SDM fiction and fact

Started Mar 29, 2011 | Discussions
awaldram
awaldram Forum Pro • Posts: 13,271
SDM fiction and fact

I decided to bite the bullet and strip down my 50-135 to see if I could reach any conclusions.

My 50-135 has always focused well and still does, But occasionally it will balk at micro AF adjustments flash the AF (no-lock) light , releasing and re-applying AF then locks. Its done this from new and hasn't really been a fault but curiosity got the better of me.

I'd read the http://bangmedia.no/pentax/sdmfix/

And used that as I guide to get me in I was particularly interested in the 'waggle the screw' idea as this just doesn't seem right.

Now everything after this is just what I observed and conclusions from that so could be dead wrong.

I noticed that the screw is locked in position with locktite this implies its an adjustment screw not connected to the rotor.

I also noticed that both MF and AF(motor) were on the same gear chain without any clutch mechanism so when MF is used the motor free wheels.

The screw drive is decoupled with a clutch mechanism and does not rotate for inmotor of Manual focusing.

I therefore cracked the the seal (noted position) and rotated the screw hard to the right.

In this state the whole focusing chain was locked solid , MF,Af(motor) and AF (screw) were jammed.

I then rotated to the hard left.

In this state everything was very free with very little friction from the motor.

I set the srew slight more left than it was originally and re-assembled.

Lens now focuses without hesitation and micro adjusts consistently (so far).

Conclusions.

It seems to me that most symptoms reported can be attributed to this adjustment screw.

1 No Focus lock = motor can overcome the friction for large movements but unable to micro adjust so no lock
2 Focus Jammed = screw has drifted right and locked up.
3 SDM failure = lock/stiffness has caused motor/electronics to fail.

The reason the screw is wrongly set seems to fall into 2 camps

1 initial setting from factory is to tight (sort of my case) leading to symptom 1 for people, This seem more prevalent in early 16-50 lens.
2 design constraints

I doubt the motor was ever specced for fast spinning so the data for locking the screw is probably inadequate for the field use.

i.e because the MF is directly couple to the motor and fast spin of the focus will wizz the motor considerably faster than it rating, This in turn will force the stator adjustment screw one way or the other (tighter/looser).

This also explain the 'warm up the lens' symptoms people report were exercising the MF brings SDm back to life.

Whether this motor is used in the later SDM solutions I have my doubts as they don't seem to have the same symptoms reported.

I suspect the failure rate is pretty low even for the affected lens as it relies on the loctite failing under very specific circumstances.
Unfortunately it would affect people who MF more than those who don't.

This would explain why I have not been affected by any SDM problems as my view on AF is why have a dog and bark yourself

 awaldram's gear list:awaldram's gear list
Pentax K-x Pentax Q Olympus PEN E-PM2 Pentax Q7 Pentax K-3 +17 more
essH Contributing Member • Posts: 880
Re: SDM fiction and fact

Fascinating stuff; thanks! It certainly sounds plausible.
--
Simon

 essH's gear list:essH's gear list
Pentax K-30 Olympus OM-D E-M10 II Pentax smc FA 77mm 1.8 Limited Olympus M.Zuiko Digital 45mm F1.8
jamesm007 Veteran Member • Posts: 5,793
Re: SDM fiction and fact

Nice stuff. Good detective work. I want to ask a question not as a smart as# but what your hypoth would be on it. Many have reported problems with SDM with the lens sitting for long periods of time? I myself went to my local store to try a DA*50-135mm and it would not AF at first, it took awhile between me and the sales man to get it to go. This came at the exact same time others started to report failures/problems with their DA*50-135mm lens. Before that time (about Jan 2009) the lens was the true star of Pentax. The only factor in common seemed to be time not use? Like the lens I tried it sat under the counter for it life before I tried it. I am sure others did try it but it was never in field use, what let go after time?

