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03-05-2007, 05:45 AM   #1
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21mm Limited Back Focus Confirmed

Just got the response from Pentax via their head office in Germany. They confirmed that there is a back focussing problem with the 21mm Limited exclusively. They offered to set up one of my bodies to work in conjunction with the lens, which I refused. I do not wish to take up one body exclusively for that lens. I am waiting on a firmware fix or a chip set change for the lens. I will use it in manual focus only. They are sending me back the two bodies and the two lenses i.e. the 21 and the 16-45 which didn't have a problem and focussed perfectly on the "0" calibration point. I will wait for the 16-50 f2.8 which I should have in ten days to use in lieu of the 21mm Limited. If it is anything like the Canon 17-40L lens which I used on 80 percent of my magazine work, I will probably do the same with it.

Ben

03-05-2007, 06:15 AM   #2
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Thanks for posting the info Ben,

I will keep my fingers crossed on a firmware fix rather than a chip fix.

In the meantime, I was out using this lens at f8 at the weekend and wow, it's sharp. I will certainly be keeping it.



Andy

Last edited by amcinroy; 03-05-2007 at 06:24 AM.
03-05-2007, 06:21 AM   #3
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Andy,

beautiful shot
03-05-2007, 06:25 AM   #4
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But there was yet another DA 21 user who reported FF instead:-

RiceHigh's Pentax Blog: DA 21 Limited Front Focusing or Back Focusing?

Also, I cannot see how a firmware or chip upgrade can help to resolve the problem. I guess it is about the weaknesses of the Pentax AF system and/or the in-born optical errors, probably the spherical aberrations which cause the problem:-

RiceHigh's Pentax Blog: AF Accuracy's Dependency on Lenses and Yellow Light

QuoteOriginally posted by benjikan Quote
Just got the response from Pentax via their head office in Germany. They confirmed that there is a back focussing problem with the 21mm Limited exclusively. They offered to set up one of my bodies to work in conjunction with the lens, which I refused. I do not wish to take up one body exclusively for that lens. I am waiting on a firmware fix or a chip set change for the lens. I will use it in manual focus only. They are sending me back the two bodies and the two lenses i.e. the 21 and the 16-45 which didn't have a problem and focussed perfectly on the "0" calibration point. I will wait for the 16-50 f2.8 which I should have in ten days to use in lieu of the 21mm Limited. If it is anything like the Canon 17-40L lens which I used on 80 percent of my magazine work, I will probably do the same with it.

Ben


03-05-2007, 06:27 AM   #5
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QuoteOriginally posted by Piotr Quote
Andy,

beautiful shot
Yes, good mood, good composition and nice colors.
03-05-2007, 06:34 AM   #6
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As to the 21mm thats very interesting.. I had a lot of problems with the focus accuracy of the DA21 on my K100d but actually don't seem to have a problem with the K10d... weird.. will be interesting to see what Pentax do since they admitted the fault..
03-05-2007, 06:38 AM   #7
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Why You?

QuoteOriginally posted by amcinroy Quote
Thanks for posting the info Ben,

I will keep my fingers crossed on a firmware fix rather than a chip fix.

In the meantime, I was out using this lens at f8 at the weekend and wow, it's sharp. I will certainly be keeping it.



Andy
Why You and Not Me? I guess 'cause you are shooting parallel to the subject...Nah, that is not the case as they tested mine and it was back focussing. I'm telling you it is my Karma for leaving Canon so abruptly. It is called the "CC" or the Canon Curse...
03-05-2007, 07:51 AM   #8
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Ben,

I'd be careful if you are considering going back to Canon after getting a Canon Curse. You may get a Pentax Curse, or does that exist? I don't want to jinx you.

03-05-2007, 08:45 AM   #9
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QuoteOriginally posted by benjikan Quote
Just got the response from Pentax via their head office in Germany. They confirmed that there is a back focussing problem with the 21mm Limited exclusively. They offered to set up one of my bodies to work in conjunction with the lens, which I refused. I do not wish to take up one body exclusively for that lens. I am waiting on a firmware fix or a chip set change for the lens. I will use it in manual focus only. They are sending me back the two bodies and the two lenses i.e. the 21 and the 16-45 which didn't have a problem and focussed perfectly on the "0" calibration point. I will wait for the 16-50 f2.8 which I should have in ten days to use in lieu of the 21mm Limited. If it is anything like the Canon 17-40L lens which I used on 80 percent of my magazine work, I will probably do the same with it.

Ben
The DA 21 does seem to have a generic BF problem. I have tested 2 samples and they were identical. For most purposes it doesn't come into play as the DOF of the lens is quite large even at f3.2 and in many shots it will not be noticeable as the near limit of acceptable sharpness will be just within the focus point.

Here is a test at f3.2



At smaller apertures and with reasonably distant objects (as in the beach shot posted by Andy) it wouldn't show up, in fact it will give added depth of field behind the focus point which could be beneficial.

It will obviously be quite critical at close subject distances and at, or near, max aperture, which is how I suspect Ben is working with this lens.

The FA*24/f2.0 also has a tendency to exhibit a similar charactaristic, but is less severe, but as it's an f2.0 lens it does show more readily. This is why some people claim it is soft at wide apertures, usually it is slightly OOF. Here is the test at f2.0.



Unless Pentax feature the same kind of menu focus calibration settings as allowed by the new C*n*n 1D mkIII (which they could do, the capability is in the debug subset. Maybe they will see fit to bring it in as a firmware update?), the lens itself will probably need to be re-calibrated.

Last edited by Richard Day; 03-05-2007 at 08:47 AM. Reason: changed a word for clarity
03-05-2007, 08:59 AM   #10
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Wow, so technical haha. it will be a long time till I actually get limited lenses.
03-05-2007, 09:03 AM   #11
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F9.0

QuoteOriginally posted by Richard Day Quote
The DA 21 does seem to have a generic BF problem. I have tested 2 samples and they were identical. For most purposes it doesn't come into play as the DOF of the lens is quite large even at f3.2 and in many shots it will not be noticeable as the near limit of acceptable sharpness will be just within the focus point.

Here is a test at f3.2



At smaller apertures and with reasonably distant objects (as in the beach shot posted by Andy) it wouldn't show up, in fact it will give added depth of field behind the focus point which could be beneficial.

It will obviously be quite critical at close subject distances and at, or near, max aperture, which is how I suspect Ben is working with this lens.

The FA*24/f2.0 also has a tendency to exhibit a similar charactaristic, but is less severe, but as it's an f2.0 lens it does show more readily. This is why some people claim it is soft at wide apertures, usually it is slightly OOF. Here is the test at f2.0.



Unless Pentax feature the same kind of menu focus calibration settings as allowed by the new C*n*n 1D mkIII (which they could do, the capability is in the debug subset. Maybe they will see fit to bring it in as a firmware update?), the lens itself will probably need to be re-calibrated.
I was working at about 2.5 to 3 meters from the subject at f 9.0.

Ben
03-05-2007, 10:44 AM   #12
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Wow



03-05-2007, 10:47 AM   #13
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haha if I get a Ltd, BF or FF is not going to be the first thing on my mind!
03-05-2007, 01:21 PM   #14
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That's a nice picture!
03-05-2007, 03:46 PM   #15
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QuoteOriginally posted by benjikan Quote
I was working at about 2.5 to 3 meters from the subject at f 9.0.

Ben
Wow, that DA 21 must be waaay out!

Ask for a replacement!

The DOF at f9.0 and 2.5m distance is huge, basically everything should be sharp between 1.24m and infinity.

Check it out here: Online Depth of Field Calculator
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