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03-13-2013, 04:00 AM   #1
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what makes a Pentax camera know it's a Pentax lens?

Quite simple really, what bits can I remove from the inside of Pentax-a 50 f/1.7 before a K5 stops seeing it as that lens?

I have a requirement for a k mount tube that a K5 sees as an '-a' series plus lens, the thinnest possible but with a light diameter of at least 28.12mm. Long story, but it's about the K-5 not rear syncing properly with a Metz 58-afII itself set using manual flash power. However the K-5 will sync properly with an '-a' series plus lens. So I removed all the glass from an -a f/1.7 and that gives me a theoretical maximum diameter of 29.4mm set at f/1.7. Should be enough, but it's not, it still slightly vignettes an infinite focused beam for me still. This is macro using an infinite relay lens, so I need a bigger maximum diameter.

So I could quite easily remove all the aperture mechanism to create a bigger 'hole', but if that stops this thing from being seen as an -a lens it's a bit useless . Anyone know when a lens stops being seen as a lens by the body?

Many thanks!

03-13-2013, 04:16 AM   #2
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Speculating here, but is there not a "chip" on it addressed by the pins. I know Nikon call their lenses with this ability "cpu" lenses. My money is on being able to strip out everything except that connected to the pins. You may, however have a problem is the aperture arm is missing.

I would be very interested in the answer. Not that I would use the information, but still.
03-13-2013, 05:04 AM   #3
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QuoteOriginally posted by Nass Quote
what makes a Pentax camera know it's a Pentax lens?
I whisper it as I'm putting it on, along the lines of oh you xyz, your a fine piece of glass.

Last edited by Kerrowdown; 03-14-2013 at 07:08 AM.
03-13-2013, 05:04 AM   #4
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Yes, there is a PCB in the lens with some information on it, relayed by the digital pins. It tells the camera what the lens is, the current focal length (if it's a zoom), the aperture, it manages AF communication, transfers the MTF function, etc. Without crippling the lens, it's hard to imagine a way to remove just the focal length info.

03-13-2013, 05:08 AM   #5
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'A'-series lenses don't have digital pins. The contacts tell the body the range of apertures the lens covers, but that's it. The exception is the '*' pin, which points the other way and retracts when the aperture ring is taken out of 'A' mode.

See here for information on the KA-mount:
http://kmp.bdimitrov.de/technology/K-mount/Ka.html

Last edited by THoog; 03-13-2013 at 05:26 AM.
03-13-2013, 05:09 AM   #6
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There's no chip as such in the A series lenses I've looked at. The aperture range information, as well as the A-position indication is encoded in connections between the pins and some common ground (which I guess is the mount ring). As such, insulating all the pins will make the camera think the lens is a K or M series. Actually, since A lenses mechanically move a pin to connect when A-position is engaged, just insulating or removing that pin is enough. But setting the lens to the widest aperture (f/1.7) should prevent the camera from moving the aperture anyways, both mechanically and electronically. I'd guess your vignetting is not because of the aperture.

For information on how the KA mount works (and differs from the original K mount), I usually read Features and Operation of the Ka Mount.
03-13-2013, 05:11 AM   #7
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You can remove the iris mechanism, just leave the bayonet electrical contacts and associated electronics in place.
03-13-2013, 05:50 AM   #8
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Hmmm, interesing, lot of responses.

To clarify, the vignetting is because the maximum diameter possible with f/1.7 is 29.4mm. APSC diagonal is 28.12mm, and I suspect some tiny 1.3mm misalignment is making it vignette. Now it's possible to widen the maximum diameter by removing aperture blades and the inner plastic cylinder that holds the diagphragm in place, which is basically what I mean. Ie, gut the innards so the light hole diameter is wider, that's what I mean. Does this make better sense?

