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12-27-2008, 09:03 PM   #1
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Front focus issue, but unable to correct - What now?

I've had my K20D with a 50mm FA f1.4 for over a month now. I generally use MF because I was finding my AF a little off. So I finally decided to test my AF accuracy using the chart and instructions from PENTAX DSLRs.

After many tests at different apertures ranging from f1.4 to f4, I determined that my lens is front-focusing by about 7 mm. This amount has been just enough to prevent razor-sharp focus on my subjects even at smaller apertures. So I went to custom function #35 and kept adjusting it back all the way to -10, but the front focus is still occurring by about 3-4 mm now.

I'm sure I've done the tests properly to the best of my ability, but I wonder if this is a lens issue or a camera issue if it is indeed not a "me" issue. What can anyone suggest I do now?

12-27-2008, 11:09 PM   #2
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That sounds bewildering. One way to discern whether the problem lies with your lens or the AF tweaking function is to borrow another Pentax AF lens. Run it through the paces. Few lenses require a +/- 10 adjustment.
I had to adjust the FA 50mm lens as well, but it was more a -2 situation.

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12-28-2008, 01:13 AM   #3
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Just do the test and calibration using a well defined near infinity target with tripod and 2s timer. Take at least 3 shots for every adjustment you made before drawing any conclusion.
12-28-2008, 06:00 AM   #4
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You might need to adjust more than once. There is only so much adjustment you can do in one go (doesn't the range go from -100 to +100?). If you've adjusted the maximum and it wasn't enough, start the process over again and you'll find the adjustment is at 0 again and you can do another round.

12-28-2008, 06:17 AM   #5
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QuoteOriginally posted by Miserere Quote
You might need to adjust more than once. There is only so much adjustment you can do in one go (doesn't the range go from -100 to +100?). If you've adjusted the maximum and it wasn't enough, start the process over again and you'll find the adjustment is at 0 again and you can do another round.
Are you sure? On my camera you can only make an adjustment +/- 10. When I start over again, the lens is on the same setting as I last adjusted it to. Only goes back to 0 when I reset it.

enoxatnep - I had an issue with the FA 50 as well. Needed more than a 10 adjustment. Sending the lens + camera to Pentax for adjustment/alignment was the only option for me.
12-28-2008, 10:13 AM   #6
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QuoteOriginally posted by dazman Quote
enoxatnep - I had an issue with the FA 50 as well. Needed more than a 10 adjustment. Sending the lens + camera to Pentax for adjustment/alignment was the only option for me.
Just curious, what was the verdict: was it the lens, camera, or both?
12-28-2008, 10:37 AM   #7
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QuoteOriginally posted by Miguel Quote
That sounds bewildering. One way to discern whether the problem lies with your lens or the AF tweaking function is to borrow another Pentax AF lens. Run it through the paces.
Yeah, that's the problem right now; I only have the one lens but will likely buy my next one in a few days, the DA 12-24. I'll run the same tests on it. If I get the same AF results, I'll have narrowed the problem down to either me (i.e. test method) or the camera.

But one thing I didn't note in my original post is that this AF problem has occurred with real-world shots as well, where I took one with AF and it was slightly off whereas using MF with the exact same settings, composition, etc was bang-on in every case.

Anyhow, thanks for the good point.
12-28-2008, 03:06 PM   #8
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QuoteOriginally posted by enoxatnep Quote
Just curious, what was the verdict: was it the lens, camera, or both?
Camera body needed "a depth adjustment to suit all lenses" & then the lens needed a small AF adjustment via the custom menu

12-28-2008, 03:39 PM   #9
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QuoteOriginally posted by enoxatnep Quote
I've had my K20D with a 50mm FA f1.4 for over a month now. I generally use MF because I was finding my AF a little off. So I finally decided to test my AF accuracy using the chart and instructions from PENTAX DSLRs.

After many tests at different apertures ranging from f1.4 to f4, I determined that my lens is front-focusing by about 7 mm. This amount has been just enough to prevent razor-sharp focus on my subjects even at smaller apertures. So I went to custom function #35 and kept adjusting it back all the way to -10, but the front focus is still occurring by about 3-4 mm now.

I'm sure I've done the tests properly to the best of my ability, but I wonder if this is a lens issue or a camera issue if it is indeed not a "me" issue. What can anyone suggest I do now?
I have a few lenses that -10 doesn't quite do it for, and no lenses that require more than ~+2. At some point, I'll send my K20 in for mechanical AF calibration on my most uncalibratable lens, and then see if I can bring the rest of them in after that.
12-28-2008, 05:13 PM   #10
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I'm going to stick in my 2cents here. First what light source are you using to adjust the lens with? you will get a slight focus shift going from tungsten to florescent to daylight, try and calibrate to something that works well in daylight first.

Next you can adjust the camera and lens to -10 each ( or as neded ) to try and reclaim that 3-4mm, but me personally if I need to adjust anything past the -10 I replace it or send it for service, but thats just me. try setting the camera to like -5 and the lens as needed, helps sometimes just to set the camera to like -1 even if the lens seems spot on and adjust.

Next do note that the center AF secor is alot larger than the red dot implies, it is actually the size of the spot meter circle, I notice a few times trying to focus on something at small apertires it can lock on to the wrong subject, remember it's a cross that is the size of the whole spot meter circle, make sure when doing tests it is not locking on to some text or even paper texture by accident.


try focusing on a brick wall, yes the imfamous brick wall test works well to calibrate " real world ".

if all else fails send it to service.
12-28-2008, 08:13 PM   #11
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QuoteOriginally posted by Torphoto Quote
Next you can adjust the camera and lens to -10 each ( or as neded ) to try and reclaim that 3-4mm, but me personally if I need to adjust anything past the -10 I replace it or send it for service, but thats just me. try setting the camera to like -5 and the lens as needed, helps sometimes just to set the camera to like -1 even if the lens seems spot on and adjust.
I don't think what you're saying here would work. When you set the adjustment for "All Lenses", then if you set acorrection for individual lenses the camera would ignore the setting for the individual lens.
12-28-2008, 10:34 PM   #12
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QuoteOriginally posted by Stratario Quote
I don't think what you're saying here would work. When you set the adjustment for "All Lenses", then if you set acorrection for individual lenses the camera would ignore the setting for the individual lens.
I've tried the all lenses adjustment, what it does is adjust all lenses by your setting, but still allows additional adjustments per lens. IE you can set -5 in all lenses ( since most K20D's just seem to FF ) and then he can dial in say -8 on his lens, presto he's adjusted. Worked for me with my decentered 50-135 ( which is going back regardless, I hope the replacement fits in the normal +/-10 ).
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