will either be stuck with the lens wide open (in the case that you have a modern lens without an aperture control ring), or you will have to adjust the aperture manually in the case of an older lens. In my case, I have no manual aperture ring on the lens in question, so the electrical contacts were a must.

Incidentally, although it turns out I did not need to take the teleconverter apart to get the glass out, I did disassemble it before I figured that out. So I was able to see all the secrets inside and, in the case of this converter at least, the contacts pass information straight through between lens and body. There is no chip, or opportunity for information processing or modification of any kind. Given that, I was surprised that the metering and aperture control work without needing to correct for the extension tube.

In other words, I don't think the camera body has any way to know that the "18-135mm" lens it is talking to is more remote than normal. Given that, I expected the body to close down the lens-aperture too much, requiring an overexposure setting to get the exposure right. But as far as I can tell, that doesn't seem to be the case.

If someone knows what bit of logic I've got wrong, I'd love to know what it is.
12-29-2013, 03:51 PM   #6
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QuoteOriginally posted by BretW Quote
If you go with tubes that have no contacts, then you will either be stuck with the lens wide open (in the case that you have a modern lens without an aperture control ring), or you will have to adjust the aperture manually in the case of an older lens. In my case, I have no manual aperture ring on the lens in question, so the electrical contacts were a must.
Bret,

Thanks for the quick reply. In my case, I happen to have two 2x converters lying around--no contacts on them--that I never use. I also have plenty of manual lenses lying around to play with. My only hesitation now is the thought that the teleconverters might be valuable to someone. One is a Quantary and the other is a Cosina. I'll check Ebay, I guess, but if anyone has an opinion on whether or not to turn one or both of them into ersatz extension tubes, please offer it now!

v/r,

Rick
12-29-2013, 11:28 PM - 1 Like   #7
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PK, PK/A and even M42 teleconverters are so common today that any one that can be acquired for less than $30 used isn't going to be missed if converted to extension use. If you have a real use for a robust ~20-30mm extension ring feel free to convert it.

Theoretically you could even screw the optical element back in place if desired but it's a very fiddly task to get it re-registered for proper focus.

I've converted eight of 'em over the years and all worked just fine. Those with the PK/A contacts are convenient with modern lenses but the deliberate type of shots made with extension rings and the availability of immediate, post-shot exposure analysis makes AE functions of limited utility IME.

I'd note that with cannibalized lens or body mount rings and a little epoxy glue one or the other end of an ex-TC tube can be converted to use non-Pentax mount lenses from some of the fine, older makes like Minolta, Yashica, Ricoh, etc. With extension rings exact registration distance is generally not a factor.

H2
01-01-2014, 04:54 PM   #8
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Thanks for an awesome idea! Here's me putting it into execution:


2014 01 01 Ersatz Macro Tube - a set on Flickr

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