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06-19-2019, 05:27 AM   #1
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Disappointment due to Sigma 17-50 f2.8 on K50

Hello Pentaxians,

I am kinda disappointed with the results of Sigma 17-50 f2.8 on K50. Even after calibration, still there lacks some sharpness which I am expecting, the side parts of the photo are soft, so sharpness is not constant throughout the frame. Low light conditions also makes the focus problems.

Any expert could shed some light as to how to counter these problems or should I change the glass?

06-19-2019, 05:47 AM - 1 Like   #2
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please show us some examples of the problems, with EXIF data , so we can see the issues you describe...
06-19-2019, 05:53 AM   #3
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A picture would help. But you should not expect to get corner to corner sharpness and a high level of homogeneity in a non professional zoom. In fact, even pro zooms costing thousands of $ aren’t perfect in this regard.

So, yes, softness is expected from a zoom like the Sigma. It has some sweet spots and others that are so-so. It’s for you to get the best of it.

If you want the best possible IQ, you should get prime lenses. Even a cheap DA50 1.8 or DA35 2.4 will be much better. Zooms are used for convenience and versatility, not for the best performance.
06-19-2019, 06:10 AM - 1 Like   #4
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QuoteOriginally posted by haroon88 Quote
Even after calibration, still there lacks some sharpness which I am expecting
Do not fall into the trap of expecting AF Fine-Adjustment to do anything other than get the camera/lens combo to focus accurately to the best ability of the lens. Even a very soft lens can be calibrated accurately but that won't make it any sharper. Likewise AF-FA can do nothing for characteristics of the lens itself like edge softness or field curvature.

06-19-2019, 06:45 AM - 1 Like   #5
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If you want corner to corner sharpness, DFA 50 1.4 and DFA 100 macro are good choices. Apart from the DA* 60-250 and DA* 50-135I'm not aware of any zooms with excellent corners, but someone else might.

But there's a theme there, most 16-50 type zooms are much sharper in the centre than they are at the edges. Zooms don't get edge sharp until they start at at least 50mm.
The 31 ltd at ƒ/4-ƒ/8 might be your best option.

Just as general rule, 16-50, 17-50 type lenses do not have excellent corner sharpness.

For good edge sharpness throughout you have to go to big heavy and heavily corrected lenses like the DFA* 50 1.4 or lenses like 31 ltd, 43. ltd or DA 70.

Probably the best deal for a standard type edge to edge sharpness would be the 50 macro....


Even among stellar wide angle lenses very few are excellent on the edges at ƒ/8 for landscape. Most are very centre sharp, and close to excellent on the edges but you should be able to see a difference between centre and edge pixel peeping.

So the questions is not so much about your dissatisfaction with one lens, you may be a victim of sample variation, and in my experience older third party lenses are more susceptible to de-centering so that could also be factor. The question is, what could you buy that would possibly be better?

Based on reputation and the charts, I'd say you're looking for the DA 20-40 ltd.
https://www.ephotozine.com/article/hd-pentax-da-20-40mm-f-2-8-4-limited-dc-wr-lens-review-23898

Good solid edge to edge numbers f5.6 to ƒ8.

If you're looking for good edge to edge number at wider apertures, you're looking for the DFA 50 1.4 type lenses, and that is great from ƒ/1.4 to ƒ/4, it's not a spectacular ƒ/5.6 to ƒ/8 for landscape. Edge to edge sharp from wide ƒ2 to ƒ8, I don't know that it's ever been done.

Most are like the 31 ltd. excellent centre sharpness with softer borders for portraits ƒ2 to ƒ4, excellent edge to edge ƒ5.6 to ƒ8 for landscape.



https://www.ephotozine.com/article/smc-pentax-fa-31mm-f-1-8-al-limited-lens-review-16866

Last edited by normhead; 06-19-2019 at 07:49 AM.
06-19-2019, 08:07 AM   #6
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It certainly depends what and how you are trying to capture and what you expect out of your approach. I see that you have shot with the M 50/1.7 before, which is a very sharp manual prime lens. I don't think you can challenge this lens with the Sigma 17-50/2.8 when it comes to sharpness, but I am quite pleased with the results I get from it.

