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04-22-2017, 03:14 PM   #1
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DA 18-135mm ED AL IF DC WR Zoom Lens Focus problems

Hi
I have been reading about this lens on the forum and learned a great deal from the many contributions however I have seen very few comments about auto-focus problems with this lens.
This is perhaps a tribute to the quality of this versatile lens however my own one has a problem but only above 50mm focal length. It occasionally does auto focus OK but mainly the motor drives it well off focus then stops. It can be focused manually over the full range of focal length and it does beep and give a green LED when focused. Beneath 50mm it operates correctly using auto-focus. I have tried it on two different bodies with the same result so it does seem that the lens is the problem.
There is a short thread referring to this problem a couple of years ago but no clear solution was identified although one contributor did cure the problem by dismantling the lens and tightening some screws. Does anyone have any suggestions?
My lens is a second hand one without any repair guarantee and since it is still usable, albeit manually in the upper range, it would not make economic sense to send it to a repairer. I am quite happy to accept the risk of a DIY attempt to fix it but would welcome some advice on this, in particular dismantling guidance. I have stripped and cleaned lenses previously but not a DC motor version.
In addition perhaps some knowledgeable person could explain the basic operation of this auto-focus system. I assume the measurement of focus takes place in the camera body and it then feeds power through the two power contacts to the lens to focus in one direction and if that starts to make it worse it then reverses polarity to change focus direction until good focus is achieved. Is that the way it works? Is there any focus inputs through the lens contacts in particular the seventh one introduced with KAF3?
Many thanks for any help and I will of course feedback details of the outcome of my attempt to overcome this problem.

04-22-2017, 08:22 PM   #2
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QuoteOriginally posted by mcewen Quote
Hi
I have been reading about this lens on the forum and learned a great deal from the many contributions however I have seen very few comments about auto-focus problems with this lens.
This is perhaps a tribute to the quality of this versatile lens however my own one has a problem but only above 50mm focal length. It occasionally does auto focus OK but mainly the motor drives it well off focus then stops. It can be focused manually over the full range of focal length and it does beep and give a green LED when focused. Beneath 50mm it operates correctly using auto-focus. I have tried it on two different bodies with the same result so it does seem that the lens is the problem.
There is a short thread referring to this problem a couple of years ago but no clear solution was identified although one contributor did cure the problem by dismantling the lens and tightening some screws. Does anyone have any suggestions?
My lens is a second hand one without any repair guarantee and since it is still usable, albeit manually in the upper range, it would not make economic sense to send it to a repairer. I am quite happy to accept the risk of a DIY attempt to fix it but would welcome some advice on this, in particular dismantling guidance. I have stripped and cleaned lenses previously but not a DC motor version.
In addition perhaps some knowledgeable person could explain the basic operation of this auto-focus system. I assume the measurement of focus takes place in the camera body and it then feeds power through the two power contacts to the lens to focus in one direction and if that starts to make it worse it then reverses polarity to change focus direction until good focus is achieved. Is that the way it works? Is there any focus inputs through the lens contacts in particular the seventh one introduced with KAF3?
Many thanks for any help and I will of course feedback details of the outcome of my attempt to overcome this problem.
While I regularly disassemble all sorts of electronic gadgets, great care must be taken with something like a lens -- you don't want to cause alignment problems that you will be unable to easily correct.
Have you tested autofocus beyond 50mm on a tripod? And can we assume these problems occur in good light?
You are correct that the camera body evaluates focus and commands lens focusing group motion via the various electronic contacts (so they should be clean and exhibit similar projection distance from the body).
You can read up on the KAF electrical contacts here:
Features and Operation of the Ka Mount

