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12-10-2010, 01:45 PM   #1
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AF inaccuracy..

Hi,

I'm experiencing quite some front focusing with my FA 35 and FA 50 on my K-5 under tungsten lightning, and in my case that means most of the situations where I really need a wide aperture, ie under poor lightning. In ordinary daylight I have no such problem.
After doing some detective work on google this seems like a typical problem for DSLRs. And also for specific lenses.
I read, for example, a review pointing out that the FA35 had quite inaccurate and inconsistent AF.
( Pentax smc FA 35 mm f/2 AL review - Autofocus - Lenstip.com )

The main problem is the inconsistency, which makes it hard for me to make an adjustment of the AF that I can rely on. As of now, I really cannot benefit from the larger apertures, which leaves me with too many of my shots being in- and out of focus in the wrong areas.
And even if I had an adjustment that worked consistently under tungsten, it still would be quite a drag having to change it everytime I change scenery.

So, I'm pretty much wondering how much of this is caused by the lens being used, and if there are any specific lenses in the Pentax range known for better or worse focusing consistensy..

Any help will be much appreciated!

12-10-2010, 01:51 PM   #2
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I believe ALL DSLRs have this problem under Tungsten lights .... so maybe you need to find a workaround of sorts not a different lense / camera.
12-10-2010, 02:22 PM   #3
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Maybe your AF module is defective. Most Pentax DSLRs will front focus under Tungsten lighting but the K-7, K-5, and 645D have an colour temperature sensor to compensates against this normal property of AF modules. So your observation would have been normal if you didn't reported it about a K-5. I suggest to have the camera checked.

Do you have other lenses you could try? If they also show a difference of focus point depending on light temperture the case is a clear cut one. If they don't then you may want investigate a bit further before you send the camera in. I've never heard that the AF correction doesn't work for these lenses though.

Regarding the influence of the lens, I reckon that the problem will be worse for lenses with higher amounts of longitudinal CA.

Last edited by Class A; 12-10-2010 at 02:29 PM.
12-10-2010, 02:28 PM   #4
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QuoteOriginally posted by Class A Quote
Maybe your AF module is defective. Most Pentax DSLRs will front focus under Tungsten lighting but the K-7, K-5, and 645D have an colour temperature sensor to compensates against this normal property of AF modules. So your observation would have been normal if you didn't reported it about a K-5. I suggest to have the camera checked.

Regarding the influence of the lens, I reckon that the problem will be worse for lenses with higher amounts of longitudinal CA.
The K-5 and K-7 are better in this regard (and I assume the 645D, but I've never seen/tried one) than the older models without the extra module. However, if the temperature of the light is very low (<2500K), you're likely to still see some degree of front focusing.

12-10-2010, 03:21 PM   #5
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First of all, thanks for the rapid responses.

I haven't had the time to test all my lenses side by side since I got the K-5 very recently.
On my 16-45 I haven't had any troubles at all, though. But I guess f4 is quite a bit different than f2.

I haven't gone below 2500K but around 3000K, so the variation might not be that strange after all?

I will do more thorough tests with the information that you've kindly provided in mind and see if I can make any conclusions!
12-10-2010, 09:59 PM   #6
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Try setting your WB manually to a low colour temp and see what happens.
12-11-2010, 12:22 AM   #7
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QuoteOriginally posted by Wheatfield Quote
Try setting your WB manually to a low colour temp and see what happens.
What do you expect to happen?
I understand the K-r has a poor man's approach to AF correction but AFAIK, the K-5 doesn't resort to such tricks. Hence I wouldn't expect a different WB setting to change anything in the case of the K-5.
12-11-2010, 01:12 AM   #8
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QuoteOriginally posted by Class A Quote
Most Pentax DSLRs will front focus under Tungsten lighting but the K-7, K-5, and 645D have an colour temperature sensor to compensates against this normal property of AF modules.
The funny thing is that my K-m never front focused under tungsten light, but now with the K-5 I have similar experience as the OP. This is with f/1.4 and f/2.8 lenses. It's like if my K-5 tried to correct focus under tungsten in the wrong direction, i.e. instead of eliminating the theoretical front focus (which I've never experienced with K-m) it's creating it. I verified it by shooting both cam with same lens side by side. What's even worse, it's inconsistent. Under exactly the same conditions, in my living room (where the majority of my indoors photos are taken).
I wonder if the replacement K-5 will do the same... if not then my previous body's AF module might have been faulty.

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