How to check camera's meter?

Discussion in '35mm Cameras and Accessories' started by film_guy, Feb 10, 2007.

  1. film_guy

    film_guy Member

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    I just recently received my EOS 3, a camera which unfortunately has lots of history of underexposure problems. Is there a way I can check the accuracy of its meter besides sending it to Canon? I've heard of shooting slides, using the F/16 rule or using a grey card to check for meter problems. Which one's the most accurate and the best way of doing so?
     
  2. David H. Bebbington

    David H. Bebbington Inactive

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    Have you got a handheld meter? If so, easiest thing by far is to meter a gray card with this, meter the same card under the same lighting with the camera, and compare. If you don't, then a quick check is to make some exposures outdoors (not necessarily with film in the camera) and see if these are plausible according to the sunny 16 rule. This will give a rough guide, but if a camera has a tendency to underexpose, for example, due to peculiarities of its matrix metering system, then only a film test with varying subjects will really show if the meter is OK.

    Regards,

    David
     
  3. Ray Heath

    Ray Heath Member

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    g'film
    who says it has a 'history of underexposure'?

    under what conditions?

    shoot a roll, make your own detrmination
     
  4. Roger Hicks

    Roger Hicks Member

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    Well, which one involves real pictures? Sunny 16 is a get-you-out-of-trouble measure when the meter doesn't work. Grey cards are a complete waste of time: there's a module about why in the Photo School at www.rogerandfrances.com but the basic reason is that no speed determination system on earth is based on grey cards. This leaves shooting slides, which has the double advantage that you might get some pictures out of it.

    Shoot as wide a variety as possible of your normal subjects, bracketing at least +/- 1/2 stop; preferably +/- 2/3 stop (the optimum, if the camera lets you do it); and failing +/- 2/2 stop, 1 stop.

    Remember however that the metering algorithms for slides are completely different from those for negatives, because slide exposure is keyed to the highlights (so they don't 'blow' to a featureless white) while with negs it is keyed to the shadows, to get enough shadow detail: it is difficult or impossible with most subjects to 'blow' the highlights with neg, though you may need reduced development, softer paper or dodging and burning to retain the highlight detail.

    In other words, if you normally shoot neg, testing with slides won't tell you much except that it over- or under-exposes with slides. If you normally shoot neg, shoot neg film, bracketing +/- 1 stop; look at the shadow detail in your negs (you don't need to print them); and base your future exposures on that.

    Cheers,

    R. (www.rogerandfrances.com)
     
  5. marsbars

    marsbars Member

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    The few books that I have say that most meters can vary from 1/3 to 2/3 stop from camera to camera even in the same brand. Most of the advice I have found is to meter something that is average in color like green grass or concrete or a gray card if you have one in bright sunlight. Using the sunny 16 rule and adjust the ISO till it reads what ever shutter speed corresponds to the ISO you plan to shoot. I have an F2 with a DP1 and DP2 finder and they are about 1/3 stop different from each other. But they always match my Mint Pentax ZX-L that also matches my Luna Pro handheld meter. And as a note they also meter right about the Sunny 16 rule. I have calibrated both meters to the f/16 rule and my slides come out great and my B&W prints come out splendid as well. That being if I do my part on determining exposure.
     
  6. David H. Bebbington

    David H. Bebbington Inactive

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    Can't quite agree here. A gray card has the average reflectivity of a normal scene and above all provides a consistent standard for comparison readings which eliminates the effect of variables such as different angles of view of an built-in meter versus and handheld one, different metering patterns (with a gray card, integrating, center-weighted, matrix and spot metering should give the same result each time). I thus regard it as ideal for COMPARISON purposes - it will also give good results for practical photography in most cases, being exactly the same from a measurement point of view as a incident reading. Whether you might get a slightly better result in practical field work with slide film by metering a highlight and placing this on zone VIII (you probably would) or with neg film by metering the deepest shadow and placing this on zone II is another question, but I think to go straight for a film test with varying subjects is to risk confusion (even some matrix metering systems are only semi-intelligent insofar as they only work with horizontal subjects). Above all, gray card metering is quick and gives you an indication in seconds of whether your meter is in the ballpark as regards accuracy - if it is, THEN shoot some film!

