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05-23-2017, 02:25 PM   #1
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M42 lenses with f stop communication to body??

I've recently got a Spotmatic F and a f 1.4 50mm. Happily the 50mm can communicate the set f stop to the body so full-aperture metering is possible. I'd like to acquire a 28mm and probably a 135mm. How does one tell which of the M 42 Pentax lenses can "talk" to the body in similar fashion?

05-23-2017, 02:44 PM   #2
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The SMC Taks that can do open aperture metering have a small square lug on the back of the aperture ring that tells the body what the aperture is.
05-23-2017, 03:10 PM   #3
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QuoteOriginally posted by grhazelton Quote
I've recently got a Spotmatic F and a f 1.4 50mm. Happily the 50mm can communicate the set f stop to the body so full-aperture metering is possible. I'd like to acquire a 28mm and probably a 135mm. How does one tell which of the M 42 Pentax lenses can "talk" to the body in similar fashion?
I think most, if not all such lenses are SMC or S-M-C Takumars. They also have newer styling for their focusing ring. The spec tables in our lens database indicate the type of metering available:

Pentax M42 Lenses | PentaxForums.com
Pentax M42 Lenses | PentaxForums.com

Click on "specs" and look for open aperture metering in the second row of the table.

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05-23-2017, 04:13 PM   #4
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QuoteOriginally posted by microlight Quote
The SMC Taks that can do open aperture metering have a small square lug on the back of the aperture ring that tells the body what the aperture is.
Thanks! However most listings don't give a clear view of the rear of the lens.

---------- Post added 05-23-17 at 07:22 PM ----------

QuoteOriginally posted by Adam Quote
I think most, if not all such lenses are SMC or S-M-C Takumars. They also have newer styling for their focusing ring. The spec tables in our lens database indicate the type of metering available:

Pentax M42 Lenses | PentaxForums.com
Pentax M42 Lenses | PentaxForums.com

Click on "specs" and look for open aperture metering in the second row of the table.
Thanks Adam! However...how do I tell which lenses are full aperture metering in a listing at KEH or Adorama or B&H? The newer focus ring is a good clue, but listings with the mount shown would be more informative. Many of the SMC or S-M-C lenses listed in the tables don't mention full aperture metering; my particular 1.4 cites full aperture only in a note field. Can I assume that lenses listed as Electro "compatible" are good to go?

05-23-2017, 04:32 PM   #5
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I think the only SMC or Super Multi Coated Takumar lenses which don't have coupling are either zooms or rare and unusual (e.g. very long) primes. Any SMC or Super Multi Coated 28mm or 135mm Takumar will have the coupling.

Remember that KEH, B&H and Adorama are reputable stores which should be both willing and able to check these details out for you, perhaps even send you photos of the rear mount face. And if they get it wrong, they should be the sort of seller that will accept a return without too many troubles.
05-23-2017, 05:21 PM   #6
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QuoteOriginally posted by pathdoc Quote
I think the only SMC or Super Multi Coated Takumar lenses which don't have coupling are either zooms or rare and unusual (e.g. very long) primes. Any SMC or Super Multi Coated 28mm or 135mm Takumar will have the coupling.

Remember that KEH, B&H and Adorama are reputable stores which should be both willing and able to check these details out for you, perhaps even send you photos of the rear mount face. And if they get it wrong, they should be the sort of seller that will accept a return without too many troubles.
Many thanks! Would be nice if there were a database by serial number which would inform us. I've dealt in the past with KEH, Adorama, and B&H for both new and used items with general satisfaction. While these good folks and many others will accept returns, it does make life somewhat more difficult....
05-23-2017, 06:55 PM   #7
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QuoteOriginally posted by grhazelton Quote
Thanks! However most listings don't give a clear view of the rear of the lens.

