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  1. #1
    dmr
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    Recalibrating the QL17 GIII for 1600 ASA/ISO ...

    Further discussion about this little project is over at "that other place" at the URL:

    http://www.rangefinderforum.com/foru...ad.php?t=25305

    History:

    I love the Canonet QL17 GIII. However, it's not ideal. One shortcoming
    that has always bothered me was the limitation of ASA/ISO 800 as the
    fastest film that could be used with its internal meter. Since the GIII
    is one of my primary low-light cameras, this is of great importance to
    me.

    The one question which was brought up when I first mentioned the idea of
    the recalibration in some on-line forums (fora?) was that of the
    adequate range of sensitivity in low light conditions to properly work
    with 1600 speed film at the slower hand-held speeds. A quick check at
    ASA/ISO 800 at 1/4 second (two speeds slower than I ever expect to use
    hand held) showed that yes, the meter was responding to light of that
    level.

    The intended procedure is to perform a "two-point calibration" where
    adjustments (if needed) are performed on both the low ends and the high
    ends of the scale. This is similar to the "zero and span" calibration
    common on clincal laboratory equipment.

    Test equipment:

    I do not possess any test equipment whose calibration can be traced to
    any established standard. The closest I have is the meter in the Pentax
    K1000, a camera which over the years has consistently given good
    exposures on both print and slide film under daylight and artificial
    light conditions.

    Two light sources are used, a blank (eggshell, neutral, but slightly
    warm) bathroom wall with both the overhead lights and vanity lights on
    dimmers, and an east-facing window overlooking a grove on the edge of a
    forest preserve. Depending on the exact conditions, this gives exposure
    readings of 1-2 stops less than pure "sunny 16" conditions.

    Sanity check:

    The very first test was a "sanity check" on both the GIII and the Pentax
    using the existing batteries of both, the age of which are unknown or
    long forgotten.

    ISO 400 daylight 250 f8 250 f11 Canon ~1 stop under
    ISO 400 avail light 15 f2.8 15 ~f2.5 Canon slightly over

    This is similar to what I remember the last comparison to be, with the
    Canon slightly underexposing at the high end of the scale and
    overexposing slightly at the low end of the scale.

    The next part of the test was to install fresh batteries in both of the
    cameras in order to eliminate the variables of unknown battery age and
    condition.

    I've always used the "wrong" newer 1.5 volt 625 battery in the Canon.
    That was in the camera when I bought it, and even with the wrong battery
    it's always agreed within one stop of the Pentax under both bright and
    dim conditions. As could be expected, the response of the meter with
    this approximately 15% overvoltage was "hot", meaning greater span than
    normal, which is reflected by the high meter readings under bright
    conditions and the low(er) meter readings under dim conditions.

    ISO 400 daylight 250 f8-11 250 f11-16 Canon ~1 stop under
    ISO 400 avail light 15 f2.8 15 ~f2 Canon 1 stop over

    With fresh batteries, the difference of the Canon to the Pentax is
    slightly greater, indicating a possible partly depleted battery in the Canon.

    Procedure:

    The indended steps for this recalibration are as follows:

    1. Disassemble the top part of the Canon.

    2. Set the Canon ASA/ISO to 200, corresponding to ISO 400 film.

    3. Mechanically adjust the meter on the Canon so the meter indicates
    f2.8 to agree with the Pentax using the low light test source.

    4. Electrically adjust the meter on the Canon so the meter agrees with
    the Pentax using the bright light test source.

    5. If necessary, repeat steps 3 and 4 as required.

    6. Re-assemble and shoot test rolls.

    The first step is to remove the ring on top of the film advance lever.
    Many claim that this can be done by hand using a rubber jar opener or a
    rubber washer for grip, however, when I first got the camera and needed
    to take the top off, I could not get this to budge. I carefully drilled
    two small indentations in the ring for a spanner wrench.

    Once the advance lever and hardware are removed, the rewind crank is
    removed by holding the inside of the film rewind shaft and unscrewing
    the crank.

    Removing three small screws allows the top cover to be removed.

    On the first attempt, I failed to locate the screw that allows the
    mechanical re-adjustment of the meter for the low or zero setting.
    Instead I just adjusted the electrical adjustment at the high end of the
    scale. I made several back and forth adjustments, comparing the effect
    of the adjustments on the high and low ends of the scale. It was very
    obvious that the electrical adjustment had far more effect at the high
    (bright) end of the scale.

    I decided to split the error somewhat between the high and low ends of
    the scale. I settled on a setting that resulted in approximately a 1/2
    stop underexposure at the high end (bright) of the scale and about a 1
    stop overexposure at the low (dim light) end of the scale. This leaves
    at most a one stop error on the low end, and that is in the direction of
    overexposure, which I consider to be a lesser evil than underexposure
    when dealing with low light situations.

    After recal Pentax Canon (@ISO 200)
    ISO 400 daylight 250 f8-11 250 f11 Canon ~1/2 stop under
    ISO 400 avail light 15 f2.8 15 ~f2 Canon 1 stop over

    Note that the readings on the GIII are with the ASA/ISO set to 200, for
    a true ISO of 400, adjusted for the one-stop intentional miscalibration.

