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06-03-2010, 03:23 PM   #1
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Need help regarding back-front focus test.

Hi, I have a Pentax K-x and i'm very happy with its performance except one big problem, the AF inaccuracy. The AF tend to front focus, both for the kitlens 18-55mm & 10-17mm fisheye that I own.

So is there any method to test and fix this problem? I tried these 2 methods in the links below but it seems like every method will give different results. Even if I change the angle of the camera to the chart(2nd method), then the results will be different. So, could someone suggest the best method to test the AF accuracy?

1st method: RiceHigh's Pentax Blog: K-x Debug Mode Tutorial - AF Adjustment
2nd method: Jeffrey Friedl's Blog Jeffrey’s Autofocus Test Chart

Btw, is there any Pentax K-x user in this forum having the same problem? If there is any, what is your correction value/parameter?

And last but not least, what is the cons if I change the focus correction value? It's possible the value/parameter is ok for the close subject, but far subject (infinity focus) will be affected?

Sorry for asking too much question, really need to fix this. Because I love my K-x, but this problem make me very disappointed.

Thanks in advance.

06-03-2010, 05:00 PM   #2
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Focus Testing

You should download a copy of Tim Jackson's test chart at Home , this is the one recognized throughout the industry if service is required.

Although his documentation revolves around the Nikon D70, the information and testing can be used for "ANY CAMERA" no matter who the manufacturer is.

Download the current version from his site, and if I remember correctly, it is a few years old but still very valid.

Hope this helps.
06-03-2010, 06:08 PM   #3
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I take a very simple approach to this. I set my camera up looking at a clock face about 15 feet away. I then AF on the clock and take a picture.
Then I set the AF bias for that lens to +3, and take a picture.

Edit: Note that I force AF from a stop by putting my hand in front of the lens and hitting the AF button first, since it is possible that a slight AF bias adjustment might fall within the resolution of the system. I want the AF to give full movement to try to simulate real world shooting as much as possible.

Is it sharper or less sharp?
If sharper, then I keep going until it is less sharp.
If it is less sharp than I go the other way (-3) and see if it is sharper.
I pretty much just continue doing ring arounds until I hit on the best AF bias for that lens.

Note that you want to do several pictures at a setting once you've decided on to make sure that the camera is hitting the same focus distance every time.

Last edited by Wheatfield; 06-03-2010 at 06:40 PM.
06-04-2010, 03:54 AM   #4
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I like this chart.

PENTAX DSLRs: PART-1. Autofocus Adjustment for the Pentax K20D, Custom Setting No 35

06-04-2010, 08:35 AM   #5
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I'd post the results you are getting from your focus tests. The 18-55 and 10-17 both have such large DOF it would be very difficult for the millimeter or two differences that a typicial FF or BF problem would cause to ever be visible. My guess is what you are seeing isn't FF at all but the camera simply choosing to focus somewhere other than where you intended because you didn't control your test well enough.
06-14-2010, 03:34 AM   #6
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I was playing around with this yesterday to check out my new focus screen. The printable test chart linked by oddie works a treat.
The biggest thing I found is if you switch to manual after doing an auto-focus test, you can dial in your view finder with same chart...
What can I say, I'm getting old. I found that I had my view finder two clicks out on one of my cameras.
Just switched the camera to manual, put on my 50mm 1.7 and repeated the test by taking the shot then checking it for focus. Adjust view finder, refocus and then repeat.
It's a small thing, but I shoot a lot with manual lens, now I know my view finder is right.

Last edited by opfor; 06-14-2010 at 03:36 AM. Reason: typo

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