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09-12-2019, 04:01 PM   #1
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Spotmatic F -- S.M.C. and Super Takumars reading differently by two stops

I have a Spotmatic F that meters perfectly when I put on an old Super Takumar (I use the stop down switch to meter). I have tested against an evenly lit wall against a known good spotmeter. When I put an S.M.C. (aperture coupling) lens on, it reads two stops faster (2 stops over exposed). I am not using the stop down switch with the S.M.C. Anyone else familiar with this issue? I am contemplating sending it in to Eric, but hesitating because I only have one S.M.C. lens anyhow, and my main shooter is a beater black Spotty. This one is so minty I might not want to throw it in my bag and have it travel with me everywhere.

Suggestions??

Thanks,
Chris Maness

09-13-2019, 04:37 PM   #2
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My memory is a little vague, but I recall that the Spotmatic II manual mentions to always have the meter switch in the off position when mounting or unmounting Super Takumar or SMC Takumar lenses, if not the tip of theautomatic diaphragm release pin on the lens will hit the pin release plate inside the camera body and it may get damaged. Perhaps this might explain the condition?
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09-13-2019, 05:44 PM   #3
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The danger there is that the pin that protrudes from the lens to close the diaphragm when the camera fires could get bent. The pin is fine, so that is not the issue.

Thanks,
Chris
09-13-2019, 06:19 PM   #4
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QuoteOriginally posted by kq6up Quote
I have tested against an evenly lit wall against a known good spotmeter. When I put an S.M.C. (aperture coupling) lens on, it reads two stops faster (2 stops over exposed). I am not using the stop down switch with the S.M.C.
Welcome to the Pentax Forums!

Do you have the Spotmatic F manual? If not, a free copy is available from:
Pentax Manuals | M42 Screwfit Manuals (PDF password = "Pentax")
When using an SMC or S-M-C Takumar on the Spotmatic F, it is important that the Auto/Manual switch on the side of the lens be in the "Auto" position. It is also important that the switch not be moved unless the rear lens cap is attached and never when mounted to the Spotmatic F. There is a small interlock pin (not the same as the actuator pin) on the lens flange face that enforces the rule and attempts to force the lever may break the aperture mechanism. The meter/preview switch on the camera should remain in the down (off) position.

That little bit out of the way, the meter couplings on the Spotmatic F are complex and it is possible the camera might need adjustment for accurate open-aperture readings.


Steve

09-14-2019, 07:48 PM   #5
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QuoteOriginally posted by stevebrot Quote
Welcome to the Pentax Forums!

Do you have the Spotmatic F manual? If not, a free copy is available from:
Pentax Manuals | M42 Screwfit Manuals (PDF password = "Pentax")
When using an SMC or S-M-C Takumar on the Spotmatic F, it is important that the Auto/Manual switch on the side of the lens be in the "Auto" position. It is also important that the switch not be moved unless the rear lens cap is attached and never when mounted to the Spotmatic F. There is a small interlock pin (not the same as the actuator pin) on the lens flange face that enforces the rule and attempts to force the lever may break the aperture mechanism. The meter/preview switch on the camera should remain in the down (off) position.

That little bit out of the way, the meter couplings on the Spotmatic F are complex and it is possible the camera might need adjustment for accurate open-aperture readings.


Steve
Yes, switch is in auto on both the coupled and non-coupled lens. Stop down switch is up for the non-coupled lens, and down for the coupled (S.M.C.) lens per the manual. Metering should agree according to the manual. I am aware of the interlock that keeps the switch in auto for the S.M.C. lens, and I never force things on cameras -- that usually results in some type of minor catastrophe in my experience. I was thinking that something is most likely out of whack, but I want to make sure it was not pilot error because the camera is running just fine otherwise. I have had a lot of different cameras CLA'd lately and not sure I want to do that on this one. I am not sure how much I will actually shoot it. A beat up black spotty is my daily shooter. I really like the fact that it is black and brassy (understood this has nothing to do with the actual images).

Thanks,
Chris Maness
09-14-2019, 08:41 PM - 1 Like   #6
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QuoteOriginally posted by kq6up Quote
but I want to make sure it was not pilot error because the camera is running just fine otherwise
It sounds like you have it straight.


