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11-10-2010, 08:37 PM   #1
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Problems about DA 21mm focusing at infinity

I have a DA 21mm which has been used for 3 years. Recently, it can't focus on the infinity. When I focusing on things 50 meters away or further, the lens stop at the infinity mark, but the AF indication was flickering and the shutter didn’t release. I have search on the Google and someone said the reason is that the lens inside move a bit front as lens sways, it can be fixed by adjusting the locator ring, that's sounded believable,.
Does anyone here have the same problem? How do you solve it?
Thank you.


Last edited by LihongHao; 11-10-2010 at 10:32 PM.
11-11-2010, 07:23 AM - 1 Like   #2
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光辉岁月四十年(内容已改为 DA21修复记) - 无忌论坛
This is the page I found, it's in Chinese, but the picture there can show something. This guy got a DA21 which has the same problem as mine, and he fixed it by disassembly and adjust. I want to do the same thing, and have removed the cover of the lens, but the three screw there is so small that I have not got the compatible screwdriver. I will buy a smaller one (1.5mm cross patten) and carry on tomorrow.
11-11-2010, 08:22 AM   #3
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All I can say is... be very careful.

Also be sure that you are not holding the focusing ring during auto focus... i believe quick shift requires you to wait until AFTER the AF to override it. Not 100% but that is the impression I got from the manual. It said that resistance during auto-focus could cause malfunction... possibly what you are experiencing?

Have you tried autofocus adjustment first?
11-11-2010, 08:41 AM   #4
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Thank you
I have used this lens for three years, and I am sure I did not hold the focusing ring and there is something wrong.
Envn I set the focusing mode to MF, and screw the ring to infinity mark, there is no 'beep' and the shutter not released. For things not so far away, everything is OK, so I can still use it for some 'street shoot'.

11-11-2010, 08:45 AM   #5
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QuoteOriginally posted by paperbag846 Quote
Have you tried autofocus adjustment first?
I think the autofocus adjustment is used to adjust focusing errors, in my situation, the autofocus is good, but the lens can't reach the suitable point.
point.
11-11-2010, 08:47 AM   #6
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Right. Well just be careful.... that procedure looks like something I'd rather trust a pro with.

If you decide to open it up, make sure you have a little bowl or something with you to collect any little part that might come loose.

I could be totally wrong... but I would hate for your lens to break .
11-11-2010, 06:49 PM   #7
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Thank you very much for your advice.
Maybe my unskilled English make you feel bad, sorry.

This lens is out of warranty, sent it to a pro will cost me a lot, although much cheaper than make it broken.
The guy fixed his said it's easy and safe (maybe for a pro), so I want to have a try.
I will never think about disassemble a zoom lens, but a prime lens will be easier, worth a try, I think.
11-11-2010, 08:50 PM   #8
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It's always very satisfying when you do something like this for yourself... and I'm sure you will learn a lot in the process. Best of luck, solider!

PS - your English is pretty good!

11-11-2010, 09:47 PM   #9
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This seems to be a typical problem with some Pentax primes being made in recent years. There are 3 screws from the front holding the focusing ring with the actual focusing mechanism. For some unknown reason, these screws often weren't threadlocked properly in the factory. Over time, the screws will loose themselves to the point the focus ring will separate from the focusing mechanism. This is the reason for the focus shift (not BF/FF btw). The link above dissembled way more than needed. You only need to readjust the focus ring and tighten the screws from the front. Threadlock it properly afterward and it will be good forever.
11-12-2010, 09:14 AM - 5 Likes   #10
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fixed, maybe.

Wlachan, you are very right, that's what I just found.



As show in this sketch, the blue part is the AF driver, and the purple one is the focusing ring with distance marks, they are fixed together by the black screws.
To adjust the distance marks, the screws holes on the focusing ring is made as pierced arc, so when the screws loosen, the two part will move a little bit and then can't focusing infinite.
But why infinite? I guess when the AF driver push the lens move toward infinite, there will be a sudden stop when the focusing ring hit the stop block, again and agin, the lens may be rush a little over the infinite, but the focusing ring can't go over the block, then problem come.

And, there are some photo I took when disassembling it.

This is the lens before disassembling.

Front cover Removed, they are sticked there by double side adhesive tape. The three screws there is used to fix the lens cover.


remove all the cover, then showed the three screws fixed the focusing ring, and the arc.
Its not necessary to withdraw the screw to adjust , loosen them is enough


the two part are connected only by the three screws, as they together turn and stop so often, I think loosen is inescapable.


the AF driver part.


After adjusting, I go out to find a distant thing, in midnight, moon is best.
with aperture wide open, 100% crop.
well, it seems all right, the only problem is that even moon is not at infinite marks, it's between the 1.3 and infinite. I have adjusted too much, but there will be no 'ka' noise and collision of focusing ring and the block any more.
I will test the lens more tomorrow.


Thank you all.

Last edited by LihongHao; 11-13-2010 at 06:42 AM.
11-12-2010, 11:31 AM   #11
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For adjustment like this, the moon with split image focusing screen on tripod works best. You may fine tune the mechanism b4 tightening the screws.
11-12-2010, 11:40 AM   #12
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Great thread. Thanks for posting the pictures.
11-13-2010, 06:40 AM   #13
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QuoteOriginally posted by wlachan Quote
For adjustment like this, the moon with split image focusing screen on tripod works best. You may fine tune the mechanism b4 tightening the screws.
Wo, I am too anxious to check if the adjustment works, as the marks on the focusing ring does not mean too much for me, so I adjust a little more.
I have no split image focusing screen, and to shoot the moon I must get out of my room, it was too dark there to do some adjust.

I think, after loosen the screws, turn the focusing ring to infinite, then hold the focusing ring, then use the AF to focus on a distant thing, then tighten the screws, that should be a easy and effective way.

Thank you again.

These two photos show the focusing now work well, although the maks on focusing ring doesn't match.



the 100% crop of another.

Last edited by LihongHao; 11-13-2010 at 06:32 PM.
11-13-2010, 08:21 AM   #14
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congratulation~~
it seems work well.
11-13-2010, 08:37 AM   #15
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Looks like you fixed it. Nice job.
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