Calibrating Meters

Discussion in 'Exposure Discussion' started by abhoan, Sep 9, 2012.

  1. abhoan

    abhoan Member

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    I am currently using a Hasselblad 500c with a Kiev TTL metered prism (yes, i am poor). I just started using it and I wanted to calibrate it before I go out and start shooting. I have a minolta IV flash meter and a Canon 5dmkii to make metering comparisons.

    Now I don't have a grey card, would I specifically need one if I wanted to calibrate the meter? Or is there any other way for me to do this?
     
  2. ic-racer

    ic-racer Member

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    Look up "Zone I Exposure Index Testing"
    Basically you shoot a target at Zone I, metered with the Kiev prism, at various exposure index settings on the meter. Choose the negative with a transmission density that is one-third of a stop more dense than a blank negative; use that one as your exposure index for your prism meter.
     
  3. Leigh B

    Leigh B Member

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    Forget all the zone nonsense. Go out and take some pictures.

    On a nice sunny day, go find some nice scenics (urban or rural, whatever).
    Meter the first one. If the reading seems reasonable for your film based on 'sunny 16',
    shoot a three-shot series at exposures of -1/2 stop, nominal, and +1/2 stop (or -1, 0, and +1 if you prefer).

    Do the same with the other three scenes.

    Develop the roll and see how it looks. And you'll have some decent negatives to print.

    - Leigh
     
  4. afrank

    afrank Member

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    You can customize your meter readings using the spot metering on the camera to read values on the usual shadow areas of subjects you shoot, i.e: Landscapes usually expose for the landscape and not the sy, the sky can throw off the meter.
    While calibrating avoid white surfaces, its better to meter on brick walls or alike or surfaces that are not glossy. Use different setting of reciprocity to test whether the meter is working correctly, i.e: f4@1/60. f8@1/30.... etc for 1 low light situation, one medium and high (related to the min and max of your meter [my yashica tlr goes as low as f3.5@1/30 but my minolta slr can go down to f1.7@1/30] ).

    Further testing can be done by making sure the components are working correctly using a volt/amp meter, i.e: Resistors have to be in the right range, photocells/cds/photo-resistors can go bad and give ~0.1 ohms readings on both light and darkness, ussually its around 4-10kohms o light and +1mohs on darkness.
    You can also use 3v batteries or combinations of them to make the meter more sensitive to light, and change the resistances accordingly to calibrate (a variable resistor or potentiometer works best for this.) ussually there are various variable resistors in the camera, turn them with a flat screwdriver or scissors and see the needle move as current passes through it.

    Good luck!
     
  5. Leigh B

    Leigh B Member

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    Can you think of any better ways to totally screw up the calibration of the camera?
     
  6. cliveh

    cliveh Subscriber

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    For the meter prism on my Hasselblad, I set to film speed and maximum lens aperture. It’s that simple.
     
  7. afrank

    afrank Member

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    Nothing is beyond fixing, not if you know what you are doing. I just point in the direction, its easier to go about things once you know what to look for, of course it depends on how much calibration he needs. One of my cameras needed new CDS/Photo-resistors and I just gave it some extra EV range by changing from 1.5 to 6v using 2 cr1616 and compensating on the high end of the EV range with a bigger Variable Resistor on the post-CDS section of the circuit (this controls high light situations and wont mess with low light sensitivity.) After that is just a matter of tunning (move the variable resistors) up and down to let more current or less pass through the circuit as to the meter needle align with the markings.

    As a side note, 2 photo-resistors/CDS in parallel allow more current(more sensitivity to light) and 2 in line decrease it.

    Example:

    http://www.willegal.net/photo/srt/srt-meter-functional.htm
     
  8. cliveh

    cliveh Subscriber

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    What on earth are you talking about?
     
  9. afrank

    afrank Member

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    I think he means to calibrate a meter that is not metering correctly even with the correct ISO and shutter/fstop settings.
    This involves using the variable resistors that are meant for proper calibration. You can even use the batteries of you choice (no weird conversions necessary) given that the built in resistors give you enough playing space (4-50kohms).
     
  10. BrianShaw

    BrianShaw Member

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    No offense intended to "afrank", but: +1.

    Please remember, Mr Frank, that many people here are not electrical engineers so that kind of advise could cause some to end up with a pile of scrap rather than a working meter.
     
  11. afrank

    afrank Member

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    I REALLY think he means to calibrate a meter that is not metering correctly even with the correct ISO and shutter/fstop settings.

    AND that is the only way to do it.

    I apologize if I misunderstood him. Just wanted to point him in the direction he would en up going if he tried to "calibrate" his "non-working" meter.

    If so, just ignore my nonsense. BTW, what other "calibration" is there? :tongue:
     
  12. BrianShaw

    BrianShaw Member

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    I think you and me and Clive and Leigh are on the same page about what 'calibration" means. You have no reason to apologize... this is a discussion forum and the discussion is good!

    I think where we depart is the D-I-Y aspects. Turning potentiometers and varable resistors and what-not isn't a particularly good idea if you don't know the correect settings or how the measure the correct settings. This is generally true with all sorts of electronics. I applaud the D-I-Yers (of which I am one) but always remain aware that there are some who simply don't have the skills, knowledge, and tools. That may not be the case, I don't know... but what I do know is that blindly turning adjustments often does not result in successful conclusion.