Steve
09-15-2019, 07:04 AM - 1 Like   #7
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Most of my Taks don't line-up with the focus index dead centre at the top of the lens when mounted. Is it likely the aperture simulator isn't lined up properly (I'm often amazed open aperture metering ever worked on M42 lenses)?
09-15-2019, 10:57 AM - 1 Like   #8
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QuoteOriginally posted by johnha Quote
Most of my Taks don't line-up with the focus index dead centre at the top of the lens when mounted. Is it likely the aperture simulator isn't lined up properly (I'm often amazed open aperture metering ever worked on M42 lenses)?
There are two "followers" in the mount, one to couple aperture ring position and the other to couple lens alignment. The second engages a boss on the lens rear such that the aperture simulation is always proper to the actual aperture ring position. See the description in the homepage article for details...
Pentax Forums | The Evolution of the Pentax M42 Mount

Steve

09-15-2019, 11:01 AM   #9
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@kq6up The Pentax Manuals Web site has a service sheet for the Spotmatic F meter that details adjustments for both open-aperture and stop-down metering and also includes helpful diagrams.
Pentax Manuals | Service Manuals (PDF password = "Pentax")
Note that the SP F meter service sheet also includes instructions and diagrams for shutter control on the Spotmatic ES that is not pertinent to the SP F.


Steve

Last edited by stevebrot; 09-15-2019 at 11:13 AM.
09-15-2019, 06:53 PM   #10
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QuoteOriginally posted by johnha Quote
Most of my Taks don't line-up with the focus index dead centre at the top of the lens when mounted. Is it likely the aperture simulator isn't lined up properly (I'm often amazed open aperture metering ever worked on M42 lenses)?
This is kind of what I was thinking the issue might be. Also if it is worth messing with since the camera functions well otherwise. However, it just becomes another spotty without that open aperture metering feature. The only other "features" is a bigger batter that doesn't need an o-ring, and the shutter release lock (actually fairly handy). However, it is missing the cool black paint and brassing that my daily shooter has

---------- Post added 09-15-19 at 06:55 PM ----------

QuoteOriginally posted by stevebrot Quote
There are two "followers" in the mount, one to couple aperture ring position and the other to couple lens alignment. The second engages a boss on the lens rear such that the aperture simulation is always proper to the actual aperture ring position. See the description in the homepage article for details...
Pentax Forums | The Evolution of the Pentax M42 Mount

Steve

That is actually pretty genius. I was wondering how the made the indexing consistent on a screw mount lens given that a different amount of torque will find the lens resting at a different place.
09-15-2019, 09:56 PM   #11
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QuoteOriginally posted by kq6up Quote
That is actually pretty genius. I was wondering how the made the indexing consistent on a screw mount lens given that a different amount of torque will find the lens resting at a different place.
It is very genius! I had not actually seen the ES/SF camera mount before doing research for the article and it was only on close inspection while trying to figure out purpose of the boss on the lens that I realized the elegant design in the body itself. BTW...even with the same torque, very rare is the M42 lens that lines up at 12:00 even on the same-brand body.

Note: It just occurred to me that being off two stops might be caused by the boss on the lens not engaging the coupler. You should be able to confirm by opening the shutter using the "B" setting and looking into the mirror box from the backside.


Steve
09-16-2019, 01:02 AM   #12
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Does anybody know if any of the Tamron adaptall m42 mounts copy this behaviour? IIRC there's a separate 'Pentax ES' or similar version?
09-16-2019, 09:17 AM   #13
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QuoteOriginally posted by johnha Quote
Does anybody know if any of the Tamron adaptall m42 mounts copy this behaviour? IIRC there's a separate 'Pentax ES' or similar version?
There is a P/ES adapter and a few PF members have experience with it. I have seen a copy, but don't remember if it has a boss to engage the second coupler.

Adaptall-2 Mount for Pentax-ES - PentaxForums.com


Steve
09-17-2019, 10:44 AM   #14
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QuoteOriginally posted by stevebrot Quote
It is very genius! I had not actually seen the ES/SF camera mount before doing research for the article and it was only on close inspection while trying to figure out purpose of the boss on the lens that I realized the elegant design in the body itself. BTW...even with the same torque, very rare is the M42 lens that lines up at 12:00 even on the same-brand body.

Note: It just occurred to me that being off two stops might be caused by the boss on the lens not engaging the coupler. You should be able to confirm by opening the shutter using the "B" setting and looking into the mirror box from the backside.


Steve
The boss seems to be engaging the coupler just fine. I am curious -- would this be a mechanical adjustment, or a potentiometer adjustment? I just won another miss listed Spotmatic F on the bay, $20 shipped. Hopefully its meter is accurate works. If not, I have another camera to loan students and tell them to use a light meter app.
09-17-2019, 11:28 AM   #15
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QuoteOriginally posted by kq6up Quote
I am curious -- would this be a mechanical adjustment, or a potentiometer adjustment?
Below is a link to a page that has the meter service sheet for the Spotmatic F. I commented it on in a post above, but here it is again.

Pentax Manuals | Service Manuals (PDF password = "Pentax")

It says to adjust one of two sets of potentiometers. I don't know if the mechanical offset is adjustable as well. The manuals for the SPII and K1000 (also on that page) may be helpful as well, particularly since the K1000 meter circuit is similar to that of the SP-F for open-aperture metering.


Steve
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