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04-15-2021, 08:49 AM - 5 Likes   #1
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Hack: Using M lenses in A mode

I have been around this forum for quite a while now, but wasn't a member because I never really had a need to post anything. Since I found lots of helpful info here, I decided to share a little hack I found out yesterday when I was repairing one of my SMC Pentax-A lenses that had stuck aperture ring.

I always thought that there is some kind of electronics or something inside A lenses, but I was wrong. There is literally nothing different from M lens as far as functionality goes - there is zero electronics. So upon examining the contacts on the mount I figured out that when lens is set to A on aperture ring it just pops out litte connector on the mount that connects the electrical circuit and that A lenses have a spot on the mount that is isolated/blank(plastic). That gave me the idea to replicate that scenario on my SMC Pentax-M lens and to my surprise I actually got my M lens working in "A" mode.

Picture of A lens mount:


Here is the trick:
You have to cover one of the contacts on the camera mount with tape(the one closest to lens release button) and insert something metal to make "A" connector(the only flat connector) come in contact with the metal of the lens mount. I personally used a piece of electrical tape and a thin metal strip just to test out my theory, but there sure are more elegant ways to do it. This way, with any manual aperture lens attached you can use A mode, just remember to have lens stopped down to smallest aperture(f22 or so) at all times and let the camera open it up as much as it needs.

I hope that was understandable, since english is not my first language, but just in case I added some photos. Hopefully some of you find this useful.


04-15-2021, 09:12 AM - 1 Like   #2
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Welcome to the forums

you will find friendly members and a lot of knowledge here

thanks for a very interesting first post
04-15-2021, 09:22 AM - 2 Likes   #3
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Thanks for the post.

This has come up in the past, with a number of solutions.

If I remember right, you need to make contact with the right contacts corresponding to the maximum aperture of the lens.

The real trick, though is that the relationship between movement of the lens aperture lever and movement of the aperture mechanism changed between the M lenses and the A lenses, so exposures will be wrong (when the camera commands f5,6, for example, it may get f4.5).

-Eric
04-15-2021, 09:41 AM - 1 Like   #4
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QuoteOriginally posted by vili44 Quote
I have been around this forum for quite a while now, but wasn't a member because I never really had a need to post anything. Since I found lots of helpful info here, I decided to share a little hack I found out yesterday when I was repairing one of my SMC Pentax-A lenses that had stuck aperture ring.

I always thought that there is some kind of electronics or something inside A lenses, but I was wrong. There is literally nothing different from M lens as far as functionality goes - there is zero electronics. So upon examining the contacts on the mount I figured out that when lens is set to A on aperture ring it just pops out litte connector on the mount that connects the electrical circuit and that A lenses have a spot on the mount that is isolated/blank(plastic). That gave me the idea to replicate that scenario on my SMC Pentax-M lens and to my surprise I actually got my M lens working in "A" mode.

Picture of A lens mount:


Here is the trick:
You have to cover one of the contacts on the camera mount with tape(the one closest to lens release button) and insert something metal to make "A" connector(the only flat connector) come in contact with the metal of the lens mount. I personally used a piece of electrical tape and a thin metal strip just to test out my theory, but there sure are more elegant ways to do it. This way, with any manual aperture lens attached you can use A mode, just remember to have lens stopped down to smallest aperture(f22 or so) at all times and let the camera open it up as much as it needs.

I hope that was understandable, since english is not my first language, but just in case I added some photos. Hopefully some of you find this useful.
Going to have to try this. Thanks and welcome

04-15-2021, 09:48 AM - 2 Likes   #5
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QuoteOriginally posted by vili44 Quote
I always thought that there is some kind of electronics or something inside A lenses, but I was wrong. There is literally nothing different from M lens as far as functionality goes - there is zero electronics. So upon examining the contacts on the mount I figured out that when lens is set to A on aperture ring it just pops out litte connector on the mount that connects the electrical circuit and that A lenses have a spot on the mount that is isolated/blank(plastic). That gave me the idea to replicate that scenario on my SMC Pentax-M lens and to my surprise I actually got my M lens working in "A" mode.
Good work figuring this out!

While how the contacts are wired on a KA lens is variable, the logic is always passive, a simple pattern indicating the minimum aperture, aperture range, and whether the aperture ring is in the "A" position. While it is possible to easily spoof presence of the "A" pin, full logic requires spoofing the actual functionality of the lens. There is full discussion on The K-mount Page that details how the contacts work, the possible valid combinations, and how they key out...
Features and Operation of the KA Mount | The K-Mount Page
That said, providing actual body control of the aperture requires that the aperture actuator provide a specific opening for a particular amount of action. M-series lenses are basically intended for all or nothing actuation.* A-series lenses will open according to the amount of actuator movement. You can fool the camera, but with little profit.


Steve

* There are a few exceptions where some late versions of some Pentax-M lenses feature a calibrated actuator. The late variant of the Pentax-M 28/2.8 is the most famous example. M 28/2.8 (version 2) | The K-Mount Page
04-15-2021, 09:51 AM   #6
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Thanks for the notice Eric.