Moving on, so I get that the pins somehow communicate an older -a lens' aperture range. I'm guessing that when you take say an f/8 shot on an older lens that f/8 is actually done by only moving that aperture pin part of the way during a shot. Ie it's basically a mechanical motion, controlled by some current passing through these pins saying yes its a lens because they conduct and the range is x to y (revealed by the pins). User has decided f/8 so we need that sliding aperture pin to move 3mm because the body knows this is f/8 for this lens. Is that about it? (obviously simplified, and not for later lenses).

So my q: at what point does the pins conducting telling a cam it's an -a lens with 1.7 to 22 range stop telling the cam this? Ie the conducting circuit, what bits of the diaghragm does this include that cannot be removed otherwise it is no longer conducting and recognisable?

Maybe my assumptions of how this all works is way off though? Remember this is non digital lenses

Thanks again

03-13-2013, 06:10 AM   #9
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There's a secondary reason for posting this actually that I also wanted to explain.

Some of my older lenses have quite poor CA and/or distortion. I notice that on digital lenses, some additional things are inabled in-camera that may help with these. So given I don't need infinity focus for the stuff do (macro, extreme macro) what's to stop me buying a 40xs lens (the 9mm k-01 one), removing the diaghragm and glass, attaching a female k mount on the end and using this as a small extension tube with CA correction?

For me actually it's the size of that hole that the light travels through that matters. The diameter available has to be bigger than the diagonal of APSC because in some of my stuff I use an infinite tube lens to focus what would conventionally be a cone of light fitting nicely through the hole but instead in my case is a very broad parallel beam. And the 40 xs is too small I think, itlooks like I'd have to saw away the plastic surrounding the glass. The glass itself being tiny, I have a 40 pancake and it has like a 1cm glass diameter or so with the rest plastic surround. 1cm doesn't give me the 28.12mm I need for parallel beams coming onto the sensor.

I wonder if I'm making any sense whatsoever
03-13-2013, 06:34 AM   #10
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You can rip all the guts out of an A lens; the only thing that communicates to the camera is the mount plate and the '*' pin. The contacts on the plate give the endpoints, say f/2.8 and f/16, and the body determines a slope for moving the aperture arm. There's no feedback from the lens. If you put the bare mount plate on the camera, and stuck something in the '*' hole to provide contact, the camera would think there was a lens mounted in 'A' mode. For that matter, a flanged M42-PK adapter could be modified to provide contact on the '*' pin and make the camera think it was an 'A' lens (with an aperture code of 111*11 = f/1.2 to f/22),

As to butchering a DA 40 XS to borrow its digital information, I'm going to guess that the CA correction is specific to the lens, so your mileage would probably vary widely. Also, as you say, the max aperture is much smaller than the sensor size.
03-13-2013, 06:39 AM   #11
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QuoteOriginally posted by Nass Quote
Now it's possible to widen the maximum diameter by removing aperture blades and the inner plastic cylinder that holds the diagphragm in place, which is basically what I mean. Ie, gut the innards so the light hole diameter is wider, that's what I mean. Does this make better sense?
I don't know how the lens was constructed, but generally if removeable parts get in the way of the light then it makes sense to remove them.

QuoteOriginally posted by Nass Quote
Moving on, so I get that the pins somehow communicate an older -a lens' aperture range. I'm guessing that when you take say an f/8 shot on an older lens that f/8 is actually done by only moving that aperture pin part of the way during a shot. Ie it's basically a mechanical motion, controlled by some current passing through these pins saying yes its a lens because they conduct and the range is x to y (revealed by the pins). User has decided f/8 so we need that sliding aperture pin to move 3mm because the body knows this is f/8 for this lens. Is that about it? (obviously simplified, and not for later lenses).
Well, you're partially right. The aperture is moved by the camera using the black metal lever. The contacts tell the range electronically and the pin is electromechanic, moving to close a circuit and indicate that the camera should control the aperture (the lens is in A mode). For post-A-lenses, there is a serial data protocol, but the camera moves the lever in the same way.

