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05-18-2012, 12:18 AM   #1
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Emulating an AF lens?

I'm planning on attempting a conversion of a yashica AF lens (its not the zeiss one I have previously posted about) to pentax with functioning auto focus. The mechanics of it seem pretty simple but I haven't figured out how to get the camera to recognize it as an AF lens? From what I gather the 7th contact is responsible for AF. If I have that contact only and no aperture contacts will the lens AF or do I need to get all the contacts there? Is there any way to emulate the 7th pin or some way to get the AF screw drive to function? I guess I could gut a damaged pentax AF lens for parts if I can get one cheap.

Does anyone have any idea how to get the AF to function? It will be used on an ist-ds but I will be testing it on a pentax zx-7 film camera while I'm working on it. It would be quite helpful if I could even get the AF drive screw to function with no lens attached to test out function to see if it can focus the lens before going all the way through the conversion. Is there some way to trick the contacts on the camera with foil or something?

Any suggestions or even information on how the AF works or is triggered would be very helpful. Thanks

05-18-2012, 03:27 AM   #2
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The 7th pin is a data communication pinfor which the data format is proprietary to Pentax. This pin carries all sorts of things including focal length data and focusing information from the lens to the camera. Without analyzing the data feedback I doubt it is possible to convert a lens. The remaining 6 pins are maximum and minimum aperture data, along with a single pin that tells the body the lens is in the A position (3rd pin from the lens locking pin). Without electronics to communicate to the body as required you will not get AF to work
05-18-2012, 06:39 AM   #3
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Your only possibility is to take the electronics from another lens. Even mayor companies have trouble making their own implementation of modern lens protocols. But you can test it with just the mount and chip easily enough.

You need 5 pins, not just pin 7. I have no idea why so many people are convinced it's the only one used for digital data, it isn't. All the pins present on for example the mount of an MZ-60 (google up an image for reference) are used for serial data except the A pin. (Sort of proof: The MZ-60 doesn't work with any non-AF lenses, so why would they be there if they weren't used?)

Not that you need to worry about how it works, just take the whole mount (+chip) from some broken AF lens.

Edit: Ok, four pins, since you probably don't have any use for the A pin.

Last edited by drougge; 05-18-2012 at 08:44 AM. Reason: A thought
05-18-2012, 08:54 AM   #4
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Thanks for the replys. So is there just a chip mounted on the back of the mount or would I need other electronics such as sensors that are telling it the focal length etc? I guess it doesn't matter either way but I'm just curious how difficult of a job I'm in for. The lens I'm going to atempt and convert seems to be internally functional in that the back of the lens has a cavity that goes kind of deep, that has no moving parts except the little cam for aperture control. The only major hurdle other than finding a suitable donor lens would be getting a mount think enough. The mount can not be much thicker than 2mm (at least around the flange). I guess I'll have to check out a pentax AF lens to see how it is.

05-18-2012, 10:29 AM   #5
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Chip goes over 1 contact, and contains lens information, AF, focal length, (aperture?)
then certain combination of the other contacts is use to indicate maximum and minium aperture.

you can find more info here
K-Mount Evolution, Features and Operation
05-18-2012, 10:41 AM   #6
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QuoteOriginally posted by ripit Quote
Thanks for the replys. So is there just a chip mounted on the back of the mount or would I need other electronics such as sensors that are telling it the focal length etc? I guess it doesn't matter either way but I'm just curious how difficult of a job I'm in for. The lens I'm going to atempt and convert seems to be internally functional in that the back of the lens has a cavity that goes kind of deep, that has no moving parts except the little cam for aperture control. The only major hurdle other than finding a suitable donor lens would be getting a mount think enough. The mount can not be much thicker than 2mm (at least around the flange). I guess I'll have to check out a pentax AF lens to see how it is.
If your donor lens is a zoom there will be some pins on the chip going to contact brushes for detecting the focal length. You can just solder together whatever pins gives you closest to the actual focal length of your lens. There might also (depending on age) be other contact brushes to detect where the lens is focused. I wouldn't expect to able to make that work on another lens, but AF should work without it. (So you can probably leave them unconnected.)

Your description sounds a bit like your lens doesn't have an AF screw, which is a little worrying...

Anvh: Stll no, four pins are used for digital communication. I suspect bdimitrov simply assumed the old contacts would not be used differently depending on what's on the new contact, but the assumption is incorrect.
05-18-2012, 02:28 PM   #7
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Source?
And what contact does what?

