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01-18-2012, 08:45 AM   #16
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After a bit of informal testing, I found that the phenomenon I described is limited to the Sigma 30 f/1.4. For example, even in low light, my Tamron 24-135 f/3.5-5.6 locks AF better than the Sigma at the non-cross type AF points, as well as the uppermost, leftmost AF point. The Sigma does perform well at all AF points aside from the three troublesome ones.

I've noticed some softness in the upper, left-hand corner of many of my photos. I wonder if there is some aberration in the plane of the sensor of my K-5.

01-18-2012, 09:00 AM   #17
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QuoteOriginally posted by Designosophy Quote
After a bit of informal testing, I found that the phenomenon I described is limited to the Sigma 30 f/1.4. For example, even in low light, my Tamron 24-135 f/3.5-5.6 locks AF better than the Sigma at the non-cross type AF points, as well as the uppermost, leftmost AF point. The Sigma does perform well at all AF points aside from the three troublesome ones.

I've noticed some softness in the upper, left-hand corner of many of my photos. I wonder if there is some aberration in the plane of the sensor of my K-5.
When you get into wider apertures(1.8 or less) you'll find AF accuracy takes a hit. If it's any consolation, Pentax isn't the only camera to have such issues either. I shot a 28/1.4 Nikkor that also seemed to loose accuracy as the aperture got wider. Though I didn't have any other fast lenses to test my theory with.

Anyways, I own quite a few f/1.4 lenses and my resolve(beyond candids) is to take hold of the focus myself(pressing AF disable) and focusing manually. Cause when the cards are down. you just can't beats a well focused shot imo.
01-18-2012, 02:12 PM   #18
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QuoteOriginally posted by JohnBee Quote
When you get into wider apertures(1.8 or less) you'll find AF accuracy takes a hit. If it's any consolation, Pentax isn't the only camera to have such issues either. I shot a 28/1.4 Nikkor that also seemed to loose accuracy as the aperture got wider. Though I didn't have any other fast lenses to test my theory with.

Anyways, I own quite a few f/1.4 lenses and my resolve(beyond candids) is to take hold of the focus myself(pressing AF disable) and focusing manually. Cause when the cards are down. you just can't beats a well focused shot imo.
This actualy sounds like a focus shift problem that many lenses suffer from.
Certain lenses shift there focus when they are stopped down. Normaly, the increase in depth of feild is enough to cover this aspect.
The tamron 17-50 suffers from this as you can see here Tamron AF 17-50mm f/2.8 SP XR Di II LD Aspherical [IF] (Nikon) - Review / Test Report
01-18-2012, 02:26 PM   #19
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QuoteOriginally posted by Designosophy Quote
Having just acquired a Sigma 30mm f/1.4 yesterday, I felt like it was front focusing a smidge, and I was able to correct it with a +3 adjustment.
I'm new at this, but isn't a front focus issue resolved with a - AF adjustment setting? And back focusing solved with a + AF adjustment value?

01-19-2012, 06:58 AM   #20
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QuoteOriginally posted by JohnBee Quote
When you get into wider apertures(1.8 or less) you'll find AF accuracy takes a hit. If it's any consolation, Pentax isn't the only camera to have such issues either. I shot a 28/1.4 Nikkor that also seemed to loose accuracy as the aperture got wider. Though I didn't have any other fast lenses to test my theory with.

Anyways, I own quite a few f/1.4 lenses and my resolve(beyond candids) is to take hold of the focus myself(pressing AF disable) and focusing manually. Cause when the cards are down. you just can't beats a well focused shot imo.
This focus shift was a concern to me, since the aperture is always wide open when focusing. But at f/4.5 and above the focus is still right on the money, so I don't think there is any focus shift, at least that I can detect unscientifically.

QuoteOriginally posted by jmg257 Quote
I'm new at this, but isn't a front focus issue resolved with a - AF adjustment setting? And back focusing solved with a + AF adjustment value?
You know what, I'm confused by this as well. All I know is that it focused closer to me than it was supposed to before, and now with the +3, it is on. I tried negative adjustment first, and this made it worse.

Last edited by Designosophy; 01-19-2012 at 07:05 AM.
01-19-2012, 07:52 AM   #21
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QuoteOriginally posted by Anvh Quote
I would say that the sigma 30mm has a very curved field of dof lik most f/1.4 and faster lenses have, but i'm now just stating the obvious here
+ 999

Also, if Pentax AF points are like Canon's, the inner points have greater sensitivity, work better in low light. Some of the points detect maximum contrast in horizontal plane, some in vertical plane, and some in both planes.

Sigma 30/1.4 is an excellent lens that requires more skill to use than many users seem to have, imho, which has led to deprecation of the perceived lens quality. There is a minor distortion issue which some people with 'golden eyes' have noticed, difficult to impossible to fix in PP without the lens curve. Fast wide lenses are not easy to learn to use!
01-19-2012, 08:04 AM   #22
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What's so difficult about using the Sigma 30mm f/1.4? I have absolutely no problems with mine, no matter which focus point. Haven't had a problem of the so called field curvature issue. But then again I got my K-5 calibrated by Pentax Japan and focus is spot on and haven't needed any AF adjustment with this lens at all.
01-19-2012, 08:32 AM   #23
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QuoteOriginally posted by creampuff Quote
What's so difficult about using the Sigma 30mm f/1.4? I have absolutely no problems with mine, no matter which focus point. Haven't had a problem of the so called field curvature issue. But then again I got my K-5 calibrated by Pentax Japan and focus is spot on and haven't needed any AF adjustment with this lens at all.
Good for you! Having no problems is a great situation to be in.

I'd like to say that I do adore the Sigma 30. It's wonderfully sharp wide open (except corners and far edges). It's fast. The color, contrast and subjective quality are all excellent. It's definitely not a "bad lens" as one person on this thread said.


Last edited by Designosophy; 01-20-2012 at 07:00 AM. Reason: word usage
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