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01-02-2011, 10:06 PM - 2 Likes   #1
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Might the front focus issue be a white balance problem?

Hello, everyone. I've been lurking for a few weeks. I finally have something that will (hopefully) be useful to others.

The short version: manually setting white balance to Tungsten instead of Auto goes a long way towards improving my KR's autofocus accuracy.

The long version: Some of my pictures were fuzzy. I wasn't sure if it was my technique or a camera problem. I searched, found these forums, and did a lot of reading. Then I ran some tests and confirmed that my AF was not good under incandescent/tungsten lights. It was much sharper with fluorescent bulbs and outdoors. Setting AF Fine Adjustment to the -10 limit only made a small improvement.

Something I read suggested manually setting white balance to Tungsten. I thought WB was a postprocess applied when the camera saves JPGs, and therefore shouldn't have any effect on RAW files or their focus, but it was worth a test. (Unfortunately, I can't find that original message now, so I can't give proper credit to whoever suggested the white balance solution.)

All attached images are crops taken from the 18-55 kit lens at its widest zoom. I used the center point for AF. The focus chart comes from Yvon Bourque at pentaxdslrs.blogspot.com. The 3 spice jars are arranged with the left jar in front, right jar in back, focus aimed at middle jar.

These photo pairs are arranged with the Auto WB first, followed by Tungsten WB.


Last edited by DeadJohn; 02-01-2014 at 10:54 AM.
01-03-2011, 07:04 AM   #2
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It's possible that if the camera knows the color temperature of the source light it can compensate accordingly. Having said that, my K-r sample still did a lousy job even with a custom white balance.
01-03-2011, 07:22 AM   #3
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I haven't noticed any missed focus issues on my new K-r so far, but this WB-FF discovery seems interesting though. Both test subjects are pretty much close focus range, how about something in more distant range? I will try to do my own testing on this, just need to find a place lit by tungsten light.
01-03-2011, 09:40 AM   #4
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That looks like a great tip! I will have to try it.

01-03-2011, 11:06 AM   #5
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QuoteOriginally posted by dragra Quote
I haven't noticed any missed focus issues on my new K-r so far, but this WB-FF discovery seems interesting though. Both test subjects are pretty much close focus range, how about something in more distant range? I will try to do my own testing on this, just need to find a place lit by tungsten light.
I focused near minimum distance because the 18-55 lens gives the smallest depth of field at 18mm f3.5. At greater distances the focus chart becomes too tiny to read. If I increase the zoom level to compensate, the aperture also changes and gives a greater depth of field that makes it tougher to spot focus issues.

This is admittedly going to a level of pixel peeping I'd never use for real photos, but I want to exaggerate any focus limitations. If I get things working well for a worst-case scenario then all of my real photos will be great (or, to be more accurate, any bad photos will be my fault, not the camera's).

It's very interesting that the WB affects focus at all. I originally expected to get the same exact DNG file regardless of how I set my WB. I used cheaper cameras and never bothered with DNG; I clearly have much to learn about the technology behind my new KR.

I expected: Autofocus -> expose photo -> save DNG -> apply WB -> save JPG.
However, WB seems to come into play much sooner: Apply WB -> autofocus -> expose photo -> save DNG -> save JPG. It may be splitting hairs but the tungsten front focus may be "a type of light detection problem" rather than an "autofocus problem". Manually telling my camera "hey, it's tungsten" results in pretty good autofocus.

Later this week I will try more tests, playing with distance, focal length, light type, light intensity, WB, and different lenses. If I was really smart I would just replace my last old tungsten light fixture at home (it uses custom bulbs and can't fit CFLs) and mostly forget about this whole thing, but it's too late because this has become a fun science project.
01-03-2011, 05:21 PM   #6
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DeadJohn, THANK YOU! for posting this tip! It never would have occurred to me....

