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04-26-2019, 01:33 PM   #1
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Is this Sigma 18-35 f/1.8 Art for Pentax defective?

Hello dear Members of the Pentax forum,

I have just bought & received a second-hand Sigma 18-35 f/1.8 Art lens and don't like it at all. I'm pretty sure it's defective but before I ask eBay/PayPal for a refund I'd like to ask owners of Pentax DSLR and such lens to confirm my opinion.

First suspicion are noises when moving the zoom ring. I would expect that the zoom ring moves absolutely silent, like on my identical lens with Canon mount. However, something seems to scratch inside... Here a short video showing it:

The volume is cranked up so you can clearly hear it. Anyway, it's clearly audible, you can't miss it when using the lens. Is it normal for this lens with Pentax mount or definitely a defect? I think it's just broken. But I ask, as the mechanical construction is slightly different between these two systems due to how aperture is being closed (Canon -> electronically, Pentax -> mechanically, so there are additional springs and so on).


Second problem is corner sharpness. This particular lens is very sharp in the center but already at 1/2 of the distance towards a corner it gets blurry. Sides and corners are VERY blurry. Even closing sperture to f/2.5 doesn't help. Didn't try more but anyway, I bought this lens for shooting astrophoto wide open. For comparison I took same photo with a kit lens SMC DA L 18-55 f/3.5-5.6 AL WR and consider it very nicely sharp fully opened, even at corners. My Sigma 18-35 f/1.8 for Canon mount is also sharp to the corner already at f/1.8 and this is what I expect from this lens.

Why I'm still not sure that it's defect? Another lens, which I bought new and would expect to be nicely sharp is Tamron 17-55 f/2.8 but... it's also not. Center is nice, corners are totally blurry even at f/8. Same like with this Sigma. Is it normal on K-3, that these third-party lenses offer inferior quality compared to other cams? Hard to believe. It think they are just both broken.


Last thing I have noticed were sligtly defective screws on the bayonett. Someone definitely screwed them out and it wasn't an official service. I could've lived if it was the only problem. But it's not. And signs of opening are just a big red light.

Thanks in advance!

Best regards,
ArcziPL

04-26-2019, 01:41 PM - 1 Like   #2
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Sounds like metal on metal scraping to me. So not good.
I'd send it back. All the signs point to a botched repair job.
04-26-2019, 02:47 PM   #3
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Think it should be quiet. Can you post some photos taken with it? Would expect that lens to be pretty sharp. Are the corners equal in sharpness (or lack thereof)? Sample photos are the only way to really evaluate image quality.
04-26-2019, 04:21 PM - 2 Likes   #4
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I own one and it is dead quiet and very well dampened. Sharp as a tack and silent focusing too.

04-26-2019, 06:25 PM - 1 Like   #5
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QuoteOriginally posted by VoiceOfReason Quote
I own one and it is dead quiet and very well dampened. Sharp as a tack and silent focusing too.
Ditto
04-26-2019, 11:20 PM   #6
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Thank you all very much! It answers my questions.
04-26-2019, 11:59 PM - 1 Like   #7
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Nup. That's nasty. Send it back.
It doesn't matter how good the images are.
04-30-2019, 02:00 PM   #8
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Lens returned, got a refund. Thank you all once again! All in all a happy end but cost me much time and stress.

Had to make documentation for eBay resulution center, so I publish it also here.

A sample from corner. Extremely blurry, despite focused in LiveView exactly at this point; exposure = 1/400 s so you can exclude any motion blurr)


And clear signs of opening by a non-professional:




04-30-2019, 02:49 PM   #9
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As to your Tamron 17-55mm, f/2.8 do you mean the current 17-50mm f/2.8 XR Di II LD? In any case, as an alternative I can definitely recommend the Sigma 17-50mm f/2.8 EX DC for Pentax. It is very well built, sharp, and has quiet, fast, and accurate AF performance. It also has a handy AF/MF switch which will go from and to either setting without having to also use the camera's switch.

Tests have revealed this Sigma lens as having exceptionally low field curvature, where the Tamron's was high. This can affect edge sharpness in certain circumstances.

Last edited by mikesbike; 04-30-2019 at 02:54 PM.
05-02-2019, 03:07 AM   #10
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QuoteOriginally posted by mikesbike Quote
As to your Tamron 17-55mm, f/2.8 do you mean the current 17-50mm f/2.8 XR Di II LD?
Yes, exactly. Sorry for writing it wrongly. Actually I sit in a Canon system, where all my similar lenses end at 55mm, while the Pentax belongs to my wife. She got the SP AF 17-50mm F/2.8 XR Di II LD from me and also the Sigma 18-35 Art was purchase for her, after she has seen what such lens can do with my cam. I have a pretty good experience with buying second-hand lenses (already >13 of them in total) but there is really good choice in Canon system. For Pentax you have to hunt. It's a pitty, as they make soooo good cams with so good price/quality and price/features ratio. And... yeah, it didn't work.

QuoteOriginally posted by mikesbike Quote
Tests have revealed this Sigma lens as having exceptionally low field curvature, where the Tamron's was high. This can affect edge sharpness in certain circumstances.
Correct but testing the corner sharpness I enter LiveView, enable 10x magnification, scroll to the place where I want to set focus, do it manually and then take a shot. So the field curvature should play no role.

This is how it looks for the Tamron 17-50 f/2.8 @ 17mm, f/2.8. Shots made with K-3 (24Mpx).

Full frame, focused manually in the bottom left corner


Close to the center the sharpness is perfect


Close to the corner it is also pretty good


But the very corner is bad. It is a very small area of the whole shot but still disappointing.



The kit DA L 18-55 AL WR has much better corner performance. Same it true for Sigma 10-20 f/4.0-5.6, which my wife also got from me. I find it hard to find a full-size sample from Tamron 17-50 and Pentax, where there would be something detailed in the corner and within the focal plane, for a sake of comparison. This lens is still on warranty, I wonder if it's worth sending it for a service. I don't have best experience with their service: my 28-75 for Canon has awfully bad AF performance (it's tack sharp but AF misses reproducibly). Service tried to calibrate it, they mean it's within their spec but still doesn't work good on any of my cams (and there are 3 different models, including a mirrorless one).


QuoteOriginally posted by mikesbike Quote
In any case, as an alternative I can definitely recommend the Sigma 17-50mm f/2.8 EX DC for Pentax. It is very well built, sharp, and has quiet, fast, and accurate AF performance. It also has a handy AF/MF switch which will go from and to either setting without having to also use the camera's switch.
Thanks for recommendation. I considered it before buying but finally decided to take a Tamron. Don't remember anymore why. It was a weighted decision based on reviews, taking sharpness, reflections, distortion, aberration, coma, behavior with light spots in viewing field into account. But maybe Sigma would be a better choice indeed.

Last edited by ArcziPL; 05-02-2019 at 08:43 AM.
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