    The obvious next step is to shoot some test rolls under real-world
    shooting conditions. First, a sanity check roll of ISO 200 (camera set
    to 100) and next a roll of 1600 to test the end result of this
    procedure.

    Upon studying some online photos of the GIII, I was able to locate
    that elusive screw and perform the mechanical calibration at the low
    end of the scale.

    Although the elusive screw can be seen after the top cover is removed,
    loosening this screw actually requires further disassembly, the removal
    of the film counter assembly. Once this is done, a thin screwdriver may
    be used to loosen the screw, allowing the meter assembly to be rotated.

    First, the top cover of the camera is removed as described above. Next,
    the film counter indicator dial is removed by first removing one screw
    and sliding the dial off. This leaves the film counter assembly exposed
    and ready to be removed.

    Three items must be un-done in order to remove the two pieces of the
    film counter assembly:

    1. A "clip ring" which holds the outer film counter wheel to the shaft
    under the film advance lever. This is tricky. I got this off carefully
    with two very small screwdrivers. Be careful to avoid letting this snap
    loose and fly across the room!

    2. A screw post which holds one end of the film counter spring.

    3. A screw opposite the post holding the spring. This is longer than it
    first looks and also holds the camera strap bracket in place.

    Once the elusive screw is exposed, apply a bit of nail polish remover
    to the lacquer holding it in place. After the remover has softened the
    lacquer, the screw may be loosened with a small screwdriver.

    Rotating the meter assembly is tricky. I used a combination of a dental
    explorer (that pick a dentist uses on your teeth) and a hemostat (a
    serrated surgical clamp) to gently coax the assembly one way or the
    other as needed.

    I adjusted my low light source to indicate an exposure of 1/15 at f2.8
    ay ISO 400 on the test camera. You can use any test source of similar
    brightness, measured with a meter or camera that is known to be
    accurate.

    REMEMBER TO SET YOUR QL17 ASA/ISO TO HALF THE ISO OF YOUR METER OR TEST
    CAMERA!

    Rotate the meter assembly until the reading agrees with that of your
    test camera or meter. It's best to repeat the reading several times,
    capping the lens and allowing the meter to re-indicate the exposure.

    When this meter consistently agrees with your test camera or meter,
    tighten the elusive screw and proceed to the high brightness
    calibration.

    My known good camera read right between f/8 and f/11 at 1/250 at ISO
    400. You can use any scene or light source somewhere in this range.

    Adjust the electrical adjustment (potentiometer) until the reading on
    the QL17 agrees with that of the known good camera when metering the
    bright scene. It can be tricky to adjust this, but a dental explorer in
    the "dimple" of the adjusment arm of the potentiometer moves it easily.

    Again, take several readings, capping the lens in between, and make the
    meter re-indicate each time. When you're satisfied that the QL17 reads
    the same as your known good camera, you can proceed to re-check the low
    light reading.

    You will find that the electrical adjustment affects the readings at the
    bright end of the scale far more than those at the dim end of the scale.
    The mechanical adjustment affects both readings equally. Therefore, the
    mechanical adjustment is used at the low end and the electrical at the
    high end.

    You may very well find that even after the electrical adjustment, the
    available light low end reading still agrees very closely. If this is
    the case, no further adjustments are necessary.

    If they do not agree, loosten the elusive screw again and repeat the low
    end adjustment. Then check/adjust at the high end, then the low end,
    rinse, repeat!

    After second recal Pentax Canon (@ISO 200)
    ISO 400 daylight 250 f8-11 250 f8-f11 On the numbers!
    ISO 400 avail light 15 f2.8 15 ~f2.8 Very close!

    As you can see, the Canon now agrees well within 1/2 stop of the Pentax
    under both dim and bright light.

    The final step after assembly was to paint a very conspicuous red dot
    by the ISO setting. This serves as a reminder, a "string around the
    finger" so to speak, to set the ISO to half of the actual film speed.

  2. #2

    Join Date
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    Thanks

  3. #3

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    Is this as difficult as it sounds? I may try this, so far all I've done inside my QL17 is adjust the rangefinder. I remember seeing something online about adjusting the meter and I wondered if you could change the sensitivity to include 1600. It would be way more useful to me than ISO 25.

  4. #4

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    I wondered if you could change the sensitivity to include 1600. It would be way more useful to me than ISO 25.
    This is for 1600 NOT 25 did you mean the other way round?

  5. #5

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    No, I was assuming that by readjusting the meter (originally calibrated for film speeds 25-800) that you'd lose ISO 25 in order to gain ISO 1600. The range is still the same, shifted over towards the higher sensitivity of ISO 1600. Is that correct or does this modification simply add ISO 1600 to the whole range, and if so how would you then set that on the film speed selector switch? I really don't care about ISO 25 at all, the slowest film I've ever used was ISO 50 and I'm much more interested in faster films.

  6. #6

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    In low light, I would rather the use the camera on manual rather than relying on auto. The procedure is interesting but if I ever mess around with the camera I would rather recalibrate it so it is as accurate as possible.



 

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