    Regards,

    David
     
    Last edited by a moderator: Feb 12, 2007
  7. eunkefer

    eunkefer Subscriber

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    I have a Nikon F2 with a DP-1, DP-2 and DP-3 finders an EM and FG and a JTL LM-8 Digital Light Meter. I acquired the DP-2, DP-3 and FG recently and to do a quick check of their meter accuracy before purchase I used a simple test in a bathroom in our house that has a light fixture with 10 60W clear bulbs (very bright) and dimmer with eggshell white walls. I tested the F2 finders against the EM the FG and light meter at varying light levels from 1/1000 sec at f22 to 1 sec at f2 and multiple ISO settings, all meters matched every possible ISO shutter aperture combination within 1/3 stop. These are fairly simple metering systems, but I know they accurately measure a simple light source with the same results. Obviously you have to have trusted camera/meter to run a comparison against. The DP-2&3 and the FG have worked with film in the field without any problems.
     
  8. Roger Hicks

    Roger Hicks Member

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    Dear David,

    I'll agree on two points. First, a quick-and-dirty comparison before you shoot film is indeed a good idea. Second, a sheet of grey card is indeed a useful neutral target -- but no more useful than any other neutral target such as a sheet of white paper, or indeed a field of grass, though coloured targets should be treated with caution because meter cells respond differently to different colours. An old sensitometrists' trick (which I leaned from my frends at Ilford) is to put the test target well out of focus in order to ensure an absolutely even field of illumination.

    Second, 18 per cent is NOT the reflectivity of an 'average' scene. This is a widespread myth which I believed myself until a few years ago, and I must confess to my shame that it is propagated in some of my early books -- the ones from before the days when drafts were read through as a matter of course by Mike Gristwood, long-time member of the ISO standards committee.

    Kodak's original work in the 1930s indicated that the overall reflectivity of an average outdoor scene is 12 to 14 per cent (indoors, all bets are off). An 18 per cent grey card is a Munsell mid-tone, i.e. the card that most people will pick out as a mid-tone when shown a range of cards from as back as possible to as white as possible.

    It has acquired some sort of totemic status because of its adoption by Ansel Adams, but its relevance to metering is slight, as no speed criterion ever has been based on a mid-tone. Speeds for negative materials are based on shadow detail, while speeds for transparencies are keyed to the highlights.

    Kodak first sold grey cards for use in determining exposure in studio colour photography only, in the days when incident light meters were rare but reflected-light meters were relatively common, at least among professionals. If I recall aright (I lost the instructions for my grey card some decades ago) the card was supposed to be angled between the subject-camera axis and the subject-key light axis, at which point the question of how much light it reflects becomes quite interesting. I suppose it it pretty much governed by Snell's law. Before they sold grey cards, they recommended the use of a Kodak yellow film packet (in my 1941 Kodak Dataguide, for example).

    There's one free module on ISO speeds (including history) in the Photo School at www.rogerandfrances.com, and another on why grey cards are of limited usefulness.

    Cheers,

    R.
     
  9. Mick Fagan

    Mick Fagan Subscriber

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    I agree with Roger on the grey card being a problematic thing, sometimes.

    I also agree with David that the grey card is extremely useful, sometimes.

    But have either of you used, a Wallace Expo/Disc?

    These are quite brilliant, they turn any in-camera meter into an extremely accurate incident light meter.

    I have had two of these for about 15 years and their accuracy for obtaining, among other things, a perfectly exposed, colour neutral, 18% grey C41 negative, for calibrating purposes under an enlarger, are unbelievable.

    The Wallace Expo-Disc is in my humble opinion, the modern grey card!

    I have two of them, 52mm and 72mm for my lenses. Using my system I am able to print a colour perfect and density perfect print, first go almost every time. Artistic interpretation of the negative means that I will change things a bit, but for a standard neutral 18% frame it cannot be beaten.

    I also use it for B&W.

    Mick.
     
  10. Helen B

    Helen B Member

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    History? That's my cue to trot out this picture again. Here's Prof Karapetoff with the Neutrowe Gray Card in '39 or '40. It was 14% grey, and that was decided on by looking at pictures made by metering different cards and deciding which ones looked right. As Roger says, it was intended for use with colour film: Kodachrome.

    [​IMG]

    If you want a standard reference, the palm of your hand makes a convenient alternative to a grey card, if you don't have an incident meter. The incident meter, with a dome (cardioid responce) instead of a flat (cosine response) receptor, has the advantage of being better able to measure light from a variety of directions at once.

