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09-10-2010, 12:36 AM   #1
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Equipment problem - Pentax K-x, Sigma 100-300 f/4 and AF-C mode

Hello guys,
I have problem with my new K-x. Few days ago I went to airshow in Czech Republic (CIAF 2010 in Hradec-Kralove). Unfortunately, I didn't get satisfying results with Pentax K-x and Sigma 100-300 f/4 - huge amount of pictures are bad because of missed focus (I am sure it is not motion blur).

I had my old K-m with me, and when I switched from K-x to K-m (and kept the same lens) I immediately got better results (focus on some shots missed but I have more keepers).

Is it some kind of technical problem with K-x? Do you have any experience with AF-C and continuous shooting?

Here are my settings used on both cameras:
1. Only center AF point.
2. Tv mode (from 1/2000 to 1/125 depending on plane type)
3. AF-C mode.
4. Burst mode "high".
5. Sandisk Extreme III class 10 card
6. Distortion correction and other similar stuff set to off.
7. SR off (actually I also tested K-x with SR on, but I didn't notice any difference between off and on).

Best regards, Tomasz

09-10-2010, 02:58 AM   #2
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Unfortunately, I have had the same kind of results with Kx focusing in AF-C mode. I think you should use multipoint AF anyway if you are shooting planes in the sky.

Speculating, I might say that maybe the AF speed just cannot cope with the high speed bursts or you are getting blur from the mirror slap so using lower speed bursts, you might have more keepers

Pentax is not for high speed AF jobs, that job is better delivered by other systems. I am curious about the speed and accuracy of SAFOX IX announced for K-r.
09-10-2010, 03:16 AM   #3
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QuoteOriginally posted by WoRaS Quote
1. Only center AF point.
I know that Pentax AF may not be its forte but shooting planes shouldn't be a difficult task at all (even with older bodies). I would use multipoint (5 or 11) instead of center AF point and experiment with that. I've shot planes with a slower lens (55-300mm) and it was fine.
09-10-2010, 03:45 AM   #4
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I think that the issue is the combination of AF.C and Hi continuous shooting. I do a lot of Hi continuous shooting and I prefer to use AF.S or MF. The AF.C is not always fast enough to AF at 5.2 fps.

With high-speed targets (Cars, motos, planes), my preference is to use MF with Hi Continuous shooting.

Hope that the experience may help.

08-28-2012, 10:53 AM   #5
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AF.C? Do you think that Pentax is Canon AI Servo? Just kidding, kind of.

I do not use center focus for plains, and do not use AF.C either, not with Pentax, althought I get great results with the K5. Never used it withthe K-x
08-28-2012, 10:55 AM   #6
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QuoteOriginally posted by Pepe Guitarra Quote
AF.C? Do you think that Pentax is Canon AI Servo? Just kidding, kind of.

I do not use center focus for plains, and do not use AF.C either, not with Pentax, althought I get great results with the K5. Never used it withthe K-x
Just a bit late (2 years) on the response
08-28-2012, 12:40 PM   #7
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Reviving these type of threads can have beneficial impacts on people with same problems.
08-28-2012, 01:03 PM   #8
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QuoteOriginally posted by Pepe Guitarra Quote
Reviving these type of threads can have beneficial impacts on people with same problems.
That is true - I was merely confused at first because the poster was talking about the future k-r AF. Gave me a double take there!

I've used my k-x's AF-C and center focus on some running subjects with mild success, but not airplanes. However, I imagine airplanes generally fly perpendicular to the action.

08-28-2012, 01:20 PM   #9
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one of the things that may be causing problems (this is a guess as no photos offered) is if you were looking at an edge or focusing on a subject that is smaller than the AF sensor. Combine this with the instability of an image due to shake, and you may be moving the AF sensor on and off subject. getting good focus lock in these situations is difficult.

that is one of the real benefits of optical stabalisation, it presents a stable image to the AF sensor also.

It might be that the newer body is a little faster reacting making it appear worse because it can move faster as you go off subject.
09-04-2012, 03:40 PM   #10
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Sigma 100-300m f4 EX

I have the Sigma EX 100-300mm HSM, not OS in the Canon mount. That lens rocks. I have not had a bad photo since I am using it with the 5D Mark II. I could use a better camera though, the Mark II is only 3.8 fps, while the K5 is about 9 fps.
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