I too have been having trouble with the "front-focus" issue (as well lurking for the past month). I only noticed a focusing problem under relatively dim light, when I'd be using the lens' fastest stop. Following the advice in another thread I tried several kinds of bright artificial light, to no avail. Bright daylight never resulted in bad focus, although I managed quite a few clunkers under overcast skies.

By setting the white balance specifically for the type of lighting, rather than relying on AWB, all my focus issues seem to be resolved. I've tried tungsten (halogen bulbs), two kinds of CFL's (2800K and 6500K) and the long tube-style fluorescent bulbs out in the workshop. No problems.

We should be "enjoying" some light snow flurries here tomorrow, and I'll get a chance to test the "cloudy sky" setting.

Thanks again! (maybe this will help Pentax develop a firmware update, too.....)
01-04-2011, 09:33 PM   #7
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I ran more tests and confirmed my findings from the opening post. For the one tungsten light fixture I still have at home, manually changing WB from Auto to Tungsten greatly reduces front focus to where it's not an issue for me.

New Test: How bright is your tungsten?

I started at 100% brightness. I then dimmed the light down to ~25% (shutter speed slowed from 1/80s to 1/13s in response). Auto WB gave acceptable focus in the dim tungsten but not in the bright tungsten. WB set to manual Tungsten gave acceptable focus regardless of brightness.

Bright tungsten = Auto WB might not be recognizing that it's tungsten. Front focus results.
Dim tungsten = Auto WB detects correctly and the correct autofocus calculations get triggered.

New Test: Fluorescent

Auto WB under fluorescent light was focusing fine. Manually changing WB to Tungsten under fluorescent light introduced *back* focus.

This means that autofocus gets tuned based on color temperature. If you pick the wrong color temp, you not only get odd colors that can be fixed in postprocessing, you also get focus problems that are much tougher to fix. WB is far more important than I ever realized while using point & shoots.

Limits of my testing

I think I've gone as far as I can without access to a photo studio. My photographic interests are mostly outdoor so I don't have lighting, backgrounds, etc. to test other conditions.

The walls of the room I was testing in are beige with a hint of yellow. I don't know if the yellowish tint may have influenced how Auto WB detects color temp. Testing with white, black, and other color backgrounds could be informative.

I was working with a small focus target so I had to be pretty close. All of my tests were done at one distance (12") and focal length (18mm).

If I backed up and zoomed to compensate, increased depth of field put everything into focus. That's a good thing for real photography but bad for these test conditions.

My Next Steps

Based on my camera, my lighting at home, and my expected uses for the camera, the tungsten front focus limitation is not a big problem for me. I dialed in some AF Fine Adjust to improved sharpness but I don't expect to see a big difference in my actual picture taking.

I contacted Pentax USA by email and received a prompt reply. They stated no known tungsten AF problems but said I can send my camera and lenses for adjustment. My warranty is good for another 11 months so I'm going to wait to see whether I actually notice any problems in real photos. I'm doubtful that the optics can be adjusted for tungsten without hurting focus in other lighting types. Extra time before sending my K-R allows Pentax to gather more information and maybe issue a firmware fix.

Everything I learned might already be old news to Pentax engineers. However, if anyone has good contacts at Pentax, please pass along this info to them in case it's helpful. I'll try calling tech support but I suspect that I won't get past a first-level drone.
04-23-2011, 09:43 AM   #8
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very interesting, i wonder if the k-5 people have read this...

04-23-2011, 09:51 AM   #9
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Has anyone tried experimenting with Adobe Lightroom's WB adjustments?
04-23-2011, 10:14 AM   #10
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QuoteOriginally posted by manishved Quote
Has anyone tried experimenting with Adobe Lightroom's WB adjustments?
To solve the focusing issue?
04-23-2011, 12:00 PM   #11
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With my K-r switching between awb and tungsten wb makes no difference to af results. I tested with cfl lighting and in daylight. I experience ff in tungstenish cfl lighting regardless of wb setting.
04-26-2011, 07:58 AM   #12
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Well my first thoughts on the AF with K-5 was just the WB. It messures wrong and thus AF in the same way wrong. I think.
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