How to use a 18% Grey Card with Reflected LIGHT Meter.

Discussion in 'Lighting Equipment' started by miss.annette_leigh_haynes, Dec 27, 2017.

  1. Any paper that only reflects 85% Lambertian isn't white, it's light grey.

    Do the experiment guys. Photograph a double thickness of reasonable quality copier paper and compare the histogram in the same light with white chalk, virgin snow, fluffy white clouds or any other surface that's recognised to have close to 100% Lambertian reflectance.

    I've also heard it said that the optical brighteners added to copier paper result in a reflectance > 100%. You can't have it both ways.

    Whatever, for all practical purposes, if you take a reflective meter reading from white paper and open up by 2.5 stops you'll arrive at exactly the same exposure as you'd get from metering an 18% card.
     
  2. I would think that-if that's true-it's only true under certain types of lighting.

    IF the brighteners fluoresce under UV, their fluorescence could be in the visible range. If no light of the correct excitation wavelength is present, the brighteners would do nothing to increase the visible reflectance. If the excitation is in the visible range, it would reduce the reflectance since fluorescence isn't a 100% efficient process.

    Still, though, the amount it would change is hardly worth quibbling about.
     
  3. I doubt that any "conventional" (silver halide) color papers have reflectance greater than 85% but most people would consider them to have "white" bases. In photofinishing that amount of reflectance would correlate to an optical density of about 0.07. Most finishers are accustomed to seeing "stain" densities in the vicinity of about 0.10, which is an even lower reflectance.

    What we see as "white" is relative to other things, not to its absolute reflectivity.
     
  4. When I've looked at "white paper" in search of materials for white balancing digital cameras, I've used specialized instruments not a camera. For the purpose of WB one wants a paper without brighteners added to it. I don't recall seeing any such paper with reflectivity more than the high 80s percent range, even bleached papers. Could exist, I guess, I just haven't seen any "plain" paper, by which I mean without special high-reflectivity coatings.

    I don't know how a regular photographer would know what their reference white is for a comparison photo, unless perhaps they use something likely a Macbeth Colorchecker (this has about 89% reflectance - an optical density of about 0.05).

    I concur, not worth quibbling about for the purpose of setting exposure.
     
  5. To put this in perspective: The real world is not littered with 'Speculon' or similar PTFE tablets.
    It is littered with white clouds, snow, whitewashed fences, white ceramic objects, white paper, etc., etc. Those are the 'white' highlights we're likely to encounter in our everyday pictures, and the reflectiveness of those common objects is all we generally need concern ourselves with.

    IME, the 'whitest' and brightest object in a scene can be represented by a simple sheet of white copier paper. Examples follow:
    1) Bathroom with 'white' ceramic fittings + copier paper -
    IMG_20180102_131101.jpg
    2) 'White' kitchen units + copier paper -
    IMG_20180102_130836.jpg
    3) 'White' expanded polystyrene packaging + copier paper -
    IMG_20180102_131339.jpg

    I hope you're seeing the pattern. That in every case the most reflective and neutral matt surface is the simple sheet of copier paper.
    It wasn't specially selected, just grabbed out of the pack of A3 printer paper I had handy.

    At least I'm not under the illusion, as Ansel Adams clearly was, that a matt white surface can have a reflectivity of 144%! Because if Zone V has 18% reflectance as Adams insists, then Zone VIII (3 stops or 8x brighter) must reflect 144% of incident light. Something amiss with Adams' infallible Zone system somewhere methinks.
     
  6. Good examples joe, thanks.
     
  7. The Zone System is nothing more than a tool to let you get a scene recorded on film with some sensible exposure, and development consistent with the contrast range. The resulting negs should then print with minimal pain and grief on standard grades of paper. There was a time when both film and paper curves had useful toes and shoulders, allowing one to have both shadow and highlight detail, while still maintaining sufficient contrast in the midtones to produce a brilliant print. I think those days have been gone since the '70s. Regardless, one way or another, you usually have to get a much wider range of brightness mapped onto paper that most certainly doesn't come close to 144% reflectivity! Minor White also wrote a book on the Zone System, which I'd suggest avoiding like the plague. Though maybe a bit hard core, one of the best books for people that really want to understand this stuff is Photographic Sensitometry, The Study of Tone Reproduction, by the late Hollis N. Todd and Richard D. Zakia. I may be a bit biased as Todd was one of my instructors at RIT.
     
  8. The point I was driving at is: If, after years of using his own system (presumably) and banging on about the importance of correct exposure, Saint Ansel could make a very fundamental error, then how important is accurate exposure really?

    We use a logarithmic system based on doubling or halving the amount of light with every stop. That's a ridiculously sloppy tolerance by any standard. And yet people worry about putting a 1.5 v cell in a meter designed for 1.35 v and suchlike nonsense.

    Let's get a sense of proportion and stop worrying about measurement precision that makes no practical difference.
     
  9. I'm a measurement geek, and in my electronic work anything worse than a few parts per million isn't even interesting. For photo, not so much. It always amazed me that film makers got the consistency they did. All that black art of ripening and such. Great accuracy just isn't inherent in the process, though I can say from experience that a systematic problem like a light meter reading a stop too high will trash your work until you figure it out. IMO, everybody should do a ring-around at some point just to make sure they're somewhere near the sweet spot. I see a lot of technically bad b&w work, but without the broad knowledge base we used to have, much of it is just accepted as normal.
     

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