    "...its relevance to metering is slight, as no speed criterion ever has been based on a mid-tone. Speeds for negative materials are based on shadow detail,..."

    Well, that's a bit of a grey area. There is a technique known as Aim Density that uses the density of a grey card exposure as the determining factor for speed. It's used in cinematography with colour negative materials, and there are a few variations. It uses midtones, because they tend to be the most important tones in most movies. Well anyway, that's what I think. There is no ISO for the speed of colour negative motion picture film, by the way, so a manufacturer could use midtone criterion for the determination of a recommended EI if they so wished.

    This is how one variation it works, in brief. Years ago EK were kind enough to give me a week's (free) workshop in the practical use of it for conventionally printed film and for telecine'd film.

    The film manufacturer gives you red, green and blue Status M aim densities for a grey card. You expose a grey card at different effective meter settings, using your meters, in third-stop intervals. Then you measure the Status M densities and find the setting that came closest to the recommended values for each layer. The speed is calculated by voting: eg 250, 320, 320 means 320. Then you use that as your basis for the rest of your film testing, which is a mixture of numbers and appearance, and which is the larger part of the exercise.

    Best,
    Helen
     
    Last edited by a moderator: Feb 12, 2007
  11. Roger Hicks

    Roger Hicks Member

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    Dear Helen,

    You are of course absolutely correct, as usual. I could be picky and say that this is not an ISO standard, but in cinematography, a mid-tone is indeed a de facto standard -- or, from what little I understand of the subject, a flesh tone, especially a highlight flesh tone on the female lead. Early books on spot metering devoted quite a lot of time to this. My understanding is that this is all in the interests of continuity from one shot to the next.

    I'd still say that even after allowing for your correction, my observation about neg speeds holds good for non-cinematographers.

    Oh: and thanks for the wonderful picture.

    Cheers,

    R.
     
  12. Sirius Glass

    Sirius Glass Subscriber

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    I have a spot meter capability on my camera, but if I still have questions I will check with the palm of my hand. It is more convenientthan a gray card and I almost always have it with me. :rolleyes:

    Steve

    Warning! Some postings may have a touch of irony.
     
  13. Chan Tran

    Chan Tran Member

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    What I do to check the meter of an unknown camera is the following.
    1. I put my Beseler computerized color head on its side and dial in all three filter to get an EV14@ISO100 at its diffuser. I check the light level using my Minolta flashmeter VI in spot mode.
    2. I point the camera at this about a couple of inches from the diffuser and with the lens focus at infinity. I check and see if the shutter speed and aperture combination on the camera is at EV14.
    3. I dial in 150cc of all three color and this should bring the brightness down to EV9@ISO100. I check it with the spotmeter.
    4. I point the camera at this target and see what the reading is.

    If at this 2 points the meter readout on camera is kinda OK then I think the meter is OK.
    The method leaves a lot to be desired but it work well enough for me.
     
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  15. Chan Tran

    Chan Tran Member

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    "The film manufacturer gives you red, green and blue Status M aim densities for a grey card. You expose a grey card at different effective meter settings, using your meters, in third-stop intervals. Then you measure the Status M densities and find the setting that came closest to the recommended values for each layer. The speed is calculated by voting: eg 250, 320, 320 means 320. Then you use that as your basis for the rest of your film testing, which is a mixture of numbers and appearance, and which is the larger part of the exercise."

    Helen, I like this method as it determine the exposure accuracy overall not just the light meter or shutter speed or aperture accuracy. However, I ran into trouble using this technique recently because I can't find a reliable film processor to process my C41 film.
     
  16. Ed Sukach

    Ed Sukach Member

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    Having been involved in the calibration of Photometers - sundry different levels of sophistication - I can only recommend - for sanity's sake - to return it to the manufacturer, or a well - equipped repair shop.

    Every other "at-home" method I can think about will NOT be accurate enough to determine the meters accuracy; e.g., if it falls within the manufacturer's tolerances.

    BTW ... It is interesting to note just what the manufacturer's tolerances are, information that is may be difficult to obtain. The specifications for the Gossen "UtraPro" that I have lists +/- 1/3 "Stop". That CAN mean that two "in tolerance" meters, one towards the high side; the other toward the "low" will differ by 2/3 stop.