QuoteOriginally posted by Nass Quote
So my q: at what point does the pins conducting telling a cam it's an -a lens with 1.7 to 22 range stop telling the cam this? Ie the conducting circuit, what bits of the diaghragm does this include that cannot be removed otherwise it is no longer conducting and recognisable?
When the A connector/pin is not closing the circuit (i.e. raised), the camera thinks of the lens as K or M. That is, it shouldn't even read the rest of the connectors.

Last edited by topace; 03-13-2013 at 07:04 AM.
03-13-2013, 07:00 AM   #12
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Wait, what exactly are you trying to do?I dont understand at all. Are you trying to turn a lens into an extension tube?
03-13-2013, 07:02 AM   #13
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A lenses have 6 pins, 5 to communicate min/max aperture (actually they tell the wide open aperture and the number of f-stops that can be closed down), and 1 retractable to tell if the lens is in the A position. If not in the A position then it behaves as a K or M lens. These pins are binary (contacting or non-contacting) and the bit pattern indicate the values. See this for details Features and Operation of the Ka Mount

Modern lenses (F, FA and DA) have an additional pin that transmits digital data from the embedded chip in the lens including focal length, focusing distance, lens corrections, etc.

The aperture level on Pentax A and later lenses moves linearly with the area of the iris so if a lens has a range of 5 stops, 1/5 movement is one stop. The older lenses (K and M) move the level linearly with the diameter of the iris and will result is exposure errors if converted. On un-crippled mounts, the lens tells the camera through another lever, how far has been closed down from wide open. This level changes position as you rotate the aperture ring. The cameras exposure system measures always wide-open and uses the info from the secondary level to compensate. There is no need to know the exact wide-open f-stop; just measure and adjust.

While focusing, the body keeps the aperture lever push all the way to keep the lens wide open. When taking the picture, the bodys release mechanism let the level move all the way for K and M lenses or to a predefined position for A, F, FA and DA lenses. Lenses with aperture rings set to a value (not the A position) will close down to the set aperture on the lens.

On an A lens (50mm F1.7) when you set F8 in the camera, it knows that the lens is F1.7 wide open and can go to F22 for 7.5 f-stop range. F8 is 4.5 f-stops from wide open or 60% in the f-range. The bodys release mechanism will move the aperture lever activator to the 60% position of its range and that will correspond to the correct F-stop on the lens.
03-13-2013, 07:08 AM   #14
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Cool, I think I get it now. Thanks gents for your input!

QuoteOriginally posted by THoog Quote
a flanged M42-PK adapter could be modified to provide contact on the '*' pin and make the camera think it was an 'A' lens (with an aperture code of 111*11 = f/1.2 to f/22)
So in fact I could presumably actually also do the same with the male end of a bellows (the bit that can be unscrewed on a -m bellows), as long as it's got that little stud pressing into the * making electrical contact with the body's "*" contact and it conducts all the way through to the rest of the pins? And since that then makes it 111*11 the range is 1.2-22 as per Bojidar Dimitrov's page?

Na Horuk, I want to be able to use rear sync with a metz 58 on manual settings with non-aperture linked optic on a K-5. K-5 won't do this (unlike K-7) because it requires an aperture linked optic. So I'm deceiving my K-5 into thinking it has an aperture linked optic.
03-13-2013, 07:23 AM   #15
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QuoteOriginally posted by Nass Quote
So in fact I could presumably actually also do the same with the male end of a bellows
See the thread below. After reading the text, scroll down a couple posts to my post with photos. I'm using a K-mount macro-converter rather than empty tubes, but the process is identical.
https://www.pentaxforums.com/forums/pentax-camera-field-accessories/113756-pk...ml#post1965512

...and yes this setup will work with all the various flash options the same as if you attached an A-series lens to your camera.

For what it is worth, while it takes only a few minutes to remove or replace the 'A' contact body plug, I have yet to find a reason to do so a year later.
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