Question About Film ISO and Exposure Index.

Discussion in 'Black and White' started by Vincent Peri, Jul 16, 2020.

  1. I remember this post, see link - Shooting Film at Box Speed

    Short answer: the metering standards allow the manufacturer to bias the reading over a limited range, by using different values for either a 'K' constant (reflective meters) or a 'C' constant (for incident meters). The stipulation is that they must also specify the value that they use. (See the user manual, in the "specifications" section.) One probably needs to see the metering equations from the standards to make sense of these values.

    I think that Minolta used to go with the ~18% value for their incident vs reflective values, so likely the readings would mostly agree between the two using a frontally lit Kodak 18% gray card. Whereas some other common brands want to see a less reflective card.

    I recently did the comparison with a Gossen meter. As I recall, I think this meter was looking for a nominal,14% reflectivity from the gray card. (The method I used was to rotate the card away from the light source until the readings matched. Then calculate, from the angle, the reduction in light striking the card.)
     
  2. As I've commented before, the sainted Ansel didn't seem to be able to match the descriptions of his precious Zones to actual, measured reflectance values.

    He states quite categorically that Zone V mid grey is equal to 18% reflectance, while also describing something like 'snow with texture, a white picket fence, etc.' as falling on Zone VIII, a full three stops away.
    Now 3 stops more than (8 times) 18% reflectance takes us to a ridiculous 144% reflectance!

    Something wrong somewhere.

    FWIW, a figure of 12.5% reflectance for Zone V would resolve that particular issue, but that was never, apparently, AA's argument.

    Whatever. I'm pretty sure that the 18% 'average' reflectance figure came from Kodak's research labs and not from Mr Adams.

    Also FWIW, if you take a reading from a white card - a doubled up sheet of copier paper will do - and add 2.5 stops, then you get the equivalent of an 18% grey card reading. Doing this gets an exact match for placing the white card reading at the extreme right, but not saturated, of my digital camera histogram. Co-incidence? I don't think so.
     
    Last edited: Jul 25, 2020

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