[pathdoc]

I have my authentic Pentax M42 screw-K adapter and my K5 and I'm ready to go. I found a nice little lens, and then I saw the back view with a protruding object.



Here is the view of the lens, and on the rear surface can clearly be seen a long, silver protuberance.



What is it, and what (if anything) do I need to do about it? I should add that I need this camera for work from time to time, so I'd really like NOT to have the lens stuck in place and need to be sent off for removal!!

[BigMackCam]

Hi - that's the aperture control for the lens. On a K-mount camera with adapter, it serves no purpose. I have a Helios 44m 2-58 with this same pin on my K5

---------- Post added 04-15-2015 at 01:48 PM ----------

Just to reassure yourself before fitting it, set the aperture to something like f16 on the lens and look through it, then push the pin in and out. You should see it open and close...

[Adam]

That's the aperture stop down pin. On genuine lenses it isn't a problem, so I wouldn't expect it to get in the way on that lens either.

[pathdoc]

OK, just making sure. Thanks, all.

[Spodeworld]

There is one on every M42 I own. Not a problem.

[sterretje]

On genuine lenses it isn't a problem, so I wouldn't expect it to get in the way on that lens either.

I have a SuperTak300/4 and I have a very strong suspicion that the pin touches the SDM/PZ contacts in the body at every turn (once the lens is a little 'in') while I screw the lens in. I haven't managed to bent those contacts yet, but something is definitely touching something at every turn.

I'm using the original Pentax adapter.

What is it, and what (if anything) do I need to do about it?

Ricoh pin only applies to K-mount, not to M42.

[pathdoc]

Ricoh pin only applies to K-mount, not to M42.

A valuable thing to know - thank you!!

As a side note, my adapter has "Pentax Japan" on it, rather than "ASAHI Pentax", so I'm assuming it's a later version made after they stopped putting "Asahi" on everything.

[tvdtvdtvd]

I have a very strong suspicion that the pin touches the SDM/PZ contacts in the body at every turn (once the lens is a little 'in') while I screw the lens in.

I've noticed that as well, with some lenses generating more resistance, (a more noticeable contact), than other lenses.
You're comment got me curious so I took a closer look.

The stop down pin and SDM contacts are indeed at about the same radius from the center of the mount, ( ~ 1.9mm from
center). The amount of resistance appears to be related to the amount of tension on the stop down pin, which varies
from lens to lens, (strength of the spring pushing the pin, whether pin is dirty or gummed up, etc).

For lenses with an Auto/Man switch, it's best to set the lens to 'Manual', as this takes the pressure off the spring.
For fully automatic lenses, it's best to set the aperture to minimum(smallest f-stop), as this minimizes the pressure
on the spring.

And now I feel the need to clean the stop down pin on all my M42 lenses to ensure they move as easily as possible.

---------- Post added 04-15-2015 at 10:40 AM ----------

As a side note, my adapter has "Pentax Japan" on it, rather than "ASAHI Pentax", so I'm assuming it's a later version made after they stopped putting "Asahi" on everything.

My adapter also says "Pentax Japan". It came in a green plastic sleeve with instruction booklet and a price
sticker dated Oct. 1982. Curiously, the instruction booklet includes images of the adapter that clearly read
"Asahi Pentax".

[BigMackCam]

That's interesting regarding it touching those two contacts. I wondered if that might happen, but I don't notice any change in resistance as I turn the lens when mounting it.

Re mounts, I have two. The first is a cheap no-brand infinity focus variety, from the 'bay, and it is poorly finished, plus the threading has been cut such that my Helios mounts with the the red line about 90 degrees round from the top (where it should be). I wouldn't recommend using one of these without filing the edges smooth first, but otherwise, it is functional. The second is from SRB Photographic in Dunstable. It's the same style, but properly finished, so it fits and removes very easily, and the threading is perfect. It's much cheaper than the original Pentax accessory. Highly recommended.

[sterretje]

That's interesting regarding it touching those two contacts. I wondered if that might happen, but I don't notice any change in resistance as I turn the lens when mounting it.