I do seem to read that you believe the screw was mis-adjusted from the get go for the above problem. Is this correct? And what you mean by micro-adjustments? But if I remember correct the lens would not spin (try to AF) at all!
--
jamesm007,

http://www.flickr.com/photos/jamesm007/
http://s195.photobucket.com/albums/z77/jamesm700/

 jamesm007's gear list:jamesm007's gear list
Pentax K20D Pentax K-5 Sigma 10-20mm F4-5.6 EX DC HSM Pentax smc DA 10-17mm F3.5-4.5 ED (IF) Fisheye Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical (IF) +13 more
awaldram
OP awaldram Forum Pro • Posts: 13,271
Re: SDM fiction and fact

Hi James,

Was thinking about that one, Had a few ideas but they are such hypothesis that I didn't mention them.

My thought went along the line

if the stator or commutator is an aluminium alloy and there is slight moisture in the air then it will oxidise.

If during this process the rotor isnt spun (sitting) then the motor will seize.

As there is a high gear ratio motor -> MF then a small stiffness/seize in the motor will lock the MF solid.

I think there's a good chance that forcing movement would fix it BUT the gears are only nylon so there is a real danger you could strip teeth and on such an expensive unit probably not worth risking.

 awaldram's gear list:awaldram's gear list
Pentax K-x Pentax Q Olympus PEN E-PM2 Pentax Q7 Pentax K-3 +17 more
thxbb12 Senior Member • Posts: 2,163
Good stuff!

Excellent deduction work, awaldram

Cheers for pointing out your observations. I had read that page, but never dared actually opening my lens.

After reading your message, I think I'll try to open up my DA* 16-50 as it's always had the "blinking focus light" symptoms (ie won't focus in many instances). Furthermore, it seems to not focus at all now although it doesn't happen all the time.
As it is now pretty much a MF lens, I've got nothing to loose

Thanks for sharing your findings!

-- hide signature --
 thxbb12's gear list:thxbb12's gear list
Panasonic Lumix DMC-GM5 Fujifilm X-T20 Fujifilm X-S10 Panasonic Lumix G 14mm F2.5 ASPH Olympus M.Zuiko Digital ED 40-150mm F4-5.6 R +6 more
Aristophanes Contributing Member • Posts: 560
Re: SDM fiction and fact

I have always surmised that SDM failures on Pentax lenses were partly due to WR and the added seals and friction. The thermal break the seals provide would also go some way towards an explanation as to the "warming" effect.

 Aristophanes's gear list:Aristophanes's gear list
Olympus OM-D E-M1X
awaldram
OP awaldram Forum Pro • Posts: 13,271
Re: SDM fiction and fact

Aristophanes wrote:

I have always surmised that SDM failures on Pentax lenses were partly due to WR and the added seals and friction. The thermal break the seals provide would also go some way towards an explanation as to the "warming" effect.

I don't think WR seals can have any affect on an internal focusing design such as the 50-135.

 awaldram's gear list:awaldram's gear list
Pentax K-x Pentax Q Olympus PEN E-PM2 Pentax Q7 Pentax K-3 +17 more
Success Regular Member • Posts: 218
Re: SDM fiction and fact

I've always had a sort-of unsubstantiated theory that perhaps the use of quick-shift makes a lens more susceptible to SDM problems. I arrived at this theory partially based on the fact that I never use quick-shift (or MF) with my several SDM lenses, and the fact that I have never had any AF problems at all with any of them, and I do have the infamous 16-50. I have been reluctant to put forth my theory because I'm afraid someone will be able to shoot it down, and I like my theory, it makes me feel better!

Robgo2 Veteran Member • Posts: 5,359
Re: SDM fiction and fact

I must say that you are a much braver man than I. The mere thought is dissembling a lens makes me want to crawl under a bed. I'm curious if you noticed any new dust particles inside the lens after you re-assembled it.

Rob

awaldram
OP awaldram Forum Pro • Posts: 13,271
Re: SDM fiction and fact

As I didn't remove any lens elements there was no more chance of getting dust between the glass than any normal non-WR lens.

As it take 5-10 Years for dust to get between elements on even the worst dust pumps (canon 100-400 etc) I was not overly concerned.