    Please understand, i'm not being critical of you... just discussing the flip side. Thanks for your participation on this forum!
     
  13. Chan Tran

    Chan Tran Member

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    I am sorry because I am not familiar with this prism finder but are these adjustments that you're talking about are easy to get at? and do you know the function of each? and you can really get to the photocell and measure its resistance? Most cameras to do this would require quite a bit of disassembly.
     
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  15. Chan Tran

    Chan Tran Member

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    I think the OP want to check and see if the meter is accurate before using it. Film testing is checking of the combination of both shutter speed accuracy as well as meter accuracy and possibly even aperture accuracy. To test only the meter and with the flash meter IV. I would find evenly lit, same color surface and compare reading. I would want to fill the frame with the surface. Try to find several different brightness level.
     
  16. wiltw

    wiltw Subscriber

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    It is pointless to compare any REFLECTED light meter with any INCIDENT light meter, if the reflected meters are not being pointed at 18% grey card (we will dismiss debates about the better suitability of 12% grey target, for this discussion).

    It is perfectly suitable to compare two meters, both reflected light meters, simply by pointing both of them at the same tonality surface, even one which is not 18% (or 12%) tonality...a featureless wall would be an example. After all, the point is merely to determine if the unknown meter matches the known meter. If you compare three reflected meters made by different manufacturers, you are likely to run into the fact that there is NO SINGLE ABSOLUTE STANDARD that all manufacturers follow for meter calibration...the ISO standard equation allows manufacturers to choose the actual value of a 'constant' in the equation! But they nevertheless should be within 0.3-0.5EV of one another
     
  17. Chan Tran

    Chan Tran Member

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    The flash meter IV can be used in reflected light mode with an acceptance angle of 40 degrees. This meter can measure both flash and continuous light in both incident and reflected mode. With attachment it can measure a 10 or 5 degree spot as well.
     
  18. afrank

    afrank Member

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    Indeed disassembly is required, a service manual is often the best way to go, usable found in PDF format online the more popular the camera/meter is. I do know the function of each, it can be seen on the link provided as almost all old meters based on CDS/Photocells/Photo-resistors work the same way, the only thing that changes is the values of the components and maybe extra components for custom fancy readings.

    Like most pointed out, its best to check if the metering is wrong first :wink:. Even then it could be the battery, some unclean contacts that hinder the connection to the battery or even a dirty meter window that wont let light through. Prisms usually get dirty and will set readings off.

    If someone is doing film photography developing and printing learning a bit about this wont hurt either :tongue:.
     
  19. Poisson Du Jour

    Poisson Du Jour Member

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    You're "poor"? And you have a Canon 5D Mk II?? Come on, you can't have both...

    Forget about "calibrating (TTL) meter". Your first priority is to load transparency film into the camera and shoot as the meter reads the scene: leave it all to the TTL meter to start with. At the moment, you are speculating and this will generate reams and reams of conjecture, claim and counter claim (a.k.a. the countless Kodak threads). This loading of error-sensitive film is the litmus test. Don't use neg film as the latitude is too great to pick up errors, while tranny film is sensitive to +/– 0.3 stop (or 0.5 stop for those cameras that operate to that amount). When you've got that film back, all will be revealed.

    Don't rely on a digital camera when comparing meter readings. Stick to using the Minolta Flashmeter. You have the right tools for the job, just need some basic fact finding and putting the film in will achieve that. If there are glaring errors, time to brush up on your multi-spot, incident and reflected meter skills.
     
  20. wiltw

    wiltw Subscriber

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    ...but never forget the phenomenon of 'subject failure'...a metering target which fails to match the assumption of average brightness of 18% gray. Even TTL meters cannot remain immune to subject brightness induced metering error!
     
  21. Poisson Du Jour

    Poisson Du Jour Member

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    In which case you have a reliable stand-by tool: an incident meter. :smile:
     
  22. abhoan

    abhoan Member

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    Thank you everyone for their comments. I hope I didn't start a fire with the question.

    When I said calibrate, I meant that the meter is working but because I have never used a meter on the hassey before, I am not sure if its readings are correct. I guess both comments, about the electrical configuration and about how to check the readings are correct as they end up leading to the same point of getting the correct requirements.
     
  23. Leigh B

    Leigh B Member

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    The meter was correct when it left the factory.

    Absent any visible damage, you have no reason to __assume__ that it's incorrect now.

    Shoot some average scenes, as I recommended waaay back in post #3, and see how the negatives look.

    - Leigh
     
  24. abhoan

    abhoan Member

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    Well I just loaded up the meter with fresh battery (3LR44's) and with respect to my meter, all exposures are about -4 stops. I had to set the sensitivity to ISO 25 to get -2 stops difference for ISO 100.

    Think would actually need to open it up and bring up the metering.
     
  25. Leigh B

    Leigh B Member

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    A four-stop error is certainly beyond its internal adjustment range.

    It needs to go to the repair shop. This is not a DIY project.

    - Leigh
     
  26. afrank

    afrank Member

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    Have a little faith on him :wink: