Pentax PKA-AF Extension Tube problem

Discussion in 'Pentax' started by Pedro, Feb 9, 2005.

  1. Pedro

    Pedro Guest

    Hi all,
    Can onyone help me resolve this problem please? I have a set of PKA-AF fit
    extension tubes (Jessop branded) which seem to have the necessary electrical
    contacts passing through them to suit A-Series lenses and the *istDS. If I
    fit my old standard Pentax-A 50mm f1.7 PKA lens directly to my *istDS the
    lens works as expected with the aperture ring set to 'A'. If I then place
    any or all of these tubes between the camera and lens the Av indicator on
    the camera shows only -.- and obviously reduces the capability of the
    outfit. Does anyone have any experience of these? I have set the custom
    setting to allow use of aperture ring on the camera and set the MF/AF switch
    to MF. One curious thing is that with no extension tube fitted, when
    changing the mode switch the display shows Aperture Priority in most/all
    modes as it should but with an extension ring fitted the display shows the
    information relating to the selected mode EG: Change mode switch to Tv,
    display reads shutter priority. I suspected the contact faces of the
    extension tubes might be dirty and cleaned them with IPA (isopropyl alcohol,
    not beer) but still no go. Any help and advice would be greatly appreciated.
    Best regards, Peter Smith.
     
    Pedro, Feb 9, 2005
    #1
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  2. Pedro

    John Bean Guest

    Peter, the behaviour is completely consistent with the camera thinking it
    has a non-A lens mounted, so it sounds like the contact that handles this
    data is not making it through the tube. When a non-A lens is used all modes
    except manual default to Av, so that's normal too.

    If you want to know which contact is used, look at the body flange - the one
    that looks slightly depressed compared with the others is a simple on/off
    signal that tells the camera that an the aperture ring is in the "A"
    position. The other contacts on an A lens tell the body what the maximum
    aperture is so it knows how far to stop down to get the required taking
    aperture.

    --
    John Bean

    The wine urges me on, the bewitching wine, which sets even a wise man to
    singing and to laughing gently and rouses him up to dance and brings forth
    words which were better unspoken (Homer)
     
    John Bean, Feb 9, 2005
    #2
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  3. Pedro

    Pedro Guest

    Thanks John.

    I have had a look at all of the contacts and tried each tube separately, all
    with the same result. Can you confirm that these tubes should actually work
    with my equipment as described? All of the contacts seem to match those on
    the camera body except one. The camera body has one more contact than the
    tubes. I think it is the one most anti-clockwise looking at the front of the
    camera body. I have tried insulating that contact with a piece of thin paper
    thinking that it might have been shorting to the rear face of the tube but
    that didn't help.

    I haven't checked with a volt meter yet but do you know if there are any
    measurable voltages present on these pins when the camera is powered on. I
    could then at least note the voltages on the camera, fit a tube, and measure
    again to determine which, if any, pins aren't contacting. It seems odd to me
    that all tubes behave in a similar way.

    Regards, Pete Smith.
     
    Pedro, Feb 9, 2005
    #3
  4. Pedro

    John Bean Guest

    Yes, it should work. The extra unused contact is the "digital" one used by
    modern bodies and lenses to pass additional information like focal length an
    distance, it's not a requirement.

    All the others are dumb "short to ground" contacts. The recessed one we
    already covered, the others define max and min apertures in a sort of binary
    pattern.

    The reason the "A" contact is recessed is historic. Early "A" lenses
    retracted the spring loaded contact when moved from the "A" position, so
    breaking the connection mechanically. More modern lenses leave the contact
    in place and disconnect it inside the lens, achieving the same effect. It
    seems likely that the contact on the ring doesn't protrude quite as much as
    it needs to connect to the slightly recessed body contact. If this is so,
    the camera will ignore all the other contacts and assume a dumb lens is
    mounted. It's equally possible that they may work on some bodies and not
    others. I'm not a fan of Jessops "own brand" goods I'm afraid. The Kenko (or
    is that coffee??) tubes work fine and are as well-made as the hideously
    expensive Pentax ones, though they're still not exactly cheap.
     
    John Bean, Feb 9, 2005
    #4
  5. Pedro

    Pedro Guest

    You could be right, the contacts on these tubes are smaller overall in all
    respects. Pins are thinner, nylon insulators are thinner. The camera body
    contacts are at least 25-50% larger. I'll have a close look at the mechanics
    later. I'm not a big fan of Jessop gear either, these were on eBay at a
    reasonable price.

    Regards, Peter Smith.
     
