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10-31-2008, 12:23 AM   #1
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P-TTL & Matrix with k10, m42 and PK lenses

Hello!

I've searched this forum without success about this topic, so I'll share my experience here with you (sorry if this sounds like réchauffé to you!)... I've hesitated putting this in the lens section, but this is more related to the body, IMO.

Following these sites (robertstech and kmp.bdimitrov), I've found that it's indeed possible to activate P-TTL flash and matrix metering with old m42 and PK lenses, contrary to what is said in these sites ("This modification will not work for Pentax Digital cameras because they use the digital data pin on the lens to get aperture information, not the multi-contact array discussed here.")...

The trick is to short the Data pin too, and this works! Guess they did a little step back in their algorithms...

And good news, it's surprisingly easy to use!

For m42 lenses:
- short all the contacts (the A contact is a little bit hard to short due to its recessed position), including the Data pin.
- the body then display an aperture (f/1.2 if all the pins are shorted), the value is not important.
- select the smallest value available (f/1.2 if available).
- Tadaaa! Now you can use PTTL and matrix metering...
- Want to change the aperture? Just use your aperture ring, without changing anything on the body...

For PK lenses:
- short the contacts following the tables available on the sites above (the A contact is a little bit hard to short due to its recessed position), including the Data pin.
- select the same aperture on lens and body...
- Tadaaa! Now you can use PTTL and matrix metering...
- Want to change the aperture? Use your aperture ring, and synchronize Av value on the body.

This is possible due to the shorter course of the PK actuator compared to the PKA actuator, for a given aperture...

The other modes are then available with a PK lens... Just sync the lens with whatever aperture is given by the K10...

I've not done extensive tests to check the accuracy of the matrix mode yet (and the meaning of an accurate aperture with m42 lenses), but PTTL works just fine at all apertures (given you've installed a LL60 focus screen beforehand, or you'll fall back on the non-linear exposure problem ).

But now that I think about it, this is a great solution for PK-owners having exposure problems : this method simply removes altogether the need of the Green Button, at the same time as enabling PTTL and matrix metering!!!

Damn, there must be something rotten lying somewhere, this is too good to be true...


Last edited by dlacouture; 10-31-2008 at 01:13 AM.
10-31-2008, 12:50 AM   #2
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Do you mean that in Av mode with M lenses the aperture will close now as in M mode, if we tell the body the selected aperture?

I would not really be interested in PTTL (a flash with auto setting will do for me, but it could be handy for eg macro flash) or matrix metering (center weighted average will do), but it would be nice to use an M-lens with more reliable/predictable metering than the green button provides with the standard screen.

This could in theory be provided in a firmware upgrade, so there would be no need to fiddle with short-circuiting lens contacts.
We need to tell the camera the max (wide open) aperture of the lens after mounting (like supplying the focal length). The camera meters wide open (which is accurate also with the standard screen) and compensates the exposure because we tell it the selected setting on the aperture ring by selecting the F-stop in Av mode.

But I guess that for marketing reasons Pentax does like it that the old lenses are a bit harder to use than new glass.

Last edited by tomtor; 10-31-2008 at 01:27 AM. Reason: clarification
10-31-2008, 01:11 AM   #3
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Basically, yes, as long as you sync body and lens apertures...
And exposure and flash exposure will be accurate as well...

I'm currently considering whether to short the A pin on the body itself or not, in a more or less definitive way...
This A pin simply doesn't have a meaning anymore...
10-31-2008, 03:43 AM   #4
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QuoteOriginally posted by dlacouture Quote
For PK lenses:
- short the contacts following the tables available on the sites above (the A contact is a little bit hard to short due to its recessed position), including the Data pin.
- select the same aperture on lens and body...
- Tadaaa! Now you can use PTTL and matrix metering...
- Want to change the aperture? Use your aperture ring, and synchronize Av value on the body.

This is possible due to the shorter course of the PK actuator compared to the PKA actuator, for a given aperture...
Are you sure of this. I converted an F2.0 50mm lens from K to KA (I have an F 1.4 so the F2 is redundant)

I did this by modifying the base plate to give the correct apature information to the camera achieved by drilling the mount and filling with epoxy for the insulating locations and additn a spring loaded pin for the "A" contact.

The result was that I could control the apature but not correctly. the activating lever in the body is designed for KA lenses, which have linear displacement of the lever with area, (i.e. linear with f stop) where as on K mount lenses the lever is linear with diameter, or the square of area. The result was that the body and camera did not meter correctly together. over the exposure range of the lens F2-f22 the exposure went from -1 to +2 stops, and then fell back again.

Even setting the apature manually caused problems as the activating lever interfered with lens opening

10-31-2008, 03:59 AM   #5
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Well, I tried with a Porst 55mm f/1.2 at several apertures, and didn't have any problem...

Hummm... Maybe you tried setting the lens to f/22 and controlling it via the body only? (at least, that's how I read your post...)
In this case, yes, you'll have inconsistent results...

What I did was setting desired aperture on both the lens and the body... But I don't understand you last statement about the activating lever...
10-31-2008, 04:42 AM   #6
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QuoteOriginally posted by dlacouture Quote
Well, I tried with a Porst 55mm f/1.2 at several apertures, and didn't have any problem...

Hummm... Maybe you tried setting the lens to f/22 and controlling it via the body only? (at least, that's how I read your post...)
this was one test, and yes this was a poroblem
QuoteQuote:
In this case, yes, you'll have inconsistent results...

What I did was setting desired aperture on both the lens and the body...
I tried this also with no success
QuoteQuote:
But I don't understand you last statement about the activating lever...
It is a documented fact, that the K and KA lenses have different mechanical relationship with respect to the movement of the apature activating lever. K mount lenses have the diameter of the apature change proportional to the change in position of the activating lever, KA lenses have the area change in proportion to the movement of the activating lever. This is so it is easier to control the apature, especially at low apature values, with the camera.

The problem is that on real K mount compatible bodies the activating lever moves all the way back when the shutter is tripped, allowing the lens to stop down the the preset setting. This is also true of the DSLR bodies in manual mode. But, when a KA compatible body is in any other mode but manual, with a lens that it thinks is KA or later, the activating lever only moves enough to open the lens for the exxposure set by the body. As a result, not all exposure settings on the lens will be reached before the body lever stops the lens from opening. The two systems are quite incompatible.

I had written pentax and proposed that they allow users to enter maximum and minimum apature, and re-scale the movement of the activating lever to allow for proper control of K mount lenses, but I suspect that not all lens types (i.e. read this as every different lens model) has the exact same relationship of Lever to diaphram since it was used only really as an on-off control.
10-31-2008, 05:13 AM   #7
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Hummmm....
This shows this solution doesn't work with all PK lenses...

It all depends on the lever course/aperture surface relation of each lens, and now I don't think it's really diameter-related... After all, there was no imperative on respecting this relation at the time, so some lenses are maybe surface-related, others diameter-related, and still others have a completely different lever action...

Thus, with some lenses (like you 50 f/2), a surface-driven lever position simply doesn't reach the lens' selected aperture, as you stated...

Guess it's easy to see : the aperture should close down slower than with a PKA lens...

My Porst has the opposite behavior : it closes faster than my FA50, as you should expect from a diameter-related vs surface-related lever, and thus works in fake-PKA mode...

Last edited by dlacouture; 10-31-2008 at 05:23 AM.
10-31-2008, 05:41 AM   #8
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Thinking about it, it's now clear to me...

Let's say a PK lens has an On/Off lever, diameter-related...
If its fully closed-down position (smallest aperture) is farther than the corresponding PKA position for this same aperture (which is not necessarily at the end of the actuator course!), then you will have erratic exposure on the closed-down area portion of the aperture range...

And looking at your M 50 f/2, this is the case, as it will only close to f/22 whereas the KA mount allow f/32!...

My f/1.2 closes down to f/22, and that's the limit of the PKA mount for a f/1.2 lens...

So, this method works perfectly with only some PK lenses (those with 9 stops and more)... The others will have bad closed-down exposures...
And a quick check on KMP just shows that, apart from the 50 f/1.2, there is simply no PK lens with 9 stops...

That's where the rotten thing was...


Last edited by dlacouture; 10-31-2008 at 06:02 AM.
01-22-2009, 03:54 PM   #9
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dlacouture, aren't all contacts except A automatically shorted? How do you short contacts? Can you summarize to which lenses the trick work? Can you take advantage of P-TTL by lenses in M mode i.e. mode in which aperture is always dimmed to the setting of aperture ring?
01-22-2009, 04:48 PM   #10
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note that many M42 lenses are too small in diameter to short out any pins, that is where the tinfoil comes in to the equation.

Also the thrid pin from the lens locking pin, the "A" pin is recessed so a K mount lens won't hit it. that is how the camera knows there is a K mount and not a KA mount or later attached.

while the whole thing might work randomly for certain lenses and exposures it is a real hastle, and if you are seroius about flash with legasy lenses get an *istD and use TTL, or get an AF540FGZ and use auto mode like the oldd days of the 1980's
01-23-2009, 12:53 AM   #11
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Well, two months later, I've still found no exposure problem with my PTTL shots using my m42 lenses...

This simply works so well I barely think about it anymore...
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