I have noticed that different A lenses have different positions of the blank spot corresponding to widest aperture as well. In my case it is the f1.4 lens(first pin blank) and if I remember correctly the f1.7 has blank spot on the 5th pin. I have not yet tested the accuracy of the exposures though, so thanks for clearing that up. I guess I was too excited about my discovery and didn't even think that aperture mechanism might have been changed. I might do some tests though, just to see how much "off" it is.
04-15-2021, 10:08 AM - 1 Like   #7
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the issue is not that you can drill out the contact pattern of A lenses in K/M series lenses, that part is easy, the issue is that on K and M series lenses the aperture mechanism does not necessarily follow a linear relationship if F-Stops with lever displacement, it is simply an on off switch for the aperture

many years ago i modified my XR Rikenon 50mm F2.0 to have the correct "A" contact pattern, and while i got the ability to control the aperture in Av mode on the body, the exposure accuracy was terrible. i will try to find the post i did at the time

---------- Post added 04-15-21 at 01:27 PM ----------

here is the link to this subject from years ago

Converting K/M lenses by adding an A pin? - PentaxForums.com

---------- Post added 04-15-21 at 01:29 PM ----------

Note however, o did something like this for the tamron 200-500 / 5.6 with KA mount because the KA adaptor only has the correct contact pattern for lenses up to F4.0 and i managed to paint over with hard nail polish, the correct spot to convert the adaptor to F5.6 and use flash with this huge zoom, and get correct flash metering. BUT this is a special use case
04-15-2021, 10:23 AM - 1 Like   #8
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Thanks for all the replies.

I did some digging and compared my M and A lenses. I can confirm what most of you pointed out: aperture actuator action on M lenses is different from the newer counterparts, as it does not have linear relation between motion of the aperture lever and opening of the aperture itself. So this mod is pretty much useless as it would produce exposures all over the place.

I guess this was just too good to be true.

04-15-2021, 01:26 PM   #9
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A long time ago I found a link that said M lens aperture movement was linear ratio to aperture diameter and A lens movement was linear with aperture area. Diameter changing to 2x is an area change of 4x. However I have also read that while heretically true the distance from open to closed isnít standard on M series which means that you could bottom out before you expect to. Honestly the unpredictable nature makes this fairly unusable. Thatís even if the proportion of movement is repeatable.
04-15-2021, 01:58 PM   #10
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QuoteOriginally posted by vili44 Quote
Thanks for the notice Eric.

I have noticed that different A lenses have different positions of the blank spot corresponding to widest aperture as well. In my case it is the f1.4 lens(first pin blank) and if I remember correctly the f1.7 has blank spot on the 5th pin. I have not yet tested the accuracy of the exposures though, so thanks for clearing that up. I guess I was too excited about my discovery and didn't even think that aperture mechanism might have been changed. I might do some tests though, just to see how much "off" it is.
If it's consistent, presumably you could use an exposure compensation when using the lens.
04-15-2021, 04:51 PM - 1 Like   #11
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QuoteOriginally posted by UncleVanya Quote
A long time ago I found a link that said M lens aperture movement was linear ratio to aperture diameter and A lens movement was linear with aperture area. Diameter changing to 2x is an area change of 4x. However I have also read that while heretically true the distance from open to closed isnít standard on M series which means that you could bottom out before you expect to. Honestly the unpredictable nature makes this fairly unusable. Thatís even if the proportion of movement is repeatable.
It is in the link I I posted above, I believe I actually wrote those exact words

---------- Post added 04-15-21 at 07:53 PM ----------

QuoteOriginally posted by Paul the Sunman Quote
If it's consistent, presumably you could use an exposure compensation when using the lens.
I might have a plot somewhere of the exposure error as a function of aperture, it is not a constant error but an error that changes as a function of aperture selected, it is only correct wide open and fully stopped down, it is sort of a circular error as a function of F-Stop, curving to a 2 stop error at the mid aperture
04-15-2021, 07:49 PM - 1 Like   #12
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The only M lens I have is the M 50/2 and it's not worth the trouble.
04-15-2021, 10:27 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
I might have a plot somewhere of the exposure error as a function of aperture, it is not a constant error but an error that changes as a function of aperture selected, it is only correct wide open and fully stopped down, it is sort of a circular error as a function of F-Stop, curving to a 2 stop error at the mid aperture
I have "adapted" Revuenon 55/1.2 (M-->A) and metering with this lens is wrong about +2/3 stops in f1.4-8 range but the error is constant (pictures are looking equally exposed in this range; I did not try f8+ apertures, because I don't use them). So exposure compensation -2/3 eV helps if you need correct/good exposure. In case of underexposure there is no problem to make adjustments in postprocessing.
04-15-2021, 11:18 PM - 1 Like   #14
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On the M lenses linearity was not a documented feature but the non-linearity wasn't one too. So different objectives will behave diccerent after this modification. When you look at objectives from other brands with PK mount things are different again. I have modified a Tokina 17/3,5 that had a halfway linear aperture, good enough for practical use. Only on very high aperture numers a real non-linearity showed.
I have also modified an early Tamron 90/2,5 macro, first or second model, that had a nicely linear aperture.

The isolators are easy to make, mill a flat hole, fill with epoxy. The sprung ball bearing is difficult as there is little room for a spring and the ball musn't come out.
04-15-2021, 11:22 PM   #15
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