    Pedro, Feb 9, 2005
    #5
  6. Pedro

    John Bean Guest

    There's a big advantage to be had - especially with more than one ring - in
    ignoring the "A" functionality altogether and use the aperture ring in
    manual mode. When you press the AEL button to meter it will take a stopped
    down reading of the light that actually gets through, independent of the
    uncertainty of the additional mechanical and electric contacts introduced by
    the ring(s). I usually do this even with good quality rings that work
    properly - with your Jessop brand it's a no-brainer.

    BTW: the Pentax-A 50/1.7 is *exceptionally* good as a macro lens when
    reversed, in which case you have no choice other than to use it manually :)
     
    John Bean, Feb 9, 2005
    #6
  7. Pedro

    Pete Smith Guest

    I have just spent a couple of hours trying to figure out what might be wrong
    with these PKA tubes . . . and failed. I have another set of tubes described
    as 'Auto' but that only refers to the auto-diaphragm mechanism, they are PK
    mount tubes (lacking the pass-through electrical contacts). Both the PKA and
    the PK set work in exactly the same way on my outfit (istDS + PKA 50mm
    f1.7). I have read and re-read the istDS manual and have determined that the
    only camera mode which will work properly is M; this is the only mode which
    closes the aperture down to whatever the aperture ring on the lens is set
    to. Eventually I got some respectable results. By stopping down to f16 the
    DOF is acceptible, which it certainly wasn't at full aperture (f1.7). The
    Vivitar 6000AF ring flash works great in ttl flash mode. All in all I think
    that 'M' mode and manual aperture setting is going to satisfy my macro/macro
    flash requirements. With all that said, I really would like to understand
    why these PKA tubes don't allow a PKA lens to work properly. As far as I can
    tell, all the contacts seem to be OK and they make contact with the camera
    body and lens OK. What this means of course is that I can't possibly use a
    DA series lens, like the kit lens, because it has no aperture ring.

    Peter Smith.
     
    Pete Smith, Feb 13, 2005
    #7
  8. Pedro

    John Bean Guest

    Yes, you need the contacts to use a lens with no aperture ring of course,
    but other than that there's no advantage to be had. If you want to use the
    tubes other than with flash don't forget that pressing AEL in manual mode
    will do automatic stop-down metering and set the shutter speed accordingly -
    almost as convenient as aperture priority and *much* more accurate when
    using tubes.


    --
    John Bean

    If the automobile had followed the same development cycle as the computer, a
    Rolls-Royce would today cost $100, get a million miles per gallon, and
    explode once a year, killing everyone inside (Robert X. Cringely)
     
    John Bean, Feb 13, 2005
    #8
  9. Pedro

    Pete Smith Guest

    Thanks John.

    All manual it is then and I'm happy with that. Now I know the limitations I
    can work with it. I think I'll stick to my Vivitar PK set of tubes, they are
    better quality, tighter fit, better finish, etc. I might invest in a set of
    Kenko tubes at some point but you're right they are expensive and I'd
    certainly prefer to 'try before buy' based on my current experience.

    If anyone following this thread wants a set of Jessop branded PKA Auto
    extension tubes . . . keep an eye on eBay where all my redundant Pentax gear
    will be appearing shortly under username 'Mannesty'. You will have gathered
    by now that they don't work with a Pentax istDS but I've no reason to
    suspect that they will be any trouble on an older 'analogue' PK-A compatible
    camera.

    Thanks again for your valued assistance John, regards, Pete Smith.
     
    Pete Smith, Feb 13, 2005
    #9
  10. Pedro

    John Francis Guest

    That is most certainly untrue. The problem is most likely to lie in the "A"
    pin itself failing to make contact on your body, your lens, or one of the
    intermediate contacts between the different tubes.

    That problem will show up on older PK-A compatible cameras just as much as on
    a *ist-D. I've run into exactly that problem with some lens/TC combinations
    on my PZ-1p; combinations that work without any problems on my *ist-D.

    It's *possible* the tubes would work fine on an older body. It's also quite
    possible that they will still fail to work - maybe only intermittently, maybe
    quite consistently.
     
    John Francis, Feb 13, 2005
    #10
  11. Pedro

    Pete Smith Guest

    Thanks for the info John. I'm reasonably sure it is a tube - camera body
    issue as I have a number of A series lenses all showing the same problem.
    Further, I get the problem with all 3 tubes even when fitted separately. If
    I fit the tube(s) to either the body or the lens the pins 'feel' OK in that
    the spring pressure can be felt. The pins in the tubes are not spring biased
    in any way, just free floating. When fitted to either a lens or the istDS
    however, the spring tension is definitely there. That of course doesn't mean
    that they are making a good electrical contact. I wondered if some of the
    other pins were shorting to the tubes mount but that is very difficult to
    check. It would be easier to check if I knew what voltages or waveforms
    would normally be present on those pins. I have both an oscilloscope and a
    DMM to measure such things. I think I might just fire up the 'scope and have
    a look anyway, out of curiosity.
     
