[marcusBMG]

When I first acquired and tried to use an M42 lens (a helios 44) on my K-r it was all a bit perplexing: all my pics were way underexposed, and the camera didn't work as I expected. I now know that these glitches/problems are inherent to the the use of vintage/legacy lenses on Pentax, see for example:

https://www.pentaxforums.com/forums/131-visitors-center/222413-please-help-kr...en-button.html

The point of this piece is to share some of the experience I have gleaned.
Here is my recently acquired (they're a bit scarce but look around and they can be found, I got this one with a lens) shiny plated M42 t-mount with PK adapter fixed in place*, alongside a standard black PK t-mount, a modded t-mount and a plated brass M42-PK adapter that I am now using on my Vivitar 90mm macro.
*BTW you should remove the little locking spring clip on the adapter when using the PK-M42 adapter like this. I have made a wee notch to accept the k-mount lens locking pin to hold the lens in place see pic below.



In a nutshell, with a non-conducting black anodised mount such as a typical M42 or most t-mounts:

1. Green button (set on Tv shift) no longer stops down (if this needs explaining to you read this ). But testing indicates a shutter speed IS set (note: K10D and K20D, for example - the green button always stops down, and other models may have their own idiosyncracies).
2. There are a few early third party PK mount lenses that are anodised and non-conducting, but these are pretty rare. These would, however, be problematic to use without creating an electrical connection to the contacts on the camera mount. With t-mounts and M42, because there is no aperture lever, and therefore the iris is not held open by the camera, stop down metering in Av is the mode of choice.
3. But exposures are way off - typically requiring high levels of compensation, in fact with longer focal length lenses like my 400mm Tamron Nestar or Tamron Adaptamatic 300mm I am normally maxed out at +3eV.

A lot of us use hacks/work-arounds: foil between the lens and camera mounts to short (specifically) contact 7 at the half past position, or removing the black anodising off the lens mount, as per the modded t-mount in pic.



I use a piece of foil, folded, like this it can be held in position while the lens is tightened in the mount. It doesn't seem to matter if the foil covers several contacts, just as long as the data contact is covered. The helios 44M-4 has a wide base that covers the contacts so this works well. The silver helios 44 however has a narrow M39 mount that doesn't cover the contacts. The "copper wire trick" is an alternative method.



In any case I had been looking for one of these plated t-mounts as a nicer, more convenient and better connecting solution for my t-mount lenses. As you can see the mount only just covers (half) the contacts. This is the comparison between the two t-mounts, using a Jupiter 11A 135mm f4 as a convenient test lens, on my Kr.

mount	mode	f4	f5.6	f8	f11	f16
black	green button	1/320	1/160	1/80	1/50	1/40
black	Av	1/1000	1/500	1/250	1/125	1/80
silver	green button	1/250	1/125	1/80	1/60	1/40
silver	Av	1/250	1/160	1/80	1/50	1/30

You can see the difference:



My M42 mount Vivitar 90mm f2.8 produces similar results when compared mounted using a pentax original style rimless (= non-conducting) adapter and the rimmed plated conducting adapter in the pic above.

mount	mode	f2.8	f4	f5.6	f8	f11	f16
rimless	green button	1/500	1/400	1/200	1/100	1/60	1/125
rimless	Av	1/2000	1/1250	1/800	1/400	1/160	1/100
plated	green button	1/500	1/320	1/200	1/80	1/40	1/20
plated	Av	1/500	1/400	1/200	1/100	1/60	1/30

To summarise:

1. The exposures in M mode, and with a conducting mount in Av, were basically the same.
2. The exposures in Av with a non-conducting mount were approx twice as underexposed.
3. In all modes all the vintage lenses I have used require exposure compensation, normally for underexposure, which is why I prefer Av mode, with an appropriate compensation dialed in based on test shots.

And in conclusion, I can say that if it wasn't for a glitch, using conducting adapters or a foil etc hack would be a no-brainer. That glitch is "mirror flopping". Unfortunately, using these tricks tends to set off a propensity to flop. On my K-r's its not been out of hand, like some of the bad cases documented on threads here. Typically its a routine double mirror up down, the second delayed a few secs, on taking a pic. But its variable, sometimes it hardly flops at all. The viv + plated adapter, for example, seems to work particularly well. Why? Well thats the $64,000 question isn't it!

Some more relevant threads:
https://www.pentaxforums.com/forums/120-general-technical-troubleshooting/230...g-problem.html
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/60513-quest...-m42-lens.html
https://www.pentaxforums.com/forums/120-general-technical-troubleshooting/241...ew-mounts.html
https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/257018-exposure-...0mm-1-7-a.html
https://www.pentaxforums.com/forums/120-general-technical-troubleshooting/230...g-problem.html
https://www.pentaxforums.com/forums/120-general-technical-troubleshooting/247...s-k-500-a.html

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Last edited by marcusBMG; 09-02-2014 at 01:46 PM.

[Not a Number]

Excellent. It would be interesting to run similar tests on all models to get comparison of how each behaves. It would also be of interest to see if firmware version is a factor.

It would also be of interest to see how non-"A" K-mount lenses behave on "rimless" vs conducting teleconverters or extension rings or even the odd 3rd party lens with non-bare metal mount.

[Mistlefoot]

This post is currently absolutely useless to me. But it is posts like this from the community that make this site an incredible resource. I am sure some day I will come back to this post to solve some future problem. Thank you marcusBMG.

[sherrvonne]

Wow, interesting, wish I had learned stuff like that alongside the sewing cooking from scratch! 😀

[marcusBMG]

Thanks for responses. Note my revision re flopping.

[marcusBMG]

UPDATE: the K5 I acquired as an upgrade to the K-r does not suffer from the exposure compensation issues with a black non conducting mount - exposures are the same. However the lack of electrical connection still means that the green button doesn't work, CIF doesn't work.

[RKKS08]

the K5 I acquired as an upgrade to the K-r does not suffer from the exposure issues with a black non conducting mount

My K-5 does.
It works as expected with a SMC-M 1.7/50, but neither green button nor DOF preview work with my Petri 2/28 (which works with both methods with my K200D). The Petri lens mount is NOT "black anodized".

However, after shorting pin 7 with a small piece of tin foil as a test, the K-5 will do. So there must be some kind of isolating surface which cannot be seen.

[marcusBMG]

My K-5 does.

I have edited my previous post to clarify: exposure compensation issues have alleviated.

[marcusBMG]

These are the missing pics. Also see stevebrots article/thread on how stop down exposures can be problematic anyway:

Stop-down metering, why it is a problem, and how to make it work - PentaxForums.com