[BigMackCam]

I noticed today that the rather long stop-down pin on my Jupiter 21m has gouged a groove into the edge of both SDM contacts on my K5. They still work fine and i'm not upset as this is my backup / experimenting body, but I'll need to modify / disable that pin before I fit the lens to my K3 or any future bodies. It also makes me a little wary of fitting any other auto M42 lenses to my cameras without carrying out a similar precautionary mod first.

Anyone else experienced this with the same or different lenses?

[Lowell Goudge]

There is an m42 adaptor that has an inner lip to depress this pin.

Check the b&h website

[BigMackCam]

There is an m42 adaptor that has an inner lip to depress this pin.

Check the b&h website

Thanks I actually already have a Kipon adapter with that inner lip (which I bought so I could use auto-only M42 lenses as manual), but I've generally been using a high quality Pentax-style adapter from SRB on the basis that Pentax should have got the design right Oh well, I guess I can forgive them - they can't have thought of every single lens when designing the adapter

However, I'll be using the Kipon going forward until I can perform the pin mods to the relevant lenses...

It does make me think, though, that all existing / future M42 fans ought to know that this is a possible problem (maybe they already do and I'm just slower than usual!!)

[stevebrot]

Anyone else experienced this with the same or different lenses?

The potential for some pins to contact the power zoom/SDM contacts was discussed in another thread several months back. Surprisingly, length is not the main issue.* It is the pin thickness and/or how how far out from center it is placed. Also a factor is how the mount threads are cut. Even relatively short pins will overlap the contacts. Blame for this one rests firmly on the heads of Pentax engineers who spec'd the original power zoom bodies. There were plenty of users mounting adapted lenses and use of adapted lenses should have been part of the design spec.

The interference may be easily visualized by anyone owning a Tamron Adaptall M42 mount. Pair it with an M42 adapter and mount the combination. The clearance is tight at best. Owners of Pentax film cameras with power zoom support may be able to check the clearance from the back with the shutter open on "B".**

Some M42 lenses will retract the pin when the A/M switch is in the M position, so it may be best practice to move that switch to the M position regardless of the lens mounted.


Steve

* The pin must be a certain minimum depth in order to engage the body actuator striker plate. On most of my lenses, the pin tip is about 8mm deep from the mount face.
** Don't know if the shutter will open on those cameras with the film door open.

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Last edited by stevebrot; 08-25-2015 at 05:21 PM.

[BigMackCam]

The potential for some pins to contact the power zoom/SDM contacts was discussed in another thread several months back. Surprisingly, length is not the main issue.* It is the pin thickness and/or how how far out from center it is placed. Also a factor is how the mount threads are cut. Even relatively short pins will overlap the contacts. Blame for this one rests firmly on the heads of Pentax engineers who spec'd the original power zoom bodies. There were plenty of users mounting adapted lenses and use of adapted lenses should have been part of the design spec.

Thanks, Steve - most helpful.

I assume the radial position of the pin dictates potential fouling of the SDM contacts, while the thickness would additionally dictate possible shorting of them?

Let me add to this the spring strength and finishing / end profile of the pin. A pin that is lightly sprung and / or has a smooth, rounded end (like the SDM contacts themselves) should slide easily over the contacts during rotation of the lens while fitting and removing it. A strongly sprung and / or sharp / flat-ended pin (my Jupiter 21M is both sharp-edged and flat-ended - like a pencil lead that has been cleanly snapped off) is more likely to scratch and damage the contacts as it passes over them (which is what appears to have happened to me!).

Again, I'm left wondering if use of the genuine Pentax M42 adapter (and clones) is too risky, and we might be better served using an alternative adapter with a lip, such as the Kipon I mentioned...

Just for info, see below a crop of an image I took earlier to demonstrate something else... You can clearly see how badly-grooved the SDM contacts are as a result of the stop-down pin scoring them as the lens is fitted / removed...

[stevebrot]

I assume the radial position of the pin dictates potential fouling of the SDM contacts, while the thickness would additionally dictate possible shorting of them?

I guess so, though there normally should not be voltage across the pins.

Again, I'm left wondering if use of the genuine Pentax M42 adapter (and clones) is too risky, and we might be better served using an alternative adapter with a lip, such as the Kipon I mentioned...

The Kipon-type would be nice, though they are notoriously hard to find. I would also wonder how deep the Kipon lip extends and whether it has potential to run afoul of the contacts as well.

I just mounted my Adaptall-2 M42 adapter (sans lens) to my K-3 and can confirm that the pin on that mount (8mm from mount face) overlaps the power zoom pins by at least 1mm and ever so slightly contacts both pins as it passes by. It makes a slight clicking sound. I will try and get a photo with my phone cam to post.


Steve

[K-Three]

high quality Pentax-style adapter from SRB on the basis that Pentax should have got the design right Oh well, I guess I can forgive them - they can't have thought of every single lens when designing the adapter

In Pentax's defense,
The M42-> K adaptor was designed before any power/comm contacts were envisioned in the mount. The original Pentax adaptors were designed in the 1970s with the introduction of the K mount, and I don't believe have been changed at all over the years.

