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02-22-2011, 05:13 PM - 33 Likes   #1
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LumoLabs: Pentax K-5 low light focus study is published


Fig.1: Accuracy of the Pentax K-5 phase detect AF vs. luminosity in EV. The above chart includes all measurements, i.e. various lenses, light colors, distances and apertures. The accuracy is measured as deviation of the focal plane from the sensor plane, in m. [from the study linked below]

LumoLabs (that's me ) has decided to have a closer look at the K-5 low light front focus issue. After careful evaluation and many hundred test shots I must say that the issue is real.

A preliminary copy of the paper was provided to Pentax earlier this week and the head of Pentax Europe officially receives a printed copy today. I have been told that Pentax engineering will receive a copy too.


The study

The results are too complex to be presented in the scope of a forum article. Fig.1 above may provide a first idea of the work though.


Please, refer to the full blog article and to the complete paper for our findings:
You'll gain a deeper understanding of Fig.1 too

In a nutshell, this is what we find:
  1. The K-5 as it presently ships indeed has a flaw in its phase detect autofocus module or software which causes it to front focus in low light below a lens-dependent threshold in EV.
  2. If it does, it seems to consistently focus ≈ 255 m behind the sensor plane (although with a significant 75 m scatter of results which is about twice as large as the normal scatter of result).
  3. Faster lenses seem to keep working in lower light but of course, are prone to more blur when the front focus does eventually happen. Slower lenses can already start to front focus at light levels metering as 4 EV or 6 EV even. A fast lens may work down to 0 EV in white light.
  4. Light sources other than daylight emphasize this problem as they simply appear darker to the AF module. Moreover, it seems to be moderately color blind for red which further emphasizes the effect in deep tungsten light.
  5. The effect is real and can negatively impact the daily work of a photographer. On the other hand, it is possible to run into a low light tungsten situation without the problem.
  6. The paper clarifies conditions to hit or avoid the issue. White light (halogen is not white enough though) and a wide lens stopped down help to work around the problem. AF assist light typically doesn't help though. But an LED flash light does.
  7. I don't rule out that a m-valued variable becomes 0xFF (255) and causes a false shift of the focus plane by 255 m. I call it the +0xFFm hypothesis.

Please, keep discussion within this thread related to the above study and work which is based on experiments. We have another thread for the exchange of observations and opinions.
It is here: https://www.pentaxforums.com/forums/pentax-k-5-forum/128562-relevant-news-re-...ial-light.html

Thanks everybody for their motivation. I probably wouldn't have done it without. Again it turned out to be more work than anticipated


Last edited by falconeye; 02-22-2011 at 05:35 PM.
02-22-2011, 05:23 PM   #2
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Thanks falconeye. Wow! You're a testing machine. Your value to this community is appreciated.

Compliments aside, out with it: hardware or software? Guess, if you must. Reasoned deduction preferred.

Thank-you again.
02-22-2011, 05:52 PM   #3
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Brilliant Stuff Falc !!!

Started Reading now. Much thanks for this. next Step ... Please tell Pentax so they can fix it
02-22-2011, 05:53 PM   #4
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Kudos to Lumolabs for taking on this contentious issue and trying to bring rigorous testing to bear on it! Lots to think about here.

02-22-2011, 05:59 PM   #5
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A huge thanks Falk!

With enough rumbling by users and thorough documentation like this, we can now only hope Pentax addresses and fixes the low light front focus issue.
02-22-2011, 06:07 PM   #6
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wonder if the k-r does too because of some change to the software...

now, does everyone agree that the k-x and prior did not have this issue?

And thanks for the thorough and hard work.
02-22-2011, 06:15 PM   #7
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Thanks for your hard work.
02-22-2011, 06:29 PM   #8
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QuoteOriginally posted by hawk1500 Quote
wonder if the k-r does too because of some change to the software...

now, does everyone agree that the k-x and prior did not have this issue?

And thanks for the thorough and hard work.
Well I can onyl speak my copies, but my K10D and K20D do not suffer from this issue at all and focus accurately in really low light levels. They take some time to do it of course but they do it

My K-5 OTOH ...

02-22-2011, 06:37 PM   #9
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Falk, your ability amazes me! Keep up the good work!
02-22-2011, 06:44 PM   #10
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Thanks, Falk. Good stuff. Possible error in the summary:

2. Should probably read "in front of the sensor plane"?

Jack
02-22-2011, 06:52 PM   #11
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Wow...nice job Falk. I hope you sent a copy of this to Pentax.

I'm hoping you're right about the byte overflow. Otherwise, it smells like a hardware bug to me...
02-22-2011, 06:55 PM   #12
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QuoteOriginally posted by hawk1500 Quote
wonder if the k-r does too because of some change to the software...

now, does everyone agree that the k-x and prior did not have this issue?

And thanks for the thorough and hard work.
I do not agree at all. My K10 an 20 have this issue. Its also a known issue for most camera companies. I get around the problem by remembering presets (AF adjustments) for low light levels that are different for daylight.
for example. My F50mm f1.7 lens has a +3 adjustment for daylight, but goes to a -7 for tungsten. And its safe to say I have shot thousands of photos with that setup.
So I do not agree at all.
02-22-2011, 06:58 PM   #13
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So, what would be involved in Pentax fixing this? Just a software tweak, or hardware? Because it can be brutal. And it sucks to have such a low light beast that is so troublesome in dark tungsten light, which is exactly where you would be most likely to use a low light monster. I was shooting an indoor climbing competition the other day, and it was giving me a ton of trouble. I had the focus correction for body and lens maxed out, and it was still a bit off. I was still able to get the shots I needed, but it was tough. I had to intentionally focus on something slightly further away, or focus on something, then give the focus ring a smidge of a turn.
02-22-2011, 07:04 PM   #14
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I was going to replace my Kx and K7 with a K5, but now I think I'll hold off until this problem is put to bed - but will Pentax correct the problem, let alone even acknowledge the problem?
02-22-2011, 07:11 PM   #15
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QuoteOriginally posted by Steelski Quote
I do not agree at all. My K10 an 20 have this issue. Its also a known issue for most camera companies. I get around the problem by remembering presets (AF adjustments) for low light levels that are different for daylight.
for example. My F50mm f1.7 lens has a +3 adjustment for daylight, but goes to a -7 for tungsten. And its safe to say I have shot thousands of photos with that setup.
So I do not agree at all.
I shoot with a K20D so I know exactly what you're talking about -- but I think it's a color-temperature issue rather than a light-level issue. That's why the K-7 introduced the color sensor as part of the AF sensor module, and I believe many people have reported that the K-7 goes a LONG way toward fixing that problem.

But this is apparently a new problem in the K-5.
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