Last edited by FilmORbitz; 04-25-2017 at 02:22 AM. Reason: fat fingers
04-23-2017, 02:50 AM   #3
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FilmORbitz many thanks for your very prompt and helpful reply.
The lens problem is present in good light conditions and with tripod support. The two power contacts have been cleaned and are on the same level and show witness marks from contact with the camera contacts.
I have two of these lenses and can compare the problem lens with the good one so I am fairly confident it is a lens issue.
At this stage I am trying to put together as much information as possible to understand how to dismantle the lens with minimum risk of damage and also understand the auto-focus mechanics so that I can identify the sections of the lens for investigation which are most likely to cause this problem.
My initial thoughts are that the camera body is receiving the necessary input to identify the correct focus point and it is sending the appropriate output to the lens. For some reason at longer focal lengths this output is not operating the focus motor to the correct position other than occasionally but it is operating the focus motor. I have attempted to find some pattern to this and to see if there is any correlation with other factors, for example short range v long range. So far the only thing that has been identified is that autofocus is more successful in liveview but not always successful and occasionally the camera freezes in liveview and can't be controlled with any controls or even turned off! One possible sources of problem is some kind of poor contact or earthing caused by the physical movement at longer focus but I have tested for continuity between each of the two power contacts and the lens mount ring. Continuity response is the same for the good lens and the problem one but I realise this does not rule out completely some issue within the lens electrical/electronic systems.
If anyone can offer further advice on dismantling or problem investigation this would be most welcome.
04-24-2017, 06:11 AM   #4
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QuoteOriginally posted by mcewen Quote
FilmORbitz many thanks for your very prompt and helpful reply.
The lens problem is present in good light conditions and with tripod support. The two power contacts have been cleaned and are on the same level and show witness marks from contact with the camera contacts.
I have two of these lenses and can compare the problem lens with the good one so I am fairly confident it is a lens issue.
At this stage I am trying to put together as much information as possible to understand how to dismantle the lens with minimum risk of damage and also understand the auto-focus mechanics so that I can identify the sections of the lens for investigation which are most likely to cause this problem.
My initial thoughts are that the camera body is receiving the necessary input to identify the correct focus point and it is sending the appropriate output to the lens. For some reason at longer focal lengths this output is not operating the focus motor to the correct position other than occasionally but it is operating the focus motor. I have attempted to find some pattern to this and to see if there is any correlation with other factors, for example short range v long range. So far the only thing that has been identified is that autofocus is more successful in liveview but not always successful and occasionally the camera freezes in liveview and can't be controlled with any controls or even turned off! One possible sources of problem is some kind of poor contact or earthing caused by the physical movement at longer focus but I have tested for continuity between each of the two power contacts and the lens mount ring. Continuity response is the same for the good lens and the problem one but I realise this does not rule out completely some issue within the lens electrical/electronic systems.
If anyone can offer further advice on dismantling or problem investigation this would be most welcome.
If you decide to not tear it down, here is a respected repair source:

Home-Pentaxs


Last edited by FilmORbitz; 04-25-2017 at 02:23 AM. Reason: more fat fingers
04-24-2017, 07:45 AM   #5
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The Nikkor AF 28-85 usually had that same defect. It was found that any knock or bump would dislodge the inner front element, causing to not af beyond 35mm, it would screw phase detection. Does your lens feel rough if you zoom to 100mm?
04-24-2017, 04:49 PM - 1 Like   #6
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Jeff Lopez. Thanks for your suggestion. I think the lens elements are OK but will try to check this further. The lens seems optically satisfactory with very sharp images and when operated in manual the focus system recognises a correctly focused image with the usual green LED. The problem is that in auto-focus the motor attempts to focus but drives the lens to an out of focus position. This only occurs above 50mm focal length. Beneath that it is usually OK.
I have now stripped the lens partially and the attached photo shows the pcb above the motor. There does not seem to be any obvious mechanical problem. Focus ring and zoom barrels move smoothly over the whole range. I intend removing the pcb and the casing ring on which it is mounted. This will allow access to the axially mounted "tracks" on the barrel and spring contacts which signal the zoom position back to the pcb. I need to check whether a bad contact in this system is causing my problem. Usually the electronic part of most mechanisms is reliable and it is the mechanical bits that fail so at this stage I do not suspect the pcb or it's components. The fact that this issue only occurs above 50mm also makes me suspect a zoom position feedback problem.
If anyone has had experience of a similar repair or some information about the principle of operation of the focus mechanics/electronics inside the lens their input would be welcome.
Attached Images
View Picture EXIF
TG-3  Photo 
04-24-2017, 06:06 PM   #7
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Some vintage af lens had a sort of Hall type sensor, like a ring with a lot of holes, and a diode behind it, or optical sensor, looks like a small block with a tiny bulge..nowdays lenses dont need feedback, cameras got really smart with reading whats on the mirror..let me see what I can get from other lenses..Steady hand there!