Last edited by Anvh; 05-18-2012 at 02:44 PM.
05-18-2012, 02:49 PM   #8
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QuoteOriginally posted by drougge Quote
You need 5 pins, not just pin 7. I have no idea why so many people are convinced it's the only one used for digital data, it isn't. All the pins present on for example the mount of an MZ-60 (google up an image for reference) are used for serial data except the A pin. (Sort of proof: The MZ-60 doesn't work with any non-AF lenses, so why would they be there if they weren't used?)
Very simple, those pins are used to know the max and minium aperture of the lens and so are needed for any lens/camera to work in Auto aperture.

And the MZ-60 works with non-AF lenses, it works flawlessly with for example the A lens series, as long as the lens has an auto aperture it works.
I actually ask for confirmation on this from actually users here on the forum and they all said so.

05-18-2012, 04:10 PM   #9
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I don't know what contact does what, and the source is mostly my own experimentation. If you put foil (to ground it) or tape (to isolate it) on any of the contacts that I claim are used for modern communication, you get various errors on the camera instead of working AF. I didn't take notes, so I don't remember if there is any contact you can isolate and still use the lens at all. (Except of course if you ground pin 7 it works like an A lens. I would assume at least.)

So to re-state what I've already said. Some of the same pins used for aperture indication on A lenses are used differently on AF-lenses. (But left floating/grounded as appropriate when the lens unpowered, like when it's mounted on a older camera.)

And the MZ-60 doesn't work with A lenses. It says so on its' page here on PentaxForums, it says so in bdimitrov's page, and it's obvious from the fact that not all the contacts used by A lenses are present on the lens mount. (And it's a crippled mount, so you can't use the aperture ring either.) It does strictly speaking work with non-AF lenses with a chip of course, but I don't think there are any.
05-18-2012, 06:01 PM   #10
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I would assume... that sounds hopeful.
So you've no idea what is going on actually?


I asked here for the MZ-50 which has the same mount, i believe you're saying now they are all lying there...
www.pentaxforums.com/forums/pentax-film-slr-discussion/182129-mz-50-zx-50-a.html

But you indeed seem to be right about the MZ-60, not sure why.
Btw read bdmortiv carefully because he says it does work with Auto aperture lenses so that includes A lenses.

QuoteQuote:
Bodies:
MZ-30/ZX-30, MZ-50/ZX-50, MZ-60/ZX-60, *ist
Compatibility Issues:
Bodies in this group are usable with all non-FAJ lenses only when the aperture ring is set to "A".
QuoteQuote:
The MZ-30/ZX-30 and MZ-60/ZX-60 Bodies

These "second-generation crippled-bodies" function differently — if they detect that the lens aperture is not in the "A" setting, they do not allow a picture to be taken at all. In this way no underexposed pictures can be taken, but at the same time some very desirable lenses cannot be used at all: all mirror lenses and all lenses with manually-operated diaphragm (for example the 28/3.5 shift or all "soft" lenses).
But when you read users reviews here they say they don't work so he has it wrong it seems.
Funny enough he says this as well.

QuoteQuote:
There is a simple trick that minimizes the limitations of these two bodies and make them behave like the MZ-50/ZX-50. Thanks to Piotr Szuszniak who wrote to me when noticed that his MZ-30 fires the shutter when a screw-mount mounted with the help of a certain non-Pentax screw mount adapter. This was unexpected, because neither the adapter nor the lens have an "A" setting, and yet, it worked! What was special about this combination was the adapter's surface, which was non-conductive. All Pentax K-mount lenses without an "A" aperture setting have metal mounts, and thus "short" the contacts on the body's mount. But when the lens mount is non-conductive, the camera behaves as if no lens is mounted at all, and fires the shutter.

Thus, to make the shutter mechanisms of these two cameras fire with lenses without "A" aperture settings, you have to measure which portion of the lens mount lies above the contacts, and then cover it with non-conductive Scotch tape. Michel Carrère-Gée suggests that masking all contacts is not really necessary — you just have to mask the "*" contact. If you place the camera flat on its back and llok down towards it, the "*" contact is the third one in counter-clockwise direction.

