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12-12-2010, 09:11 AM   #1
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Sigma 30mm f/1.4 Low Light AF-C Accuracy - Pentax 35mm f/2.4 Performance?

In this thread:
https://www.pentaxforums.com/forums/pentax-k-5-forum/120728-question-k5-low-l...uto-focus.html
there is talk about the low light (around 0 to 3 EV) performance and discussions with AF with fast lenses. I have tried using the Sigma at a dance (3 to 4 EV) last Friday and AF-C locked well but was very inaccurate. I used the same lens under same conditions with a K10D (hunts but locks accurately a lot of the time) and a K-7 (minimal hunting and locks accurately a lot of the time).
It seems that the K-5 under low light (0 to 3 EV) conditions with a fast lens (narrow DOF) has overall much poorer performance than the K10D and K-7 due to the inaccurate focusing performance. Other lenses are OK, just the Sigma 30mm f/1.4 is not accurate. I do seem to get more in-focus shots when I adjust the AF fine adjustment to -3, but then the shots are out of focus under normal lighting conditions.
Two questions:
1 Is anyone else having similar problems with fast lenses?
2 Has anyone tried the new Pentax 35mm f/2.4 in 0 to 3 EV?

Thanks

12-12-2010, 09:58 AM   #2
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It might not be the low light, but the type of light. I read on another forum about something having a similar problem so made a custom profile for different types of light.
12-12-2010, 10:14 AM   #3
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I was wondering about that, since dimmed tungsten light has a lot of (infra) red in the mix. So when I get home I will try and figure out how the light affects narrow DOF focus. Supposedly the camera adjusts for light color. I would be interested reading on another forum about that.
12-12-2010, 12:10 PM   #4
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The thread is here.

12-12-2010, 02:02 PM   #5
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Thanks for your response. Though I tried taking out the ****** and the link is still not working. I am not sure why. Any way you could copy and paste the text?

Thanks
12-12-2010, 05:12 PM   #6
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QuoteOriginally posted by Eric Seavey Quote
Other lenses are OK, just the Sigma 30mm f/1.4 is not accurate. I do seem to get more in-focus shots when I adjust the AF fine adjustment to -3, but then the shots are out of focus under normal lighting conditions.
Eric.

I cannot be sure but I have a theory:

Many large aperture lenses have spherical aberration. This means that the focus at f/5.6 (where phase AF takes its measure) and at f/1.4 (where the exposure is taken) do differ.

The Pentax firmware has correction data to compensate for this. But it may only include Pentax made lenses. Or it's in the lens rom and Sigma doesn't entirely support the lens rom features.

At normal lighting it would then work because you wouldn't use the lens wide open.


You may be able to test for this by doing a manual focussing at f/5.6 (use the DOF preview to enforce it and try to use a good focus target like a zone plate chart). And then shoot at f/1.4. I occasionally observed sup-par focus using this method with fast some lenses. Even a careful manual focus in 10x LV can be off.


As I said, just a theory. But maybe worth a test.
12-12-2010, 05:57 PM   #7
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QuoteOriginally posted by falconeye Quote
Eric.

I cannot be sure but I have a theory:

Many large aperture lenses have spherical aberration. This means that the focus at f/5.6 (where phase AF takes its measure) and at f/1.4 (where the exposure is taken) do differ.

The Pentax firmware has correction data to compensate for this. But it may only include Pentax made lenses. Or it's in the lens rom and Sigma doesn't entirely support the lens rom features.

At normal lighting it would then work because you wouldn't use the lens wide open.


You may be able to test for this by doing a manual focussing at f/5.6 (use the DOF preview to enforce it and try to use a good focus target like a zone plate chart). And then shoot at f/1.4. I occasionally observed sup-par focus using this method with fast some lenses. Even a careful manual focus in 10x LV can be off.


As I said, just a theory. But maybe worth a test.
It is maybe easier to test toe shoot wide open first on a subject using phase detection AF and after that taking the same shot with liveview using contrast detection, if the LV version is sharper then you know for sure its a lens issue and that focus shifts occur.
12-12-2010, 06:38 PM   #8
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QuoteOriginally posted by Rin Quote
It is maybe easier to test toe shoot wide open first on a subject using phase detection AF and after that taking the same shot with liveview using contrast detection, if the LV version is sharper then you know for sure its a lens issue and that focus shifts occur.
That doesn't work. AFAIK, you cannot force contrast AF to use a specific f-stop. It won't use the f-stop of the actual exposure. It somehow depends on the illumination level. Spherical aberration affects contrast AF as much as phase AF, at a given f-stop.

