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11-23-2017, 12:44 PM   #1
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Sigma 18-35mm + K-5: Auto focus

Hi,
It's been a long while since I last posted here. I have mostly been a M43 user but my wife's main camera is her K-5 and with the recent failing of one of her lenses, she's looking for a new lens and the Sigma 18-35mm has always been on her sights. The only problem stopping her from getting that lens has always been all the reports about the auto-focus problems.

So, the question is simply...has the problems with auto-focus been resolved? Or to be more specific, how would the auto-focus perform on her K-5?
And no, currently there is no real desire to upgrade her K-5.

If the problem is still there, she'll probably get the Sigma 17-50mm f/2.8 that is currently on sale.

Cheers,
Hubert

11-23-2017, 01:13 PM   #2
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Unless your wife needs the extra stop from the 18-35 (and doesn't mind its being half again as heavy!), I'd suggest just go for the Sigma 17-50. You get the same focal length coverage and more, at about half the price. I have one, it's a great walk-around lens, and is at least as good as the Pentax equivalent in image quality.
11-23-2017, 02:06 PM   #3
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I have two K5's and use the Sigma 18-35mm f1.8 quite frequently on both of them. I have never really experienced any significant autofocus issues with the lens. I bought my copy here on the forums, so I can't speak for newer models, but mine seems to operate just fine.
11-23-2017, 02:56 PM   #4
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Sometimes you get lucky or unlucky. You can always exchange the lens or get it serviced for free if there's a focusing issue.


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11-23-2017, 05:29 PM   #5
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I was going ho for that lens when first announced. But it just never seemed like the focus of that lens was fully reliable. Then I started thinking and realized that I would very rarely need to shoot a wide wide shot with a wide aperture. I settled for the lovely da 20-40 limited f2.8-4.

As for the 18-35, I'd ask @digitalis about his experience and thoughts.
11-24-2017, 02:57 AM - 1 Like   #6
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QuoteOriginally posted by peppermonkey Quote
So, the question is simply...has the problems with auto-focus been resolved?
I skipped the original K5. I reviewed the Sigma 18-35mm f/1.8 ART lens on the K5IIs which has an Improved AF module compared to the K5. Other than my own experiences and a few discussions with colleagues that use other camera systems with the same lens. I have to say the probability of encountering a copy with good AF is pretty much down to chance. Certain bodies - and specific models seem to provoke the inconsistent AF behavior. For the love of god, never use the Sigma 18-35mm f/1.8 lens on a K10, that camera couldn't track the movement of a Glacier with that lens.

QuoteOriginally posted by peppermonkey Quote
Or to be more specific, how would the auto-focus perform on her K-5?
Depending upon the dominant light source in the scene, assuming 5500K AF accuracy should be as good as the aforementioned inconsistent AF issues permit. The K5 has an AA filter which may obscure slight AF errors - the K5IIs does not have an AA filter so even a slight mis-focus is blindingly obvious.

AF issues aside, the Sigma 18-35mm f/1.8 ART optically superb, by far the best zoom designed for APS-C format I have ever tested on my optics bench. It fills a focal length range that is useful with an f/1.8 aperture that makes it extremely versatile. It produces IQ that in certain respects exceeds the more expensive FA31mm f/1.8 limited: However as is typical for a complex zoom lens, it fails to completely surpass the FA31.

Last edited by Digitalis; 11-24-2017 at 03:10 AM.
11-24-2017, 08:44 AM   #7
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QuoteOriginally posted by AstroDave Quote
Unless your wife needs the extra stop from the 18-35 (and doesn't mind its being half again as heavy!), I'd suggest just go for the Sigma 17-50. You get the same focal length coverage and more, at about half the price. I have one, it's a great walk-around lens, and is at least as good as the Pentax equivalent in image quality.
She somewhat does. She likes her 35mm DA 2.5 and uses that in low light situations. She has the 50mm 1.4 but she just doesn't like the focal length. And she does want something a little more wider like a 24mm and a little faster as she is constantly going into high-iso territory with the 35mm. And unless I'm mistaken, there is not 35mm equivalent (FF) with a >2.0 aperture in AF Pentax land...which seems really odd. Sure, FF, but she has a K-5....
So the Sigma 18-35 would check a bunch of needs. Fast, good optically in a focal range that she would enjoy the most (albeit, the extra range in the 17-50 would be nice at times). Of course, the only problem with the 18-35 being weight, size, price and possible AF issues...

How is the Sigma 17-50mm compared to the 18-35 optically?

---------- Post added 11-24-17 at 10:46 AM ----------

QuoteOriginally posted by SointulArt Quote
I have two K5's and use the Sigma 18-35mm f1.8 quite frequently on both of them. I have never really experienced any significant autofocus issues with the lens. I bought my copy here on the forums, so I can't speak for newer models, but mine seems to operate just fine.
That's definitely good to hear I would assume newer models would have a better chance of working fine, specially with the bundled USB mount thingie that B&H is bundling with the 18-35mm currently.