---------- Post added 04-24-17 at 06:18 PM ----------

Pentax 18-55 DA II Disassembly/Reassembly | Flickr
you may use that as a reference, just in case

http://ricehigh.blogspot.com/2011/01/diy-again-disassemble-and-repair-da-50.html?m=1

Last edited by Jeff Lopez; 04-24-2017 at 06:21 PM.
04-24-2017, 09:01 PM   #8
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QuoteOriginally posted by mcewen Quote
Jeff Lopez. Thanks for your suggestion. I think the lens elements are OK but will try to check this further. The lens seems optically satisfactory with very sharp images and when operated in manual the focus system recognises a correctly focused image with the usual green LED. The problem is that in auto-focus the motor attempts to focus but drives the lens to an out of focus position. This only occurs above 50mm focal length. Beneath that it is usually OK.
I have now stripped the lens partially and the attached photo shows the pcb above the motor. There does not seem to be any obvious mechanical problem. Focus ring and zoom barrels move smoothly over the whole range. I intend removing the pcb and the casing ring on which it is mounted. This will allow access to the axially mounted "tracks" on the barrel and spring contacts which signal the zoom position back to the pcb. I need to check whether a bad contact in this system is causing my problem. Usually the electronic part of most mechanisms is reliable and it is the mechanical bits that fail so at this stage I do not suspect the pcb or it's components. The fact that this issue only occurs above 50mm also makes me suspect a zoom position feedback problem.
If anyone has had experience of a similar repair or some information about the principle of operation of the focus mechanics/electronics inside the lens their input would be welcome.
OK, you're goin' in...I love zero-risk guts-viewing. You've probably already seen the teardowns at lensrentals.com, but in case you haven't, enjoy:

Lens Rentals | Blog

04-25-2017, 01:46 AM   #9
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FilmORbitz and Jeff Lopez. Many thanks for your comments.
The various references to other web sites have been very helpful in particular the Rice High blog spot. His problem with the Pentax DA lens, which has many similarities to the DA 18-135, is almost a carbon copy of my own problem. In addition you will see from my previous reply that I am heading in the same direction ie to investigate the spring contacts on the zoom barrel. Unfortunately with my lens the spring contacts do not seem to be accessible from the outside hence my plan to remove the pcb etc to gain assess. One other difference between these lenses is that the pick-up tracks on mine are mounted in an axial direction on the barrel whereas his are circumferential on the zoom control ring. I assume however that the principle of operation is similar in that the zoom position is fed into the pcb electronics and it somehow influences the focusing motor control signal. If anyone can explain how/why I would be most grateful.
So far disassembly of this lens, in the absence of any service manual or breakdown instructions, has not been too difficult but I suspect the next bit will be more challenging so any hints or tips from people who understand this stage of the investigation would be appreciated.
04-25-2017, 06:54 AM   #10
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same here

I have exactly the same problem - and it's a question of the lens itself:
I have a second lens and another Body to check and compare...

So: is there anything I can do by myself to solve the problem or will it be necessary to send the lens to the repair station?
How much will they take for the repair ?
04-25-2017, 10:05 AM   #11
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Hi PLEO. I am trying to find a solution to the problem but so far have not found one but hope to do so. If I succeed I can provide some photos and instructions but I am certain it will involve a significant disassembly of the lens and you will need to be confident in your ability to do this. I have no idea how much a repairer would charge but it is unlikely to be cheap in view of the time involved and possibly component costs.
03-25-2018, 02:44 PM   #12
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Same problem here - any solutions?

I have the same problem. The 18-135 mm would not focus with my K30 and am wondering if somebody managed to find a fix or got it repaired? If repaired then how much did it cost?

Thanks!
03-26-2018, 03:20 AM   #13
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AGUARAV
I am sorry autofocus is not operating correctly with this lens. I am not sure from your description if the problem is the same as my own. I did describe in a number of earlier messages the exact detail of my own problem and attempt to strip down and repair.
At present I use this lens in autofocus up to 50mm, which is the range over which it operates correctly. At longer focal lengths I use manual. This really has not been a problem since most of my shots tend to be in the lower range anyway. This successful practice however made me lazy about pursuing the repair project therefore I did not make much progress beyond that already described in earlier messages. I did establish that the problem was not the spring contacts on the zoom ring, which were originally suspected. Probably the PCB and it's components are the areas worth checking next, if I ever get round to another repair attempt.
I cannot advise on repair cost since that has not been investigated.
As a general point I would not advise stripping the lens yourself unless you have some experience of this or similar types of repair but if you do have a go best of luck.
It would be helpful if you could feed back any progress with this problem.
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