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12-15-2010, 10:51 AM   #1
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31mm Ltd AF(?) adjusting problems with the K7

Hello all.

I have this issue I am trying to solve and it is not easy.
Perhaps some of you would know how to make the proper adjustments:

Lens: FA 31 Ltd
Camera: K7
I don't have the sample images just now; I will post some of them as soon as I have the chance.

The first time I noticed that there was some BF with this lens-camera combo was this past weekend, doing some boat shots at the sea port.
The ship in question is a tugboat haulng an imposing barge, very slowly moving into port.

First, I noticed that the shots were not in focus upond reviewing them one at a time with the rear camera monitor.
I tried manual focusing, to no avail. Same results.

Once back home and visualizing the shots on the computer monitor, the shots were indeed OOF for approximately 90% of the 20 shot series I took.

So, I started to "play" with the in-camera AF adjustment option and ended up with a serious BF which needed a +10 tuning !! Can't really go higher than that, can I?
I used different subjects: charts, common house stuff (batteries, books, etc ...)
and could finally get "bang on", AF-wise, at close range.
I did use different focal lenghts during the process of testing and adjusting.

Went back out yesterday, repeated some shots on different subjects (buildings, cars ...) and the images still show somewhat not that clear, i.e.: not perfectly in focus.

Can anyone suggest something which I might not be aware of with this particular combo or is this something I am missing in the adjustment procedure?

Cheers!

JP

12-15-2010, 12:56 PM   #2
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Are you shooting at 1.8 or stopped down?
12-15-2010, 01:09 PM   #3
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QuoteOriginally posted by deadwolfbones Quote
Are you shooting at 1.8 or stopped down?
Hi!
Thanks for the reply.

I did use F1.8 to F8 and there seems to be some inconsistencies in the realized focusing:

For instance, at F1.8, it is sometimes right on focus, sometimes not. Users error?
Same thing at F2.8, 4, 5.6 and F8.
Shutter speeds of 1/50 to 1/100s. SR "ON".
Used AF and manual focusing.

I think that it is often "softer" at F1.8 but I wonder why it would also be the case when stopped down.

Other "variables" maybe? I was shooting boats ... so, there is a large body of water; the lighting conditions were not perfect: overcast, giving an overall grey cast at the overall scene but would that really matter when the boats themselves were very contrasty against the "grey" water? I was at roughly 20-40 meters away at first and then much closer once the boat approached the pier.

Cheers!

JP

P.S.: just took some shots of stationary objects (statues outside of the church) just moments ago: seem quite in focus using AF in spite of an another overcast day. AF adjustment still at +10.
12-15-2010, 01:31 PM   #4
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There has been some speculation out there for a while that the Pentax AF system is not very precise below f/2.8 or so, which leads to more inconsistent BF/FF behavior at wide apertures (f/1.4-2.4). I can't remember exactly who had this theory, but I think HerbChong wrote quite a bit about it at FredMiranda or dpreview.

For what it's worth, I had similar focusing issues with my FA31 on my K-7, particularly at wide apertures. Focus accuracy also seemed to vary a lot based on the subject distance. On my K-5 it seems quite a lot better, though I haven't used that combo enough to be really certain.

12-15-2010, 01:46 PM   #5
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QuoteOriginally posted by deadwolfbones Quote
There has been some speculation out there for a while that the Pentax AF system is not very precise below f/2.8 or so, which leads to more inconsistent BF/FF behavior at wide apertures (f/1.4-2.4). I can't remember exactly who had this theory, but I think HerbChong wrote quite a bit about it at FredMiranda or dpreview.

For what it's worth, I had similar focusing issues with my FA31 on my K-7, particularly at wide apertures. Focus accuracy also seemed to vary a lot based on the subject distance. On my K-5 it seems quite a lot better, though I haven't used that combo enough to be really certain.
Thanks again.

I agree with the fact that that lens wide open is not at its greatest for focusing. At least, that's what I am finding out.
Problem is that it does also happen stopped down... ?

I will perform more test shots during the next few days, especially in "real life" situations.
I don't think that the charts are that useful.

I will reset the AF at zero and start all over, using very small incremental corrections at see what happens.

Cheers.

JP
12-15-2010, 01:50 PM   #6
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QuoteOriginally posted by jpzk Quote
Thanks again.

