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04-09-2013, 02:26 AM   #1
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Sigma 17-50mm f/2.8 variable daylight/tungsten light back focus


I just got myself a Sigma 17-50mm f/2.8 EX DC HSM.

After some time spent adjusting its focus, it produces extremely sharp images.

However I've noticed that while in daylight a +5 focus adjustment is necessary, it isn't anymore in artificial light and a 0 adjustment produces the best results !

Now I've heard that some early K-5 copies had some front/back focussing issues in tungsten light, but I've never noticed this problem with other lenses (I might need to run some tests just in case ).

Should I exchange my lens which is otherwise perfectly fine, or is it a camera related problem ?

Thanks in advance for your insight

04-10-2013, 09:41 AM   #2
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Join Date: Mar 2013
Location: So-central, MI / Central, IN
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Hi, Please note that I am not truly qualified to speak directly to the variable AF focus accuracy under different light source issue- but I would like to give my initial thoughts:

1) I am quite certain that the lens cannot be primarily responsible for the variability in AF focus accuracy under different light sources. This should be inherent to the AF mechanism, sensor and algorithms of the Pentax body. If you are otherwise happy with your present copy of the lens, then keep it and don't gamble further in that regard.

2) If you have not already done so, I recommend to upgrade your K-5 firmware to V1.14. This isn't going to provide any significant solutions, but it may provide some small improvements. You should likely re-verify the optimum AF fine adjustment after doing this.

3) What technique are you using to determine the optimum AF fine adjustment? Based on my experience, I recommend that when you apply a new adjustment you should first focus on a more distant or a closer point prior to focusing on your target. This will improve the consistency with in your results somewhat.

p.s. I have found that the Sigma DC 17-50mm F2.8 EX HSM delivers on its claimed resolution and I recently experienced excellent AF performance using V1.14 in a mixed commercial lighting condition (mixed daylight, florescent, and some tungsten).
04-11-2013, 02:57 AM   #3
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Original Poster
Hi One3rdEV, thanks for the very detailed answer !

Regarding your first remark you must be right. Actually now that I think about it, my Sigma is one of my first large aperture AF lens, most of my other are MF (Samyang or Pentax A glass).

I should run some tests with my DFA 100mm Macro and check if I have the same inconsistency.

I was using my DA 16-45 at f/5.6 or smaller apertures, this must be the reason why I hadn't noticed this focus inconsistency between daylight and artificial light.

I have already updated my K-5 to v1.14, and found live view improved compared to the early versions.

My AF tests are conducted this way : I first use the focus chart found on this forum, on a tripod with MLU and 2 sec timer for 3 main focal lenghts. I then choose the focus adjustment which is the best compromise for these 3 focal lenghts. Eventually I use this adjustment on more distant targets (actually houses which are about 15 meters away from my balcony) to check if they are the most appropriate.

That's how i found that a +5 adjustment yielded almost perfect focus for distant targets. However when I am near MFD, the required adjustment is closer to +8 or even +10.

As you said it would not be wise to gamble and possibly end up with a worse copy, but still I'm not entirely satisfied with mine.

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