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02-24-2011, 10:44 AM   #121
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Falconeye, thanks for the help in this matter.
Kevin

02-24-2011, 11:02 AM   #122
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The studio modeling light conundrum

QuoteOriginally posted by Class A Quote
I just checked an exposure that I recently did with studio modelling lights (just the modelling light (probably Tungsten, small chance for halogen but I doubt it)
About the studio modeling (AE/ modelling in BE) light conundrum.

I see the reports for this problem coming in.

Some report to solve it with AF microadjustment.

That could mean that the peak of defocus around EV 8 in my chart (as in the OP) isn't just anecdotical.

What if there are two problems?

  1. Large focus shift (~250Ám) around EV0, not correctable by AF adjustment, as high as EV 4 or EV 6 for a few or slow lenses. Esp. in tungsten.
    -- all the dots in the upper left quadrant of the chart --
  2. Moderate focus shift (~60Ám) around EV6 to EV8, correctable by AF adjustment.
    (and it's shadow, not even halogen tungsten light -- I have no data point for tungsten in this region (shame on me))
    -- the blue dots at EV 8 reaching between 80 and 140Ám --
This two problem hypothesis needs more data to be substantiated (Pentax K-5 mid light focus study )). But there must be a reason for all the failed modeling light reports (and there are many). It is correct, modeling lights should be too bright to suffer from the problem I describe in the report.

Last edited by falconeye; 02-24-2011 at 11:08 AM.
02-24-2011, 11:10 AM   #123
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QuoteOriginally posted by falconeye Quote
@Christine: Your spherical aberration theory is good. But I fear that we have some evidence against it like the dependency on light color or that stopping down doesn't help (beyond a proportional reduction of the confusion circle of course). Maybe, an extension of your theory to include CA or Bokeh-CA would be required. It still wouldn't explain the strong dependency on luminosity though.
Thanks for humouring me, but I suspect I have no idea what I am talking about!

I started building a theory involving spherical aberration in the main lens interacting with chromatic aberration in the micro lenses focusing into the AF sensors, but quickly realised it was just wild speculation as I don't have a good working knowledge of the AF mechanism.

I do believe though that the key factors appear to be light temperature, brightness and possibly lens type (this could cover a multitude of factors including available light, aperture, lens curvature, etc.)

Would be interesting to speculate what the variance is among units. My suspicion is that the problem is more or less present in all units, but in different degrees - hence the inconsistent reports (ranging from no observable issues to severe focusing problems affecting all shots in low light).
02-24-2011, 01:18 PM   #124
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Here's my K-5 results I posted in an old thread ...

f/1.6 ISO 6400 100% crop. no PP. subject distance about 25 feet. dark street, yellow license plate lit by car halogon headlights. I think it's about EV2

Live view AF


Regular AF



02-24-2011, 01:28 PM   #125
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If live view focuses accuratly where AF fails to hit the mark, wouldn't that indicate a software flaw?

Can anyone confirm that Live View is more accurate at low light?
02-24-2011, 01:36 PM   #126
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QuoteOriginally posted by PixleFish Quote
Can anyone confirm that Live View is more accurate at low light?
Absolutely! What else can I say, it works. It IS the solution, for now.
02-24-2011, 01:47 PM   #127
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QuoteOriginally posted by PixleFish Quote
If live view focuses accuratly where AF fails to hit the mark, wouldn't that indicate a software flaw?
They use two independant AF systems. Regular AF is a module in the bottom of the camera ( reached by a secondary mirror ), while the LV AF is through the sensor.


QuoteQuote:
Can anyone confirm that Live View is more accurate at low light?
Well my images above confirm it. - Did you miss the post

LV AF certainly is accurate at low light. And yup in some circumstances is a workaround. It's a lot slower though.
02-24-2011, 02:45 PM   #128
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@smeggypants
Is your live view settings in Contrast detect or Phase detect AF?

02-24-2011, 03:25 PM   #129
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Regarding AF in LV, I tried using CDAF and PDAF and the result was CDAF is much sharper.

Can anybody try it as well. Attached are some shots taken with kit lens.

PDAF

CDAF

Focus on Amber light of mouse.
02-24-2011, 03:31 PM   #130
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QuoteOriginally posted by falconeye Quote
What if there are two problems?
Seems plausible and the data presented in Fig. 1 would be consistent with a "~ 8EV df hump" assumption (assuming other colours are experiencing the same shift, but the modelling light anecdotes support this).

How to explain the intermediate rise for df is another issue. It maybe that there is a problem with a calibration curve at ~8EV and the behaviour at ~0 EV is just a stupid bug.

Maybe Pentax has already fixed the stupid bug but is taking more time to fix the calibration issue at ~ 8EV? Hey, it is allowed to dream, isn't it?

N.B., Pentaxeros has just posted that he expects a fix soon.
02-24-2011, 03:34 PM   #131
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QuoteOriginally posted by Kenn100D Quote
@smeggypants
Is your live view settings in Contrast detect or Phase detect AF?
Contrast detect is the only logical answer and he hinted to it by saying "it is a lot slower". It is well-known that the contrast based approach works even in low light. The whole discussion only revolves around the phase detect AF module.
02-24-2011, 03:57 PM   #132
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QuoteOriginally posted by Kenn100D Quote
Regarding AF in LV, I tried using CDAF and PDAF and the result was CDAF is much sharper.

Can anybody try it as well. Attached are some shots taken with kit lens.

PDAF

CDAF

Focus on Amber light of mouse.
Your 2 shot comparison shows an obvious problem, user error...

Seriously though, it's a great example of the flawed autofocus system in the K-5. This is just another example of the many posted. Buyer beware!
02-24-2011, 04:58 PM   #133
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QuoteOriginally posted by PixleFish Quote
If live view focuses accuratly where AF fails to hit the mark, wouldn't that indicate a software flaw?

Can anyone confirm that Live View is more accurate at low light?
You post in a thread about a study which, among others, answers the question you just asked.

Please, have a look at Fig.10 and 11. Or at the topic this thread is all about. Thank You.
02-24-2011, 05:06 PM   #134
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QuoteOriginally posted by Class A Quote

N.B., Pentaxeros has just posted that he expects a fix soon.
Hi Class A,

just saw your question to him in the other thread. Let me say this much. Wednesday was the day Pentax officially received a copy of the study, face to face. And it wasn't me. I promised not to say more...

But as an additional comment, from the rumor about the SLT only strategy of Sony I learned that there is a distributor reunion in Spain going on. Maybe this is or isn't related to that. Frankly, I don't know.
02-24-2011, 05:24 PM   #135
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QuoteOriginally posted by Kenn100D Quote
@smeggypants
Is your live view settings in Contrast detect or Phase detect AF?
You can't use phase detect AF in Live view as it requires the mirror assembly to be down and trhe secondary mirror reflect some the light downwards to the phase AF module at the bottom of the camera.

The only AF method LV uses is contrast detect.

You can of course set the camera to temporarily switch out LV mode, back to mirror down mode. to use phase AF
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