[monochrome]

I bought a used 16-50 and 50-135 last year. Within weeks the SDM motors stopped working so I converted them both to screw drive and immediately sold them that way. I knew the risk going in and I didn’t pay much for the lenses but it truly annoyed me.

[SSGGeezer]

I sold cameras and lenses not just Pentax but mostly Pentax for over 20 years. Trust me they do occasionally come dead out of box new. I personally bought two back-to-back DA* 50 - 135 2.8 SDM motor lenses. The first one was squeaking with a failing SDM motor right out of the box. The second one failed within three months. They fixed it and it failed with another year. And I've had similar issues on my DA*60-250mm. Sometimes it just will not autofocus no matter how often I push the button down so I flip it to manual and now it's decided to lock itself at F-16 and become a paperweight.

Sorry to hear that. It is also a sad situation that the 60-250 is not one of the lenses that can be turned into a strictly screw drive lens.

[alfa75ts]

My own theory is that the 'clutch' mechanism used to prevent either manual focus or the screw drive from spinning the SDM motor jams.
This 'clutch' should only engage when the SDM motor is spinning. If it jams, manual focusing (or screw drive) may drive the SDM motor and damage it.

Just a theory but the 'clutch' is there for some reason. The patents seem to suggest it is to prevent wear to the gears but I'm not sure.

The 'clutch' is either roller bearings (fig 4 - 35, 43) or ball bearings (B) which wedge between two surfaces.

There have been a few patent variations which may suggest attempted improvements.

Or I might be wrong.

[RobA_Oz]

My own theory is that the 'clutch' mechanism used to prevent either manual focus or the screw drive from spinning the SDM motor jams.
This 'clutch' should only engage when the SDM motor is spinning. If it jams, manual focusing (or screw drive) may drive the SDM motor and damage it.

Just a theory but the 'clutch' is there for some reason. The patents seem to suggest it is to prevent wear to the gears but I'm not sure.

The 'clutch' is either roller bearings (fig 4 - 35, 43) or ball bearings (B) which wedge between two surfaces.

There have been a few patent variations which may suggest attempted improvements.

Or I might be wrong.

Nice find! I freed the drive by removing the lens mount, which exposed the back of the drive, which in turn had a screwdriver slot that I used to turn it. I’m guessing, but if the right-side components represent the disassembled motor, then the other side of the right-most component would hold that slot. I’m not sure if that supports your theory or not!

[alfa75ts]

but if the right-side components represent the disassembled motor

Both images represent this:



In the previous images, from 14-16 going right we have the motor and going left we have the clutch. But it's all one assembly.

[RobA_Oz]

Both images represent this:

Attachment 499764

In the previous images, from 14-16 going right we have the motor and going left we have the clutch. But it's all one assembly.

That’s what I thought. There’s a screw head on the top, which must be the one I used to free up the drive. Again, I’m not sure whether it was the clutch or the motor that seized, but brute force won the day many times, until I wearied of it and converted to in-body AF.

[photogem]

What I hate is that some SDM lenses are SDM only. Particular the 55*/1,4
There is nothing to convert.

[normhead]

What I hate is that some SDM lenses are SDM only. Particular the 55*/1,4
There is nothing to convert.

You do understand that as soon as you say "some SDM lenses" you've made the error of assuming all SDM lenses are the same? And the in buying an electrically focusing lens like the 55, you have added another component that can fail, compared to screw drive?

I recently took my 60-250 out briefly after realizing I hadn't used it for 6 months, and it worked much better than I remember it working. MY anecdotal contribution would be that it works better if you leave it in the cupboard for 6 months. Hence highlighting the problem with anecdotal evidence. It can go either way. The only thing lost is the truth about how likely the issue is too occur. Anecdotal evidence may be small part of the truth, but it's along way from the whole truth.

[photogem]

You do understand that as soon as you say "some SDM lenses" you've made the error of assuming all SDM lenses are the same?

I don't understand it like this.
When I write that some SDM lenses can't be converted to screwdrive, and that I hate that limitation, I am quite clear about it.
I mean "some SDM only" lenses and that means no screwdrive mechanism.

Where is this error you speak about?

And the in buying an electrically focusing lens like the 55, you have added another component that can fail, compared to screw drive?

What is "the in buying"?

All SDM lenses are AF lenses with an SDM motor, and yes, an SDM motor is driven by DC voltage.
Any SDM motor can fail, but all SDM lenses except one which have the screwdrive mechanism built in but not activated can be converted to screwdrive.
Makes them faster as well but louder.

I recently took my 60-250 out briefly after realizing I hadn't used it for 6 months, and it worked much better than I remember it working. MY anecdotal contribution would be that it works better if you leave it in the cupboard for 6 months. Hence highlighting the problem with anecdotal evidence. It can go either way. The only thing lost is the truth about how likely the issue is too occur. Anecdotal evidence may be small part of the truth, but it's along way from the whole truth.