Taken on a K-S2, handheld, both at f/8 (I could share some examples at 2.8 as well if you'd like to compare them with your own results to see if maybe your copy is not up to the quality of others)

17mm

Coal lorries
Top right corner is a bit softer than the rest (Bottom right corner is just too close)

50mm

Busy scene

I did have some trouble later that day when it was really dark with focussing, but that was when it was dark night with just a little artificial lighting. My attempts at manually focussing through the viewfinder were not successful
06-19-2019, 11:14 AM - 1 Like   #7
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A well-made constant-aperture 16-18mm to 50mm f/2.8 is indeed a pro-oriented zoom lens. In fact the Sigma 17-50mm f/2.8 EX DC outperforms the Pentax pro-grade DA* 16-50mm f/2.8 in edge sharpness at wider apertures in lab tests. It was also shown to have exceptionally low field curvature. I have one, and consider it to be among my best zoom lenses. I do not pay a whole lot of attention to corner performance, but good edge performance for me is of greater value, and I have found this Sigma lens to deliver in this regard. I have done my own testing and have verified the lab tests, even finding my copy to be visually superior to what the lab tests indicate. I get fine edge-to-edge sharpness in my photos. With mine I have found very good results even wide open, while even a very slight stopping down to f/3.2 yields noticeable improvement at the edges. They come in very well.

The other lenses suggested above are excellent, but they are not really comparable because they are not zoom lenses, and do not cover the same needs and focal lengths.

Even with the best zoom lenses, and with a good many of highly-regarded prime lenses, most will not generally be quite as sharp at the edges of the frame as in the central area, especially when wide open, but results will still be decent to very good here. We need some examples of images showing the results you are unhappy with, including exif information, so we can be of better assistance.

If you are having problems with focus in low light, this is another issue, and of course will not result in sharp images.

Last edited by mikesbike; 06-19-2019 at 01:15 PM.
06-19-2019, 12:04 PM   #8
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Issues could be due to the lens, missed focus, camera settings, etc. Try to rule things out with a controlled test. I would start by testing on a tripod. Use Av mode set at f8 (that's where many APS-C lenses look sharpest). Manual focus by zooming in using liveview. Use the countdown timer shutter mode to avoid camera shake. Take some test photos.

If you can't get any photos that are sharp enough, the lens might be defective or just not meet your standards.

If you get sharp photos on the tripod at f8, then you can start testing f2.8, handheld, autofocus, etc. to narrow down the problem.

06-19-2019, 06:12 PM - 3 Likes   #9
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My copy is nice and sharp, Haroon, but if you have 'brick wall' shots to prove yours is decentred or otherwise faulty, of course you must return it for repair, exchange or refund!
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06-19-2019, 09:30 PM   #10
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Better buy a Tamron 17-50mm F2.8
06-19-2019, 09:38 PM   #11
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QuoteOriginally posted by yuenslhk Quote
Better buy a Tamron 17-50mm F2.8
It's not bad, but the Sigma's better, IMHO I've got both.
06-19-2019, 10:05 PM   #12
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Clackers' assessment is also reflected elsewhere, including the Tamron shown to have quite high field curvature by Optical Limits, compared to exceptionally low for the Sigma lens.
06-20-2019, 12:06 AM   #13
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Dear Pentaxians,

Thank you very much for such an overwhelming response.

Let me share some photos for you.

The first photo shared below of the lake in front of the dolomites mountain range. That is the photo from the Sigma 17-50 f2.8. Just to mention that the lens is not new but with the one with built-in OS, normally switched off.

For those who suggested that I use a prime lens, the second photo of the Tuscan hills just made me extremely sad after buying a high quality Sigma 30mm f1.4 which came from Japan in immaculate condition.

I recently bought the Pentax 55-300 f4-5.8 lens and took lots of photos, one of which is the flower photo, and I am more than impressed from the IQ obtained from it. I don't know what in particular I can do. I was thinking to spend more money to buy the DA* 16-50 f2.8 of Pentax but even here our friend mikesbikes also said that Sigma outperforms the Pentax lens. I am not sure about Tamron. But if the Sigma 30mm f1.4 prime lens can't give good photos then I don't know if the K50 isn't compatible with the Sigma lenses. Also not to forget, the Sigma 30mm f1.4 is very very poor in focusing. I might have to get them calibrated with the K50 by a professional photographer.

Last edited by haroon88; 12-10-2019 at 05:09 AM.
06-20-2019, 01:09 AM   #14
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I gotta be honest with you, Haroon, I'm disagreeing with what you said in your original post about the sides being softer.

Nothing is particularly sharp in the first image, that's a scene that stretches for many many kilometres … it's only shot at f8, where did you focus? Was it on a tripod or handheld? Were you using the two-second delay?
06-20-2019, 01:14 AM   #15
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Well I wasn't referring my post to exact this photo, this first photo of mountains with lake is just one example in which sometimes nothing is in focus. This photo was on tripod and a remote control shutter release was used. I focused manually using live-view. I am referring to overall experience.

The second photo is handheld. I can declare but I didn't expect his much out of focus or less sharp images from my prime lens at 1/250 shutter speed.
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