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09-27-2009, 04:58 PM   #1
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Pentax SMC DA* 16-50mm question

Hello,

I own a pentax k20d and I use a Sigma 18-50mm EX DC on it. This lense has always been great for my reportages. However today I took a picture of a building here in NY and for the first time i saw that at f2,8 the left side of the frame/lense is not sharp at all comapred to the right side. The effect decreases when I stop down but it never completely dissapeares. Now I wonder whether it is worth purchasing/exchanging my current lense for the Pentax SMC DA* 16-50mm. It is a little bit pricy but I want to get equal sharpness from corner to corner. What is your guys experience with this lense? Is it worth upgrading?

thanks
Alex

09-27-2009, 05:41 PM   #2
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Hi Alex.
Your 18-50 is not bad for a go-to lens. This problem you mention has also been seen on the 16-50 (but not on a good copy). Mostly due to decentering AFAIK...

If your lens is under warranty this can be repaired, but I don't know the success you'll get with the repair.
Getting a 16-50 to me is worthwhile, as pricey as it is, but just ensure you've tested it out extensively and are happy with it under those circumstances before settling for it.
09-27-2009, 06:55 PM   #3
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I would definitely see about getting the 18-50 fixed, unless you need weather sealing or the extra 2 mm on the 16-50. Even it isn't under warranty, you'll save quite a bit of money getting it fixed and you probably wouldn't see much difference between the lenses under normal shooting conditions.
09-27-2009, 07:40 PM   #4
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Ok thank you so mucch so far. I decided to post two images on imagehost: please take a look at them in full resolution and tell me what you think . They are both around 800kb. The first on is 18mm, f5,6, 125th and although the sharpness decreases towards the edges I find that the left side is even more "un-sharp" than the right side. The other image is at 18mm, f13, 30th and please tell me what you think.

ImageShack -

And another test, again the left side looks awefully un sharp. When I swith to f8 in the second image its gets a little sharper but not as good as he right hand side

ImageShack -


Last edited by Burgerbrater; 09-27-2009 at 07:48 PM.
09-27-2009, 07:57 PM   #5
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Sure, it's quite clearly soft more on the left than the right of the f/5.6 shot - it's worth having the lens sent in for to correct since f/5.6 should be in the region where the lens should perform sharply throughout the frame.

But also be weary that other lenses, including the 16-50 are not totally sharp across the frame wide open, especially at the short end. The softness should indeed be symmetrical, though, and should be corrected by f/4, definitely by f/5.6.
09-27-2009, 09:10 PM   #6
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The Sigma 18-50mm f/2.8 EX DC was reported in several reviews as being soft at the edges, needing f/8 before its anywhere near the centre sharpness. It was tested as Very sharp in the centre.

However, if the softness is not symmetrical, its likely to be a quality issue, not part of the lens design.

BTW, the DA*16-50mm f/2.8 had many de-centering problems reported as well....

All the Wide-to-Normal fast f/2.8 zooms available for Pentax mount seem to have a lot of problems going for them....Sigma, Tamron, Pentax.... (
09-27-2009, 10:12 PM   #7
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But the phenomenon is not totally explained by lens design - it's also due to the thin DoF and non-uniform subject distance esp. at the wide end that brings out this softness.

So it's not flaw at all, but any unevenness about the softness means a slight decentering problem with the lens.
09-28-2009, 04:20 AM   #8
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The initial crop of DA* 16-50's did have a lot of trouble with decentering, but I haven't heard much about it lately. More recent problems have more to with SDM and its durability (or lack there of).

09-28-2009, 05:24 AM   #9
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Hey Alex!

Owning both lenses in question and having had the opportunity to compare them on the K20 (and the K-7), I would encourage you to buy the 16-50. The Sigma's corner weakness, usually equally to be seen on both sides of the photos, is quite substantial even at f/5.6, which obviously reduces its usability for, say, group portraits. Of course, as already stated by others, you would need to find yourself a good copy of the DA*16-50, but I say it's well worth the effort. The 16-50 is the superior lens, and though it lacks the macro function, I consider it more versatile than the Sigma. There's this French pro photographer who uses it for fashion shoots, I can't remember his name right now, he's a member here as well. I did portrait shots with it and was more than pleased with the outcome, not to mention landscape or cityscape photography or pictures of architectural detail. Not only because of the weather and dust sealing, it makes a very nice, high quality travelling kit together with the 50-135 and the DA*200.

Best wishes
Daniel

Last edited by travieso profundo; 09-28-2009 at 05:56 AM.
09-28-2009, 10:22 AM   #10
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I used to own a Canon system, and with it a 17-40L lens. It was terribly asymmetrical when I got it, even on cropped sensor, not to mention on full frame, but it was a fairly straight forward procedure to align it. You should definitely try to get it fixed if it is still under warranty.
09-28-2009, 11:25 AM   #11
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QuoteOriginally posted by Rondec Quote
The initial crop of DA* 16-50's did have a lot of trouble with decentering, but I haven't heard much about it lately. More recent problems have more to with SDM and its durability (or lack there of).
The centering defects that plagued early copies of the DA* 16-50 seem to be a thing of the past. My copy, purchased new about 7 months ago, is excellent.

I cannot recommend highly enough using this lens in conjunction with DxO Optics Pro for RAW conversions. DxO has a specific lens correction module for the DA* 16-50 matched with the K10D and K20D. The results far surpass those of any other RAW convertor that I have used or tested in terms of detail, clarity and sharpness.

Rob
09-28-2009, 01:49 PM   #12
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Another vote for the 16-50. I have a more recent copy, and it is absolutely exquisite. I consider is more than acceptably sharp at all settings. What's this about SDM failure? Mine has been a little squeeky lately...
09-28-2009, 02:01 PM   #13
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As far as sharpess goes, the Sigma is about as good as the Pentax.

I have tried both and i feel that the Pentax has better out of focus rendering (boke) than the Sigma does , so if you value that, weather seals and silent AF(but not faster AF) GO PENTAX!

If this doesn't matter to you and 16-18mm isn't important, then keep the Sigma.
09-28-2009, 06:59 PM   #14
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Ok but I really hate the fact that on the sigma you have to set it to f8 to get it sharp from corner to corner. I want the best possible option in terms of sharpness.

By the way what do you guys think of the 16-45mm from pentax. Not a star lense but it has a high rating?
09-28-2009, 07:05 PM   #15
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The 16-45 is probably as close as you'll get to the 16-50 in terms of image quality, but just no SDM or weather sealing. It's a lot lighter, but has decent build quality.

If you're happy with the range, then it's definitely a lens to consider.
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