[BigMackCam]

I guess so, though there normally should not be voltage across the pins.

The Kipon-type would be nice, though they are notoriously hard to find. I would also wonder how deep the Kipon lip extends and whether it has potential to run afoul of the contacts as well.

I just mounted my Adaptall-2 M42 adapter (sans lens) to my K-3 and can confirm that the pin on that mount (8mm from mount face) overlaps the power zoom pins by at least 1mm and ever so slightly contacts both pins as it passes by. It makes a slight clicking sound. I will try and get a photo with my phone cam to post.

I got my Kipon via Amazon UK - wasn't cheap, about GBP 25 as I recall. I was disappointed with it initially, as I'd bought it to allow use of auto M42 lenses as manual, but the lip is too deep - the few auto-only lenses I have won't stop down fully with this adapter. However, as a protective measure for the SDM contacts, it makes good sense. That said - looking at the camera with that adapter fitted, it's difficult to make out whether-or-not it also fouls the contacts. I'll check it out with a multimeter tomorrow. Time for bed now

[stevebrot]

Ok...here is what it looks like with 8mm depth from the mount face:




As you can see, there is some wear on the inward surface of the power pins, possibly from another lens or as a manufacturing mark. The slight ledge is detectable by running a fingernail over the surface. I don't know what made the scratches to lower right of the pins.


Steve

[stevebrot]

n Pentax's defense,
The M42-> K adaptor was designed before any power/comm contacts were envisioned in the mount. The original Pentax adaptors were designed in the 1970s with the introduction of the K mount, and I don't believe have been changed at all over the years.

I agree. This is not an adapter design issue or an adapter quality control issue. The adapter works fine. There is nothing about the adapter design that contributes to or causes this issue. It is a camera body issue. The power contacts were added to the mirror box 15 years after the adapter was designed. A slightly different placement of the power pins in the mirror box and the issue would not exist. Remember the pin "descends" into place if the adapter is inserted first. Even a 20 degree offset counterclockwise would likely provide adequate clearance with most lenses. The interference is the result of design error in 1991, not 1976.


Steve

[stevebrot]

It does make me think, though, that all existing / future M42 fans ought to know that this is a possible problem (maybe they already do and I'm just slower than usual!!)

I will see what I can do to write up a fresh thread as the basis for a potential sticky.


Steve

[BigMackCam]

I just mounted my Adaptall-2 M42 adapter (sans lens) to my K-3 and can confirm that the pin on that mount (8mm from mount face) overlaps the power zoom pins by at least 1mm and ever so slightly contacts both pins as it passes by. It makes a slight clicking sound. I will try and get a photo with my phone cam to post.

Yes, your picture demonstrates it perfectly. I also get the faint clicking sound with a couple of lenses. In fact, when I initially fitted the Jupiter 21M, it got stuck until I gently moved it back and forth to release it, and I now wonder if that was as a result of the sharp, flat-ended stop down pin fouling the SDM contacts.

The Kipon-type would be nice, though they are notoriously hard to find. I would also wonder how deep the Kipon lip extends and whether it has potential to run afoul of the contacts as well.

OK, so I had a look to see if the Kipon adapter touches the SDM contacts, and it does (see first pic below). However, while I can't be 100% sure of this, it appears that the part that is touching the contacts is a chamfer (second pic). Regardless, there is *much* less risk of damage to the contacts from this smooth ring as opposed to the sharp edge of the stop down pin.

My Jupiter 21M and Helios 44M seem to have the poorest finish on the stop down pins. By contrast, the pin on my old Sun Systemat 35mm has a very smooth, rounded end, much like the SDM contacts themselves, so that is less likely to scratch / gouge as it passes over them.

I will see what I can do to write up a fresh thread as the basis for a potential sticky.

Great I think this is essential knowledge for anyone experimenting with M42 lenses. The potential for damaging both camera and lens is clearly there. Let me know if I can help?

[Not a Number]

The contacts are spring load so the adapter - with smooth edges - should not cause a problem. You could always take the pin out and round the end off, perhaps shortening it just a touch. Or leave it off altogether - if you don't have and don't plan to have any screwmount cameras what difference does it make?

Looking in the mirror box of my MZ-50 I don't see much other places where the contacts could be located. 6 o'clock to 9 o'clock needs to be clear for the auto diaphragm stop-down lever. And anywhere else might interfere with the mirror or aperture control linkage. The alternatives would be to put the contacts further in so they don't stick out so much, put them on mount ring as with the "A" and data pins or on the side of the box like Nikon did.

Well Asahi/Pentax didn't so as noted before we're stuck with the design decision.

[stevebrot]

The contacts are spring load so the adapter - with smooth edges - should not cause a problem.

Agreed, unless by some strange accident the pins go "hot" (turned on).


Steve

[stevebrot]

The alternatives would be to put the contacts further in so they don't stick out so much, put them on mount ring as with the "A" and data pins or on the side of the box like Niion did.

...or put the pin part on the lens and the flat part in the mount.


Steve