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06-14-2011, 02:23 AM   #1
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Converting Sigma 170-500mm F5-6.3 APO DG from Nikon to Pentax

Hi,

I'm new to SLRs and am still a bit confused about the mounts. I'm getting a K-5 in a couple of days, and have already started to look for some lenses to go with it (besides the kit lens and a Pentax 55-300. Specifically, I'm interested in a Sigma 170-500mm APO DG with a Nikon mount. Can I just unscrew the Nikon mount and put a Pentax mount on this lens, or will it be more complicated?

Thanks!

06-14-2011, 02:45 AM   #2
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QuoteOriginally posted by Lemonhead Quote
Can I just unscrew the Nikon mount and put a Pentax mount on this lens, or will it be more complicated?
it is considerably more complicated than that, when you buy a lens it is best to get it in the same mount as your camera has (pentax K mount) and you are"locked" into only being able to use lenses in that particular camera mount.

Of course there are some examples of lenses being adapted to fit other mounts - it often involves forgoing the luxuries such as auto focus or automatic metering, and in some cases requires extensive alterations to the lens that can make it inoperable on the system it was originally designed for.
06-14-2011, 05:36 AM   #3
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Forget it. I have modified a few manual-focus Nikon-mount lenses to fit (somewhat) onto Pentax cameras. That autofocus Lil'Bigma you are interested in is NOT suitable for modification, period.

I have one in Pentax mount. I'll sell it for US$650. Yes, the Sigma APO DGL 170-500/5-6.3, like new, complete package with original box and case and everything, completely functional. I bought it new for my K20D and I just don't use it.

As Digitalis said, you want to look for Pentax-mount lenses, either bayonet or screwmount. Short lenses in certain other mounts *can* be modified for Pentax, more-or-less satisfactorily: Olympus OM, Contax-Yashica, Exakta. But you shouldn't start with those. And other mounts can't be modified for Pentax at all: Canon, Sony-Minolta, m4/3, and most others. Forget those. Start with Pentax-mount lenses. Save yourself a lot of grief, eh?
06-14-2011, 10:18 AM   #4
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Thank you for the replies. With all of the threads I've been reading, I was confused and thought the lens had a T mount. I let one go to another bidder last night thinking I could just get one of the less expensive offerings with another brand's adapter and change out the mount. Thanks for saving me lots of headaches.

That's a bit higher than I was planning to go RioRico, but I'll definitely keep it in mind. It's not like these are being sold everywhere.

06-14-2011, 08:37 PM   #5
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What about Sigma mounts? There's a Bigma on Ebay that doesn't seem to be getting much attention. (of course that may change now that I've just told everyone about it)
06-14-2011, 08:53 PM   #6
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QuoteOriginally posted by Lemonhead Quote
With all of the threads I've been reading, I was confused and thought the lens had a T mount.
You won't find AF on any T- or T2-mount lens. I think Adaptall2 supported AF, but I don't think Sigma made any such lenses.

QuoteQuote:
That's a bit higher than I was planning to go RioRico, but I'll definitely keep it in mind. It's not like these are being sold everywhere.
I bought it new a couple years ago for full retail, US$1k. Duh. At US$650, I'm only taking a US$350 loss. Ouch. Yet another learning experience...
06-14-2011, 09:16 PM   #7
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QuoteOriginally posted by RioRico Quote

I bought it new a couple years ago for full retail, US$1k. Duh. At US$650, I'm only taking a US$350 loss. Ouch. Yet another learning experience...
Ouch! I didn't realize it was that expensive new, maybe $750.
What about converting Sigma mount to Pentax?
06-14-2011, 10:41 PM   #8
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Sorry about the double post (essentially). I didn't see the post appear on the forum after a reasonable amount of time.

I found out the answer in the meantime. Apparently Pentax lenses can be modified to fit Sigma cameras, but Sigma lenses can't be modified to fit Pentax.

06-14-2011, 10:47 PM   #9
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QuoteOriginally posted by Lemonhead Quote
Ouch! I didn't realize it was that expensive new, maybe $750.
What about converting Sigma mount to Pentax?
That won't work without losing significant functionality. To quote wikipedia: "Physically, the SA mount is identical to the PK mount with a reduced back-focus of 44.0mm (equals Canon EOS) as opposed to the 45.5mm of Pentax PK and M42. The Sigma SA lens protocol is a clone of the Canon EF protocol. This provides a not so obvious but very interesting option that most Canon EF-mount lenses will work seamlessly after exchanging the mount plate with the Sigma DSLRs, but without image stabilization." And to quote Bojidar Dimitrov: "The KAF mount features the same six electric contacts as the KA, and thus remains fully compatible with the older mounts. However, in addition to those (electric) contacts, it features a seventh electronic one. Using a yet-unknown serial protocol, the body uses this contact to communicate with a digital chip contained inside the lens housing." Obviously Pentax does not use the Canon EF protocol, which the Sigma SA mount uses, so no AF going from SA to KAF, assuming all physical hurdles were dealt with.
06-14-2011, 10:56 PM   #10
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QuoteOriginally posted by Lemonhead Quote
Apparently Pentax lenses can be modified to fit Sigma cameras, but Sigma lenses can't be modified to fit Pentax.
That's not quite accurate - people who modify Pentax lenses to use on Sigma mount are not getting AF, it is strictly manual focus (just like modifying Pentax lenses for any other mount, and vice-versa).

Other than EF-SA mount plate swapping, there is only one mount adapter for any camera I am aware of that supports AF: the Sony LA-EA1 adapter. It adapts the Sony A-mount (Alpha) to the Sony E-Mount (NEX), but it doesn't work with all A-mount lenses, and autofocus performance is significantly lower on a NEX for a given compatible A-mount lens than it is on an Alpha body.

Last edited by nater; 06-14-2011 at 11:07 PM.
06-14-2011, 10:58 PM   #11
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If you really want to do a mount conversion of some sort look for a manual focus, manual aperture lens with a flange-focal distance very close to or longer than that of the Pentax mount. It helps if you happen to have access to a machine shop.

Converting an auto-focus, auto-aperture lens designed for another camera brand will be more hassle and expense than it is worth, especially if the lens you want is available in Pentax mount to start with.
06-14-2011, 11:28 PM   #12
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The conversion from Sigma or Nikon mount to Pentax mount is very easy. You sell it on ebay and buy the Pentax mount version. Seriously.
06-21-2011, 08:24 PM   #13
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Thanks for all of the advice! I purchased a SMC Pentax 400mm 5.6 from the marketplace, and a Tokina 80-400mm 4.5-5.6 from Ebay. The dedicated 400 was purchased first, but I still needed something in between that and the 18-55 kit lens, hence the Tokina. After a couple of months, I'll probably be researching TCs.
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