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08-02-2010, 10:21 PM   #1
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Sigma teleconverter & Pntx mount problem

I just purchased the Pentax 300mm and a Sigma 1.4x teleconverter (TC) from B&H last week. The locking pin doesn't lock into place between the lens and the TC. I'm not sure if the hole or the pin is at fault. A Sigma lens fits fine on the front of the TC, and the 300 has no issues mounting the camera.

One could take a file to shave off the edge of the the Sigma's pin, in principle, of course, since the alignment id sooo close.

Has this issue come up before?



08-03-2010, 08:15 AM   #2
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I called Sigma and they do not support using their Tele-converters on any lenses but Sigma. The person I spoke to said the TC will not attach correctly to your lens and suggested not using it.

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08-04-2010, 10:08 AM   #3
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QuoteOriginally posted by Chuck-B&H Quote
I called Sigma and they do not support using their Tele-converters on any lenses but Sigma.
- because that could be interpretated as making them liable for any damage that ocurred using the converter with other lenses. The converter normally works with other lenses so long as the protruding front element of the converter doesn't foul anything on the back of the lens being mounted. I've never handled a DA* 300 (avoids LBA!) to know how deeply recessed the rear element is - the Sigma won't mount a DA*50-135 because the rear element isn't recessed. It will, however, happily mount a Tokina 400mm where the rear element is half way up the lens tube. The Tamron and Kenko converters don't have protruding elements and should fit more-or-less anything, if you can find one.
08-07-2010, 11:14 AM   #4
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Bottom lines:

Yes indeed, the Sigma tech rep said that the company may have created the pin-hole misalignment intentionally.

1. GOOD: Unlike the two standard zooms of the Pentax kits, the rear element of the 300 mm Pentax DA stays well away from the Sigma converter.

2. GOOD: Optically, the Sigma teleconverter works well with the 300DA Pentax lens after you put it on, twist back a little so the manual lens mm adjustment pops up on the Pentax screen, click on "450mm" and turn the lens a couple of mms more until it can go no further. The combination is sharp (manual focus only).

2. BAD: The lens did not lock onto the TC. In contrast, a Sigma lens I have did lock in place. Without the locking pin in place, one stands the likelihood of hearing an expensive lens clattering on the floor, so what could have been a working solution becomes an expensive risk (yes, it will easily turn back and fall off). The only solution might be (untested, as judged seeing how little bits of tape lined up) to file off some of the advancing edge of the Sigma locking pin and it may become able to lock. The Sigma converters are $250, and modifying them blows any return warranty.

It really is disappointing to be so close and yet so far from having a decent TC.

I've been able to find a Tamron 1.4x TC and am returning the Sigma TC. Perhaps someone else can try modifying a TC locking pin and report back. Right now there's nothing else out there. Expectations of Pentax releasing their promised TC (late 2008) have evaporated, just as did the 1.7x TCs made by Hoya (Pentax' parent company). That last is a mystery given that the 300DA is made in Vietnam, the last place where 1.7s resembling the Hoya TC were being sold at a very high price...

Last edited by allart; 08-07-2010 at 11:21 AM. Reason: clarifications

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