Last edited by Anvh; 05-18-2012 at 06:20 PM.
05-18-2012, 06:46 PM   #11
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QuoteOriginally posted by Anvh Quote
I would assume... that sounds hopeful.
So you've no idea what is going on actually?
I'm basically a caveman poking sticks into a computer, of course my understanding is a bit limited. But I do think my observations support my conclusions. The specific assumption here depends on if the camera can tell it's not an A lens with pin 7 shorted, which is certainly possible.

QuoteOriginally posted by Anvh Quote
I asked here for the MZ-50 which has the same mount, i believe you're saying now they are all lying there...
www.pentaxforums.com/forums/pentax-film-slr-discussion/182129-mz-50-zx-50-a.html
No, but since they don't have the same mount I don't have to. (Look at images. One has 5 contacts, the other has 7. They are very clearly not identical.) There is also a post in that thread where Adam points out that the MZ-60 (as the only MZ camera, apparently) doesn't work with A lenses.

QuoteOriginally posted by Anvh Quote
But you indeed seem to be right about the MZ-60, not sure why.
Btw read bdmortiv carefully because he says it does work with Auto aperture lenses so that includes A lenses.
Not on the camera-specific page though.

QuoteOriginally posted by Anvh Quote
But when you read users reviews here they say they don't work so he has it wrong it seems.
Funny enough he says this as well.
(Cut away quote about isolating the pins)
I'm sure that's true, but irrelevant to how pin-use differs between A and later lenses.
05-18-2012, 07:07 PM   #12
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QuoteOriginally posted by drougge Quote
If your donor lens is a zoom there will be some pins on the chip going to contact brushes for detecting the focal length. You can just solder together whatever pins gives you closest to the actual focal length of your lens. There might also (depending on age) be other contact brushes to detect where the lens is focused. I wouldn't expect to able to make that work on another lens, but AF should work without it. (So you can probably leave them unconnected.)

Your description sounds a bit like your lens doesn't have an AF screw, which is a little worrying...

Anvh: Stll no, four pins are used for digital communication. I suspect bdimitrov simply assumed the old contacts would not be used differently depending on what's on the new contact, but the assumption is incorrect.
The lens does have an af screw and it actually looks like it might mate up with the pentax screw drive well enough (regarding shape of the ends). The only problem is the screw drive position is a little farther out on the lens (the original mount is a little bit bigger around). Thats why I wish I could just trick the camera AF into working, so I could see if the screw drive is going to be compatible before I put too much work into it.

I'm not really worried if metering works right or not. I often prefer manual exposure so I have no issues using it. I'm not sure I'll have a working aperture anyway and if I do it will most likely be manual control.
05-18-2012, 07:08 PM   #13
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QuoteOriginally posted by drougge Quote
I'm basically a caveman poking sticks into a computer
I'm pretty good with mechanics but that's about how I feel with the electronics aspect of this.
05-18-2012, 08:19 PM   #14
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But look at the contacts that are on your lenses, my DA lenses have different contact formation some have 5 others have 6 but they are all missing the contact next to the 7th contact but thats a contact the MZ-60 has and that contact is used on A mount for the aperture.
The contact is also missing on the DFA macro which works on A mount bodies so things are very confusing.

Look at the MZ-60 mount and look at the contacts that are missing.
It are the two M contacts that are use to indicate the minimum aperture, most likely it doesn't use them since it's crippled.

Beside that you run in the problem if what you say is true, that those 4 contacts are needed for digital information, since 3 of these contacts are certainly used to indicate the maximum aperture on older bodies it means you cant use F, FA, FAJ and DFA lenses on bodies with A mount and we know that isnt true.
So what do you think is the work around for that?
05-18-2012, 08:23 PM   #15
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QuoteOriginally posted by ripit Quote
The lens does have an af screw and it actually looks like it might mate up with the pentax screw drive well enough (regarding shape of the ends). The only problem is the screw drive position is a little farther out on the lens (the original mount is a little bit bigger around). Thats why I wish I could just trick the camera AF into working, so I could see if the screw drive is going to be compatible before I put too much work into it.

I'm not really worried if metering works right or not. I often prefer manual exposure so I have no issues using it. I'm not sure I'll have a working aperture anyway and if I do it will most likely be manual control.
Does the screw turn the same way around as Pentax?
You might damage your lens and camera if motor turns counter clockwise to focus forward but the lens needs to turn clockwise for the same motion.
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