The SuperTak 300/4 is the only M42 lens that I have where it happens; other ones seem to be fine.

[BigMackCam]

The SuperTak 300/4 is the only M42 lens that I have where it happens; other ones seem to be fine.

Gotcha. Definitely something to watch out for, though. Thanks for the heads-up.

[tvdtvdtvd]

The SuperTak 300/4 is the only M42 lens that I have where it happens; other ones seem to be fine.

You are indeed correct. There does seem to be some variance in the relative length of the
stop down pin from lens to lens. Most of my genuine Taks screw on without any noticeable contact,
but many third party lenses do make contact.

I took some measurements. On the 85/1.9 & 55/2 Super Taks, the stop down pin protrudes 8mm
from the base plate of the lens, (not the base of screwmount). On my Sears Auto 28mm the pin
protrudes just a hair over 8mm. On my Helios 44-4M, the pin protrudes 8.5mm. On the camera,
the SDM contacts are about 8mm below the base of the lens mount.

However, there appears to be more at play than just the stop down pin. My Helios 44-4M hits several
points of resistance on the last couple turns when being mounted. There's enough resistance to even
give the impression that the lens is stuck, but a little extra force will push the lens past the resistance.
Pressing the bayonet release will immediately eliminate the resistance. It appears the AF screw drive
and/or the K-mount locking pin are catching on the base plate screws of the Helios. With the bayonet
release on the camera pressed in, I can screw on the Helios without that sensation that the lens has
jammed. However I can still hear a series of soft clicks just as the lens is coming around it's final pass,
which I believe is the baseplate screws rubbing over the electrical contacts of the lens mount.

I honestly can't tell whether the stop down pin on the Helios is hitting the SDM contacts. If it is, the
resistance felt is minor compared to the contact made by the base plate screws with the screw
drive and electrical contacts.

[stevebrot]

but something is definitely touching something at every turn.

Push in on the K-mount release while screwing on/off and see if that makes a difference. I am not familiar with the flange face on your lens, but I have several M42 lenses that have screw holes on the flange that contact the AF drive pawl and/or the K-mount retaining pin.


Steve

[stevebrot]

However, there appears to be more at play than just the stop down pin. My Helios 44-4M hits several
points of resistance on the last couple turns when being mounted.

This talk got me curious. I have several M42 lenses including a couple with longer pins and don't remember any indication of contact. So, I did a quick survey and pulled the two lenses with the pins having the deepest penetration (Helios 44-M and Vivitar 200/3.5) and mounted them to my K-3. I usually mount adapter to lens first and sure enough, with the Helios I could hear a faint little click-click as the pin passed the two power contacts! At least I think I did.

I repeated and heard nothing

I looked at the pins of my favorite M42 lenses and none had any hard evidence of having done battle with the power pins. The power pins themselves were unscratched. This calls for drastic action!

Time to get access to a Pentax AF film camera having the power contacts so that I can watch from the rear as the lens is mounted.


Steve

[stevebrot]

Time to get access to a Pentax AF film camera having the power contacts so that I can watch from the rear as the lens is mounted.

I remembered that I have an M42 Adaptall-2 mount. With no lens attached, I should be able to have a good view into the mirror box to visually check the clearance. I measured the pin depth at about 8mm and saw that sure enough there was at least a millimeter of overlap between the adapter stop-down pin and the power pins. The adapter pin was set inboard just enough to clear, but the amount of clearance was very small.

Conclusion?

• Most auto-aperture M42 lens have pins that are long enough to overlap the power pins of the KAF2 and KAF2 (crippled) mounts.
• The position of the pin inboard of the near edge of the threads is the strongest indication of risk. My Tamron adapter's pin is about 1mm in from the edge. The same is true for my other M42 lenses.
• For lens where the pin retracts when the A/M switch is in the M position, it would seem prudent to flip the switch before mounting or removing the lens
• Risk may be somewhat less when mounting the adapter to the camera and then screwing the lens on due to the pin depth being somewhat less, though the amount of difference is only about 1/8 mm at best
• Caution would be in order when first using a lens you are unfamiliar with

Steve