That is not my experience with my two DA55SDM's. One has to use them constantly, leaving this lens unused makes it worse, which is actually common and accepted knowledge in the Pentax community in Germany/Switzerland/Austria (Pentaxians Forum Germany)

[UncleVanya]

I don't understand it like this.
When I write that some SDM lenses can't be converted to screwdrive, and that I hate that limitation, I am quite clear about it.
I mean "some SDM only" lenses and that means no screwdrive mechanism.

Where is this error you speak about?

What is "the in buying"?

All SDM lenses are AF lenses with an SDM motor, and yes, an SDM motor is driven by DC voltage.
Any SDM motor can fail, but all SDM lenses except one which have the screwdrive mechanism built in but not activated can be converted to screwdrive.
Makes them faster as well but louder.

That is not my experience with my two DA55SDM's. One has to use them constantly, leaving this lens unused makes it worse, which is actually common and accepted knowledge in the Pentax community in Germany/Switzerland/Austria (Pentaxians Forum Germany)

1) Pentax uses the sdm label on several different designs of motors some of which are reported as less failure prone.

2) my da 55 doesn't care if I leave it unloved, it just works every time. Statistical data is lacking, single sample data is hard to draw anything from, and complaints are lodged more than "no problem" reports.

[normhead]

When I write that some SDM lenses can't be converted to screwdrive, and that I hate that limitation, I am quite clear about it.

No one, absolutely no one beyond Pentax still makes screw drive lenses. Maybe you should be happy with what they provide.
I guess you hate Pentax because all their lenses can't be converted to screw drive, but bought in because some of their lenses can be converted to screw drive as opposed to none for other manufacturers?

Any SDM motor can fail, but all SDM lenses except one which have the screwdrive mechanism built in but not activated can be converted to screwdrive.
Makes them faster as well but louder.

Correction, any electric motor will eventually fail. (And cheaper screw drive motors strip their gears eventually.) Producers give you a great option, replacement motors. The worst offender, the DA* 16-50 already has it's replacement on the road map.
Your comments ignore the trends in the camera industry over the last 10 years. Who besides Pentax even support screw drive on all their camera bodies? Not one. You can be grateful for what you have, or you complain because the company that still champions your cause isn't up to your standards. Your choice.

[UncleVanya]

I will also point out that "faster" with screw drive is not entirely proven. With good working SDM (not aging and failing) I have not seen a major difference in speed. Note also that SDM and Screwdrive both are different speeds depending on which system LiveView (CDAF) or Viewfinder (PDAF) of focusing is used and the camera itself also matters.

As an example, on my K100D Super both SDM and screwdrive lenses are much slower focusing than on my K-3. On my own I have tested several convertible SDM lenses that are working well and I have not seen a drop in speed in either focusing system, but they do function differently. My experience seems to show they are similar in speed but in some circumstances (lighting, subject, etc) you will see differences but these are statistically hard to quantify.

[normhead]

you will see differences but these are statistically hard to quantify.

Results change with circumstances. Conclusive research would be expensive. Auto-focus isn't necessary to good photography. Neither is knowing exactly what lens would be faster in given circumstance. There is no knowledge out there that saves you from putting the time into getting to know a lens.

I marvel at how fast the AF is on my 55-300 PLM, until nights like last night, when the light levels were so low it wouldn't focus at all. My DFA 100 macro 2.8 would have been better. Yet in good light the DA 55-300 will clean the 100 macro's screw drive clock. There are many similar scenarios where a simple raw speed comparison makes no sense.

[UncleVanya]

Results change with circumstances. Conclusive research would be expensive. Auto-focus isn't necessary to good photography. Neither is knowing exactly what lens would be faster in given circumstance. There is no knowledge out there that saves you from putting the time into getting to know a lens.

I marvel at how fast the AF is on my 55-300 PLM, until nights like last night, when the light levels were so low it wouldn't focus at all. My DFA 100 macro 2.8 would have been better. Yet in good light the DA 55-300 will clean the 100 macro's screw drive clock. There are many similar scenarios where a simple raw speed comparison makes no sense.

No disagreement here. Good points sir.

[leekil]

The DA55 seems to fail less than other SDM lenses, based on what I have heard anecdotally; but maybe less people have the DA55 than some of the other lenses. But that doesn't really help anyone who has a lens that has failed.

It doesn't seems like it is an electrical problem, though; more of a mechanical one, since the lenses will stop working, but you can fix it temporarily again with enough effort. Until maybe it gets past the point of no return.

---------- Post added 08-21-20 at 03:09 PM ----------

Your comments ignore the trends in the camera industry over the last 10 years. Who besides Pentax even support screw drive on all their camera bodies? Not one. You can be grateful for what you have, or you complain because the company that still champions your cause isn't up to your standards. Your choice.

You're focusing on the screw drive, but his real complaint is that the SDM seems pretty unreliable. I mean, there are many more complaints about SDM failing than screw drive failing, even though screw drive also has the possibility of failure.