I did re-assemble the aperture spring backwards though first time , That was annoying lens worked brill till you stopped it down then it overexposed as you stopped down.

It fairly safe if your dexterous and not gung ho.

Stripping further than the Ultrasonic motor and some knowledge is required or you'll bend the zoom fingers when you remove the zoom barrel.

Stripping further gets even scarier. When you've had 9 aperture leaves fall out of a lens then the brain starts think they'll never work again.

But a steady hand and bluetack on a screwdriver tip and even the leaves can be re-assembled without issue.

If anyone does attempt it from the previous link then a few tips

1 stick a bluetack sausage over the contacts this will hold them in place and make re-assembly a breeze.

2 the O-seal should be fitted to the mount plastics not the zoom barrel that way you can be certain not to mis seat it, If cant get it to stay the a little vaseline will help.

3 ensure the rubber seal of the k-mount does not come unseated prior to re-assembly

4 watch out for the earthing spring this will not be held by the bluetack and is very small.

5 the SDM contacts have to be teased back into place ensure you hold them straight as the bend easily (tweezer or small pointed pliers).

 awaldram's gear list:awaldram's gear list
Pentax K-x Pentax Q Olympus PEN E-PM2 Pentax Q7 Pentax K-3 +17 more
il_alexk Senior Member • Posts: 2,824
Re: SDM fiction and fact

awaldram wrote:

Unfortunately it would affect people who MF more than those who don't.

Great investigation work, seems plausible.

Anyway, it is the most sophisticated "user error" theory I ever saw. Bravo!

-- hide signature --

Cheers,
Alex

 il_alexk's gear list:il_alexk's gear list
Pentax K-5 IIs Sony a7R III Fujifilm X-A5 Fujifilm X-T100 Pentax smc FA 50mm F1.4 +8 more
snake_b Veteran Member • Posts: 4,680
Re: SDM fiction and fact

Great thread for a potential buyer. I'm hoping to get my K5 or K01 in the coming months and the 16-50 is on my list, as is the 60-250 or 50-135 at a later time.

It's too bad that the K5 doesn't go back to screw-drive if the SDM fails. And unfortunately, I read about how the SDM isn't particularly fast, so it's looking like it might have just been a marketing thing to go up against Supersonic specced lenses from others.

awaldram
OP awaldram Forum Pro • Posts: 13,271
Re: SDM fiction and fact

il_alexk wrote:

awaldram wrote:

Unfortunately it would affect people who MF more than those who don't.

Great investigation work, seems plausible.

Anyway, it is the most sophisticated "user error" theory I ever saw. Bravo!

Think you'll find I plumb lay the blame on Pentax's design

"I doubt the motor was ever specced for fast spinning so the data for locking the screw is probably inadequate for the field use. "

If operating a tool within it design specifications breaks it, that is not the users fault.

-- hide signature --

Cheers,
Alex

 awaldram's gear list:awaldram's gear list
Pentax K-x Pentax Q Olympus PEN E-PM2 Pentax Q7 Pentax K-3 +17 more
il_alexk Senior Member • Posts: 2,824
Re: SDM fiction and fact

awaldram wrote:

Think you'll find I plumb lay the blame on Pentax's design

If operating a tool within it design specifications breaks it, that is not the users fault.

You don't need to convince me, but I do see a case when some users may creatively start quoting you by saying - "Your fault, you probably MF-ed instead of using the AF, you should have read the awaldram's thread".

-- hide signature --

Cheers,
Alex

 il_alexk's gear list:il_alexk's gear list
Pentax K-5 IIs Sony a7R III Fujifilm X-A5 Fujifilm X-T100 Pentax smc FA 50mm F1.4 +8 more
shane gerrish
shane gerrish Veteran Member • Posts: 5,980
Re: SDM fiction and fact

did it look like you could turn it into a screw drive only ?
--
Shane,Gold Coast Aussie. Lens list in profile.
http://bigred4x4.blogspot.com/2008/01/welcome.html

 shane gerrish's gear list:shane gerrish's gear list
Pentax K-1
Keyboard shortcuts:
FForum MMy threads