    I have read the treatise on "Grey Cards" in the mentioned web site - and I will confess that I felt something like a ping-pong ball - bouncing from "completely useless" to "pretty good in some circumstances."
    My opinion? A useful tool. Not perfect - but then again - nothing is (not even a "spot meter"). It is a good thing to understand the theory and variabilities involved in their use... as it is with everyhing else in photography.
     
  17. MattKing

    MattKing Subscriber

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    One final point - if you use the palm of your hand, and you are light skinned, and you spend a lot of time outdoors in the summers, you may need to compensate for the "tanning" effect during certain parts of the year.

    Matt
     
  18. jmal

    jmal Member

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    Matt,

    What's your secret? I have long desired tan palms, but they continue to elude me. One day I hope to have tan palms to match the rest of my hand.

    Jmal
     
  19. Roger Hicks

    Roger Hicks Member

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    Get enough hair on them and the luminance drops to the same level.

    I have to admit that I've never suffered from tanned palms either, even when living in Malta, one of the sunniest places on earth.

    I'll have to lend one of my West Indian chums a meter...

    Cheers,

    R.
     
  20. MattKing

    MattKing Subscriber

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    Jmal:

    They are never as tanned as the back, but they are usually darker at the end of the summer then the paste-white colour they achieve by winter's end.

    Matt
     
  21. Roger Hicks

    Roger Hicks Member

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    Well, that's it. As long as people understand that (a) grey cards aren't a panacea (b) they're not a lot more use than white paper and (c) they don't represent the reflectance of an 'average scene', they might as well buy one. I own two or three, though I seldom use them.

    As so often, people need to stop worrying that there's some Big Secret they don't know, because usually, there isn't. It's more often a matter of assembling small bits of information; beginning to have some idea of the theory behind what you're doing; and practise, practise, practise (or practise x3, in American). Totemism and its associated jargon are a short cut to lousy pictures.

    Cheers,

    R.
     
  22. jmal

    jmal Member

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    Roger,

    I use the hair for metering shadow detail. Mid tones and shadows on one portable device.

    Jmal
     
  23. Roger Hicks

    Roger Hicks Member

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    Portable, but a wasting asset in my case and with many other middle-aged/old men. No longer as dark as it was, either.

    Cheers,

    R.
     
  24. dmr

    dmr Member

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    Maybe you can answer this, and this has been bugging me lately.

    Is there anything readily available for a reasonable cost that will produce a light of a known intensity, kind of a "standard candle" sort of thing.

    I mean, for other values we have known physical standards. For length we have rulers, for volume we have measuring cups or graduates, for temperature we have thermometers or lacking that, the freezing point of H2O, for time we have quartz watches or lacking that the high point of the sun.

    For luminance or illuminance, it appears we have nothing more precise than "Sunny 16" or "check it with a known good meter".
     
  25. David H. Bebbington

    David H. Bebbington Inactive

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    I entirely agree with you, Roger, but let's recall that the OP's question was how he could judge whether his camera's built-in meter is working correctly within its design limits. The question was NOT whether the meter is delivering perfect exposures for both slide and negative film in all cases without operator intervention of any kind, which as we all know is impossible. The reading from any meter will need creative interpretation to give an optimum result.

    Regards,

    David
     
  26. Roger Hicks

    Roger Hicks Member

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    Dear David,

    Well, I was making the not unreasonable assumption that the reason he cared about his meter's accuracy was that he wanted to take pictures with the camera.

    Checking it against another meter using a sheet of white paper or (as another reader suggested) an eggshell white wall or even a grey card will not tell him that his meter is within specification: it will merely tell him how well it agrees with another meter. Unless the other meter is known to be accurate AND reads the same way, this does not tell him a great deal. And even if both meters are within specification, as Ed (I think it was) pointed out, they might disagree by 2/3 stop.

    Hence my advice to take pictures, and hence my observation (which many people do not realize) that a meter which gives perfect exposures for trannies (chosen for their lack of latitude) will not be giving the optimum exposure for negative at the same reading.

    Yes, I'd do a quick-and-dirty comparison test with a large sheet of white paper, but I wouldn't rely on that to do much more than tell me that the meter was working reasonably well, not that it was accurate.

    Cheers,

    R.