---------- Post added 05-23-17 at 07:22 PM ----------



Thanks Adam! However...how do I tell which lenses are full aperture metering in a listing at KEH or Adorama or B&H? The newer focus ring is a good clue, but listings with the mount shown would be more informative. Many of the SMC or S-M-C lenses listed in the tables don't mention full aperture metering; my particular 1.4 cites full aperture only in a note field. Can I assume that lenses listed as Electro "compatible" are good to go?
The lens name and a photo of the lens are the best ways to tell if you can't get a photo of the back. After taking a closer look, it seems all SMC takumars have open aperture metering, some Super-Multi-Coated takumars have it, and other takumars don't. So IDing the lens model will be key. As far as I know all variants with a different aperture system have a different name too.

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05-23-2017, 09:19 PM   #8
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I'm pretty sure all auto diaphragm Super-Multi-Coated Takumars except the 85-205 zoom. Tamron made an Adaptall 2 mount "for Pentax ES" that will work on Adaptall lenses. I have a Tamron 70-210 SP 3.5 that I use with that adapter.

05-24-2017, 02:37 AM   #9
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To complicate matters, there were some late model Super Takumars that also had it, but this probably represents parts changeovers at the end of the Super Tak run rather than a design feature. I wouldn't buy one of those for open-aperture metering without actually having the lens in hand to mount on an SPF/ES/ES-II and test it.
05-24-2017, 04:13 AM   #10
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QuoteOriginally posted by microlight Quote
The SMC Taks that can do open aperture metering have a small square lug on the back of the aperture ring that tells the body what the aperture is.
Thee is also a very small pin, on the back of the lens, that is interlocked with the auto/manual aperture switch.

inam not sure how the lenses index with the body to ensure correct aperture reporting, or whether it matters, since TTL metering only needs to know the change in aperture, not the absolute value.
05-24-2017, 09:06 AM   #11
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Insofar as I know, the basic principle stayed unchanged between last-generation M42 and the K-mount. The square lug is there to slap up against a stop in the camera body and keep the SMC lenses in angular register so that the aperture reporting arm on the lens matches up with its receiving arm, which I think is near top dead centre in the M42 mount (it's on the left for K-mount). It then signals the number of stops away from full open, so the meter reads through the full open lens and then applies the correction reported by the aperture ring.

The Nikon system is somewhat different - I don't own any of the pre-AI bodies, but that system seems to more explicitly report the actual aperture. I'm given to understand that their retro Df can actually make use of this to handle pre-AI lenses as if they were modern top-of-the-line, and if I had been a Nikon shooter instead of Pentax that probably would have been my pathway to full frame.

Fujica's system near the end of their M42 run used a similar arrangement on the outside of the lens mount; a small tab on the lens (routinely filed off by M42 shooters because it interferes with infinity focus on other cameras) catches a knob on a rotating ring. This turns through about a sixty degree arc on the left hand side of the mount, but I don't know whether it reports the aperture in explicit or relative terms or I would be very tempted to create some sort of lashup on a Takumar lens that would engage the camera-side register and give me open-aperture metering of Takumar lenses on the Fujica. Alas, I suspect the stops per degree of arc are different even if it's still a relative system, and that would significantly mess the exposure up.

Last edited by pathdoc; 05-24-2017 at 09:11 AM.
05-24-2017, 09:13 AM   #12
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You're exactly right, Pathdoc. The aperture follower on the camera body is a variable resistor that adjusts the meter to the number of stops down from wide open that the lens is set to, while the stop-down pin holds the lens open for focusing.

Last edited by Dartmoor Dave; 05-24-2017 at 09:36 AM. Reason: Transposed "number" and "of".
05-28-2017, 10:50 AM   #13
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Many thanks for all the responses! The Spotmatic F with the 1.4 is a sweet combination! Would be nice if it had a meter on/off switch. This could have been incorporated into the shutter lock.
05-28-2017, 04:41 PM   #14
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QuoteOriginally posted by grhazelton Quote
Many thanks for all the responses! The Spotmatic F with the 1.4 is a sweet combination! Would be nice if it had a meter on/off switch. This could have been incorporated into the shutter lock.
You're welcome, and amen to that.
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