    Pete Smith, Feb 13, 2005
    #11
  12. Pedro

    John Francis Guest

    I don't know whether you'll be able to test anything useful.
    My primary suspicion would focus on the actual "A"-pin-to-body
    contact. I suppose you could try just mounting the tubes,
    with no lens attached, and see if things changed as you
    pushed on the lens end of the "A" pin.

    On an "A" lens mount there would be no voltages or waveforms
    on the pins. The only difference is whether or not the pin
    is short-circuited to the lens mount, or open circuit.

    The presence of an "A" lens (at the "A" setting) is detected
    by the fact that the "A" pin is shorted to the mount. On an
    actual "A" lens this is done by mechanically moving the pin;
    on the later mount variations it is switched internally.

    You should be able to measure this easily enough on a lens,
    but how you measure whether the "A" pin is making contact
    with the body is rather harder.
     
    John Francis, Feb 13, 2005
    #12
  13. Pedro

    Pete Smith Guest

    Easy, with lens & tube removed short the 'A' pin to the camera body, fit the
    tube to the body and repeat. That should 'fool' the body. I'll try it and
    report back.
     
    Pete Smith, Feb 13, 2005
    #13
  14. Pedro

    John Bean Guest

    That's a new one, you didn't say that before. Points to a problem with the
    body in that case.
    All the contacts that go through the tube are just switch to ground signals,
    not helpful. They determine min and max apertures as I said earlier and are
    ignored anyhow if the "A" pin is not grounded.
     
    John Bean, Feb 13, 2005
    #14
  15. Pedro

    Pete Smith Guest

    This is what I have discovered so far.

    'A' Series lens fitted to camera in Auto Picture mode shows both shutter and
    aperture info. Expected behaviour.

    With no lens fitted, only shutter info appears, aperture info shows -.- and
    when shutter pressed halfway, nothing much else happens, shutter will not
    release if shutter button fully pressed. Expected behaviour.

    With no lens fitted, short the 'A' pin to camera lens flange, press shutter
    button halfway and flash pops up. Shutter releases if button fully pressed.
    This means that shorting the 'A' pin to the camera body 'fools' the camera,
    but not completely because aperture info is still showing -.- This is
    certainly a different behaviour but I don't know if it is normal.

    Fit a tube and a lens, same behaviour as with no lens fitted and shorting
    'A' pin to body, therefore 'A' pin must be connecting through the tube to
    the camera body. Still the aperture info shows -.-

    There are 2 other pins on my 'A' series lens. one looks like a polythene
    insulator (2nd pin anticlockwise from the 'A' Pin looking at the camera
    body), the other a solid (grounded) contact (2nd pin clockwise from the 'A'
    pin looking at the camera body).

    I need about 3 pairs of hands to try and simulate all three, but I'll try.

    In the meantime, if the above info triggers any thoughts, I'll be happy to
    hear them.
     
    Pete Smith, Feb 13, 2005
    #15
  16. Pedro

    John Francis Guest

    But maybe you're moving the "A" pin onto the contact when you create
    the short? Are you sure you're not pushing the pin any further than
    mounting an actual "A" lens mould push it?
    These indicate maximum & minimum aperture (f1.7 & f22 in this case).
    See <http://www.bdimitrov.de/kmp/> for further technical details.
     
    John Francis, Feb 13, 2005
    #16
  17. Pedro

    Pete Smith Guest

    Eureka, I have found the answer. This problem is caused by the extra pin on
    the camera body. It needs to short to ground. This it does when a lens is
    fitted, but fit an anadised (maybe painted) aluminium extension tube . . .
    it doesn't short to ground any more. The remedy . . . grind off the
    anadising on each ring in the place where the extra body pin is. I was a
    little worried that I would not be able to use more than one ring at a time
    because of the lack of pass-through connectivity. Having tried all
    combinations it works like a dream.

    Sorry guys, these tubes will not be appearing on any auction site (we can't
    mention eBay) in the near future.

    John & John, thanks for your feedback. It would have taken me a lot longer
    to 'fix' this without your collective help.

    Regards, Peter Smith.
     
    Pete Smith, Feb 14, 2005
    #17
  18. Pedro

    John Bean Guest

    Very odd, but I've learned something new. The "extra" pin is the digital
    interface used by chipped lenses, but of course the metal mount of an old
    lens will short this to ground. The camera shouldn't care one way or the
    other but clearly it does. I'll remember that, useful information.
     
    John Bean, Feb 14, 2005
    #18
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