I checked that my method would work. Fortunately, activating the DoF preview does not terminate the zoom level. I currently don't see another way to test it out.

In an actual low light condition, contrast AF might actually focus with the lens wide open. But you can't be sure. And contrast AF (K-5) isn't reliable in low light. Sometimes, it is spot on, sometimes, it is significantly off.


Last edited by falconeye; 12-12-2010 at 06:53 PM.
12-12-2010, 06:53 PM   #9
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QuoteOriginally posted by Eric Seavey Quote
Thanks for your response. Though I tried taking out the ****** and the link is still not working. I am not sure why. Any way you could copy and paste the text?

Thanks
it seems that this site blocked the url, change the ****'s for d p r e v i e w . c o m without the spaces.
12-12-2010, 10:01 PM   #10
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QuoteOriginally posted by falconeye Quote
This means that the focus at f/5.6 (where phase AF takes its measure) and at f/1.4 (where the exposure is taken) do differ.
Yes, that makes sense but do you have any idea why the K10D and K-7 did better with the same lens?

Do you think the K-5 somehow invalidates Sigma's (reverse engineered) way to communicate with the camera regarding focus shift?

I've heard from other users that e.g., their Tamron 17-50/2.8 front-focuses on one camera but was fine on another (and it appears the reason is not that the cameras differed in their general FF/BF behaviour).

Is it possible that somehow different cameras react to slight lens imperfections in different ways, e.g., because their AF modules are aligned slightly differently?
12-12-2010, 10:02 PM   #11
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QuoteOriginally posted by Eric Seavey Quote
Any way you could copy and paste the text?
Here's what you should copy into your browser:
http://forums.dpreview .com/forums/readflat.asp?forum=1036&thread=37038360&page=1
Just remove the space before the ".com".

In this thread someone writes
With DA 35/2.8 limited, DA 70/2.4 limited, DFA 100/2.8 limited and 18-55 WR the K5 focuses perfectly in bright daylight (camera calibrated for all lenses). In dim light the K5 consistently front focuses with all these lenses.
Perhaps there is more to it than an incompatibility with Sigma lenses?

Last edited by Class A; 12-12-2010 at 10:09 PM.
12-12-2010, 10:11 PM   #12
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Could this be a problem with the Sigma 300mm f2.8 lens and the Pentax camera?

I have been raising cain with Sigma over this issue. Now they rechipped it, and it is much better. but the chip set is very new they told me. But now in poor light and at 2.8 it hits pretty regularly. Before it was a nightmare... I am not kidding.

I have learned the hard way. It is best to stick with the companys lens. Problem is Pentax doen't make any big glass.

Sadly i am slowly switching now.

Last edited by garyk; 12-12-2010 at 10:22 PM.
12-12-2010, 11:02 PM   #13
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QuoteOriginally posted by garyk Quote
It is best to stick with the companys lens.
I don't agree. Some third party lenses are superior or offer better value for money to what Pentax has to offer.

QuoteOriginally posted by garyk Quote
Sadly i am slowly switching now.
Why? Your story had a good outcome, didn't it?
12-13-2010, 03:29 AM   #14
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QuoteOriginally posted by falconeye Quote
That doesn't work. AFAIK, you cannot force contrast AF to use a specific f-stop. It won't use the f-stop of the actual exposure. It somehow depends on the illumination level. Spherical aberration affects contrast AF as much as phase AF, at a given f-stop.

I checked that my method would work. Fortunately, activating the DoF preview does not terminate the zoom level. I currently don't see another way to test it out.

In an actual low light condition, contrast AF might actually focus with the lens wide open. But you can't be sure. And contrast AF (K-5) isn't reliable in low light. Sometimes, it is spot on, sometimes, it is significantly off.

For soem reason then it works more correctly in LV mode. Because when i use phase detection it has a slight back or front focus but LV focus 100% accurate since at least in good light it uses the aperture that i set the camera on.
12-13-2010, 05:36 AM   #15
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QuoteOriginally posted by Class A Quote
Perhaps there is more to it than an incompatibility with Sigma lenses?
Yes. I said a theory.

Without carrying out further experiments, we will not know.

QuoteOriginally posted by Rin Quote
For soem reason then it works more correctly in LV mode. Because when i use phase detection it has a slight back or front focus but LV focus 100% accurate since at least in good light it uses the aperture that i set the camera on.
You're not the OP reporting the original problem. And you describe a condition which is different from the OP's one.
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