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01-16-2010, 02:36 PM   #1
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Front Focus and tungsten light question

Hi All,

I've done quite a bit of searching through the forums and I can see that it's very well documented that Pentax DSLRs (including my K20D) are prone to front focus under tungsten light.

I just wanted to ask, does the same thing happen under halogen lightning (posh tungsten I suppose)? The colour temperature is roughly the same in that the correct white balance would be to se it to tungsten, but I wanted to check with the forum community.

The reason I ask is that I've been calibrating my Sigma 100-300mm with the focus test chart from focustestchart.com and when at 300mm and focusing from about 2 metres away, it does indeed front focus under halogen lightning (not a huge amount, but deffo front focus). Setting the K20D autofocus adjust to -10 doesn't seem to correct it fully. As per the tungsten discussions I was fully expecting this to happen.

So - should I see the same with halogen lighting as others have seen with tungsten / yellow light ?

01-16-2010, 03:31 PM   #2
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My k100d front focus too under tungsten light! But ok with other lighting!
My k 20d seem ok but I didnt test it fully under the tungsten light.
01-17-2010, 02:47 AM   #3
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Noone else help me out here?
01-17-2010, 03:07 PM   #4
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I don't knowthe answer, but would suggest you assume any strongly colored lighting - including halogen - is a poor choice for focus tests. Perform them in daylight for optimum results.

01-17-2010, 07:42 PM   #5
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I've tested K10D/K20D/K-7 under tungsten light, the K10D/K20D indeed had front focusing problem under tungsten light, but seems Pentax has fixed this problem on K-7

If you wanna calibrate AF, better do it under the light condition where you want to use your lens. Or you can do it twice, one under daylight/window light, one under tungsten. Then just remember the AF adjustment value and adjust accordingly.
01-17-2010, 09:38 PM   #6
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I understood the front focus issue happened with anylight type that has higher amounts of infrared.

I know my K10D front focuses big time with the subject is lit with halogen work lights.
01-18-2010, 05:22 AM   #7
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Thanks guys, I repeated the test under daylight and I'm still seeing a small amount of front-focus. I also did some googling on the Sigma 100-300mm f/4 and it seems to be quite prone to front focus, particularly at the longer lengths when focusing close to subjects.

I think I'm going to return it and get it calibrated by Sigma as the autofocus correction on the K20D doesn't seem to correct it enough.
01-18-2010, 12:10 PM   #8
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QuoteOriginally posted by Big G Quote
Thanks guys, I repeated the test under daylight and I'm still seeing a small amount of front-focus. I also did some googling on the Sigma 100-300mm f/4 and it seems to be quite prone to front focus, particularly at the longer lengths when focusing close to subjects.

I think I'm going to return it and get it calibrated by Sigma as the autofocus correction on the K20D doesn't seem to correct it enough.
Did you try the contrast auto-focus in live view? Perhaps, the phase detection AF and/or mirror need adjusting.


Last edited by asdf; 01-18-2010 at 12:25 PM.
01-18-2010, 12:25 PM   #9
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I seem to remember Rice High (who is not too popular around here) went on and on about AF problems of Pentax cameras in tungsten light. I think that has been fixed on the K-7 though since it can measure color, if I am not mistaken.
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