---------- Post added 11-24-17 at 10:48 AM ----------

QuoteOriginally posted by Adam Quote
Sometimes you get lucky or unlucky. You can always exchange the lens or get it serviced for free if there's a focusing issue.
Exchanging the lens is harder for us as we are thinking of getting it from B&H yet we live in Canada. Can't just take it back, we would need to ship it over the border. Not sure if there is any Sigma service centres around my parts.

---------- Post added 11-24-17 at 10:51 AM ----------

QuoteOriginally posted by UncleVanya Quote
I was going ho for that lens when first announced. But it just never seemed like the focus of that lens was fully reliable. Then I started thinking and realized that I would very rarely need to shoot a wide wide shot with a wide aperture. I settled for the lovely da 20-40 limited f2.8-4.

As for the 18-35, I'd ask @digitalis about his experience and thoughts.
Are you saying that you had focus problems with the Sigma 17-50mm or the 18-35mm?
And yes, the Pentax 20-40 is a possibility. The main reason the 17-50mm is in play (besides the focal range) is because B&H is currently selling it for $369 which is significantly cheaper than the 18-35mm (and 20-40).

---------- Post added 11-24-17 at 10:56 AM ----------

QuoteOriginally posted by Digitalis Quote
I skipped the original K5. I reviewed the Sigma 18-35mm f/1.8 ART lens on the K5IIs which has an Improved AF module compared to the K5. Other than my own experiences and a few discussions with colleagues that use other camera systems with the same lens. I have to say the probability of encountering a copy with good AF is pretty much down to chance. Certain bodies - and specific models seem to provoke the inconsistent AF behavior. For the love of god, never use the Sigma 18-35mm f/1.8 lens on a K10, that camera couldn't track the movement of a Glacier with that lens.


Depending upon the dominant light source in the scene, assuming 5500K AF accuracy should be as good as the aforementioned inconsistent AF issues permit. The K5 has an AA filter which may obscure slight AF errors - the K5IIs does not have an AA filter so even a slight mis-focus is blindingly obvious.

AF issues aside, the Sigma 18-35mm f/1.8 ART optically superb, by far the best zoom designed for APS-C format I have ever tested on my optics bench. It fills a focal length range that is useful with an f/1.8 aperture that makes it extremely versatile. It produces IQ that in certain respects exceeds the more expensive FA31mm f/1.8 limited: However as is typical for a complex zoom lens, it fails to completely surpass the FA31.
Well, it's the optical qualities that has drawn my wife to the lens even if it's a little big for her tastes. But she's not the type who likes manual focusing (specially on a zoom), so if the lens doesn't behave on her K-5, it would become a big expensive paper weight. Also, unfortunately, upgrading the body (to say the K-3) is currently not in the cards so the lens must work with the K-5.
B&H is currently selling the 18-35mm with the Sigma USB Dock. Could the AF issues be resolved with firmware updates via the dock?

Last edited by peppermonkey; 11-24-2017 at 08:58 AM.
11-24-2017, 09:58 AM   #8
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QuoteOriginally posted by peppermonkey Quote
Are you saying that you had focus problems with the Sigma 17-50mm or the 18-35mm?
And yes, the Pentax 20-40 is a possibility. The main reason the 17-50mm is in play (besides the focal range) is because B&H is currently selling it for $369 which is significantly cheaper than the 18-35mm (and 20-40).
I have not used either. I was interested in the 18-35 when it was first announced and the focus issues (multiple brands) kept me away. Digitalis is an expert on this lens in my opinion.

Given your wife's need for faster - the DA 20-40 won't help. It's 2.8 only at 20mm and f/4 by 40mm. The DA 16-50 and the Sigma 17-50 are both well respected but again only f/2.8. The FA 35mm f/2 is on sale but it sounds like you are looking for something in the 20mm range and f/2 or faster - perhaps the 16mm Samyang - but no that's manual focus.

You say an upgraded body isn't in the cards - but an upgrade to a KP would give you much better low light performance with the existing lenses. Maybe that's a better path?

11-24-2017, 01:28 PM   #9
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QuoteOriginally posted by peppermonkey Quote
Exchanging the lens is harder for us as we are thinking of getting it from B&H yet we live in Canada. Can't just take it back, we would need to ship it over the border. Not sure if there is any Sigma service centres around my parts.
B&H should provide a label if the lens is defective, AFAIK. Wouldn't hurt to ask. I know they can take care of the customs stuff so you don't have to pay twice.

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11-24-2017, 04:26 PM   #10
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QuoteOriginally posted by peppermonkey Quote
Could the AF issues be resolved with firmware updates via the dock?
From my own experience with calibrating the lens with the sigma dock, No. The only camera system that has a firmware update for this lens is Canon EF mount.
11-27-2017, 11:51 AM - 1 Like   #11
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Just an update.
Thanks for all the info. My better half decided on the Sigma 17-50mm f/2.8. Couldn't pass up getting it for $369, what with the potential AF issues of the 18-35mm, plus the better focal range, even though we give up the faster lens with presumably better optical quality. Still, for her photography, the 17-50mm is probably good enough. Again, thanks for the info, much appreciated.
Hmm...may need to dust off my K-r that's sitting lonely and try out the Astrotracer (have the GPS) one day.
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