I agree with the fact that that lens wide open is not at its greatest for focusing. At least, that's what I am finding out.
Problem is that it does also happen stopped down... ?

I will perform more test shots during the next few days, especially in "real life" situations.
I don't think that the charts are that useful.

I will reset the AF at zero and start all over, using very small incremental corrections at see what happens.

Cheers.

JP
The best way to test for consistent FF/BF is with something like LensAlign Pro, if you can afford it...
12-15-2010, 02:36 PM   #7
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QuoteOriginally posted by deadwolfbones Quote
The best way to test for consistent FF/BF is with something like LensAlign Pro, if you can afford it...
Yep! Expensive!!

Oh, by the way:
once the settings for AF have been "corrected", do you have to go back to the custom function (37) for AF adjustment (which now shows "2" for "ON") and return to 1 "OFF" or just leave at ON .... ?
I have no manual right here!

JP
12-16-2010, 01:32 AM   #8
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QuoteOriginally posted by jpzk Quote
return to 1 "OFF" or just leave at ON .... ?
Leave it at ON. OFF means that your focus adjustments are disregarded.

12-16-2010, 03:20 AM   #9
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QuoteOriginally posted by deadwolfbones Quote
The best way to test for consistent FF/BF is with something like LensAlign Pro, if you can afford it...
i would recommand somethink like the LensAlign too
got mine second hand and i found most of my "handmade" AF-adjustment
had been close, but not perfect
12-17-2010, 01:54 PM   #10
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QuoteOriginally posted by simico Quote
Leave it at ON. OFF means that your focus adjustments are disregarded.
Thanks Simico.

As usual, no user's manual handy here!

Done ... left at "ON".

Cheers.

JP
12-17-2010, 02:23 PM   #11
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QuoteOriginally posted by iFoto Quote
i would recommand somethink like the LensAlign too
got mine second hand and i found most of my "handmade" AF-adjustment
had been close, but not perfect
I have just browsed the site where they sell the LensAlign tools.

This seems very accurate and professionally built, with great users' guides, but after reading through their User's Guide PDF file, I find this to be a rather complicated task.
Just the alignment set up for the camera/tool is several pages long.

But, if this is really going to make a difference with the lenses I am using, sure, I will consider it, especially when one has to do the testing/adjusting for a good number of lenses and two camera bodies.

So, thanks to you and Deadwolfbones for the reference.

Cheers.

JP
12-17-2010, 04:38 PM   #12
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I have to say that my K10+31ltd are very erratic combo. And at f1.8 past 2-3meters the lens is not at it's best. Combine those two and there you go...
12-17-2010, 04:53 PM   #13
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QuoteOriginally posted by axl Quote
I have to say that my K10+31ltd are very erratic combo. And at f1.8 past 2-3meters the lens is not at it's best. Combine those two and there you go...
Axl, that may not be a great combo (K10 + 31 Ltd) but I am starting to believe that it isn't that great either with the K7/31Ltd combo.
One thing I will also do is to compare this with a K20D /31Ltd combo over the weekend.

Annoyingly, it seems to get worse when the subject to camera distance increases. Worse, in the sense that I get less keepers (subjects in focus) whether it is with AF or manual focusing, even at F5.6 .. F8.
And then, there is the obvious softness at F1.8 (with the K7); not much better than with your K10D/31 Ltd combo.

Seemingly, the lens/K7 camera performs better using a close distance-to-subject with flash, indoors!
I took some shots at a party this past weekend and the results were rather satisfying.

Which now means that I am pretty much mixed up as far as fine adjusting the AF.

More or that after the weekend.

Cheers.

JP

P.S.: I wonder what the says are with the K5 and the FA 31 Ltd ??
12-17-2010, 05:24 PM   #14
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I've not noticed focus issues with my K20D + FA 31 combo, in fact not with *any* of my lenses. No AF adjustments necessary and I seem to hit focus on all my shallow DoF shots in this regard.
Can't tell what the issue may be, but examples posted here of your methodology and results would help.
12-18-2010, 02:57 AM   #15
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QuoteOriginally posted by Ash Quote
I've not noticed focus issues with my K20D + FA 31 combo, in fact not with *any* of my lenses. No AF adjustments necessary and I seem to hit focus on all my shallow DoF shots in this regard.
Can't tell what the issue may be, but examples posted here of your methodology and results would help.
You are one lucky guy in that case Mr Ash!
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