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08-09-2011, 12:54 AM   #1
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Sigma 17-50 exposures with K5

I am comparing a Sigma 17-50 f2.8 and a Pentax 16-50 f2.8. I got the Sigma first, and I thought that its exposures were OK, although a little bit high for my taste. Now I find that the Sigma and Pentax lenses show different exposures in the K5. The Sigma is about 2/3 of a stop over exposed. I think that the Pentax is correctly exposed (although I can't prove this). Has anyone else experienced this issue with the Sigma lens? Perhaps I have a defective copy (that yields very sharp images).

This slight overexposure is part of the explanation for the flash overexposure problems that I was having with my K5, but the external has problems even after this issue is taken into account. See https://www.pentaxforums.com/forums/pentax-beginners-corner-q/153865-flash-ov...re-new-k5.html

08-09-2011, 01:03 AM   #2
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So we're now talking about global overexposure with your copy of the Sigma.
Sample images here with EXIF intact would help with the comparison.
Also have the camera on tripod shooting with the two lenses with everything the same (WB, subject, aperture, lighting) and there will be a more accurate picture of the issue.
08-09-2011, 01:09 AM   #3
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QuoteOriginally posted by dh87 Quote
I am comparing a Sigma 17-50 f2.8 and a Pentax 16-50 f2.8. I got the Sigma first, and I thought that its exposures were OK, although a little bit high for my taste. Now I find that the Sigma and Pentax lenses show different exposures in the K5. The Sigma is about 2/3 of a stop over exposed. I think that the Pentax is correctly exposed (although I can't prove this). Has anyone else experienced this issue with the Sigma lens? Perhaps I have a defective copy (that yields very sharp images).

This slight overexposure is part of the explanation for the flash overexposure problems that I was having with my K5, but the external has problems even after this issue is taken into account. See https://www.pentaxforums.com/forums/pentax-beginners-corner-q/153865-flash-ov...re-new-k5.html
Actually you Can prove it. Take a photo of a gray card using spot metering. With the proper white balance setting, the Red, Green, and Blue channels will all be at 128 for a perfect exposure (most editing programs have the ability to show this).

08-09-2011, 05:47 AM   #4
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In my experience it is normal to have some EV compensation on at all times, and it seems to be different from lens to lens.

08-09-2011, 06:47 AM   #5
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I've got the Sigma 17-50/2.8 and I haven't notice any overexposure issues with it. Granted, I haven't done any thorough testing with it, but all I do is go out and use it to take photos.
08-09-2011, 07:51 AM   #6
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Thanks for all the responses.

QuoteOriginally posted by Ash Quote
So we're now talking about global overexposure with your copy of the Sigma.
Sample images here with EXIF intact would help with the comparison.
Also have the camera on tripod shooting with the two lenses with everything the same (WB, subject, aperture, lighting) and there will be a more accurate picture of the issue.
I did use the tripod, and I tried to set up exactly the same image. Posting images exceeds my digital knowledge, I am afraid.

QuoteOriginally posted by pop4 Quote
I've got the Sigma 17-50/2.8 and I haven't notice any overexposure issues with it. Granted, I haven't done any thorough testing with it, but all I do is go out and use it to take photos.
I will be taking some actual photos this week. I wasn't planning to spend the first two weeks with my new camera testing lenses. I have to decide which lens today. The Sigma is sharper, but seems to have some problems, most of which I can deal with by changing settings, but this is inconvenient at best. The Pentax has CA issues that I don't know how to fix easily (I do not see that the in-camera correction is fixing the blues fringes). The Pentax costs somewhat more ($900 vs. $670) and interferes more with the built-in flash. I called both Sigma and Pentax, and sent images, but neither company is providing much help.

Last edited by dh87; 08-09-2011 at 08:10 AM.
08-09-2011, 09:51 PM   #7
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Update:

Pentax says that the chromatic aberration that I see is normal. They recommend using the in-camera CA correction. However, I tried it, and it doesn't help much.

Sigma says that there is a firmware update that will fix the exposure problems. They are willing to pay shipping both ways to apply this update.
08-09-2011, 09:59 PM   #8
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QuoteOriginally posted by dh87 Quote
Update:

Pentax says that the chromatic aberration that I see is normal. They recommend using the in-camera CA correction. However, I tried it, and it doesn't help much.

Sigma says that there is a firmware update that will fix the exposure problems. They are willing to pay shipping both ways to apply this update.
Then I would let Sigma fix it. The In Camera corrections, to my knowledge, apply to Pentax lenses only. Specific lenses are known to exhibit CA in certain conditions and even with those in the firmware database, the correction is only mildly effective. Much better off taking it out in post processing.



08-10-2011, 01:06 AM   #9
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QuoteOriginally posted by dh87 Quote
Sigma says that there is a firmware update that will fix the exposure problems. They are willing to pay shipping both ways to apply this update.
If it's going to be at no cost to you, then I'd take them up on their offer. When looking around for my fast normal lens, I had to make a choice between the Tamron 17-50/2.8, Sigma 17-50/2.8 and Pentax DA* 16-50/2.8. I ruled out the Tamron since it was screw driven. Then ruled out the DA* because I didn't want to run the SDM lottery, and also, I didn't need the weather sealing. And I've been pretty content with my Sigma 17-50/2.8 so far.
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