12% or 18% – The problem of the calibration value of the exposure meters

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In the previous sections, there was repeated talk of the light meters interpreting everything as medium gray. This is not a coincidence or a result of divine providence; rather, it stems from an agreement between camera and film manufacturers. They realized one day that they had to bindingly standardize the relationship between the amount of incident light and the resulting blackness. How else could users obtain consistent results, irrespective of the type of device they use? The standard was established by defining the average brightness value of an average subject. We reliably find these properties in a medium-brightness grayscale. For camera and accessory manufacturers, this agreement means that their light meters must treat all color and brightness values as this medium gray. For film producers, it means that a film exposed with this value and developed exactly as specified must have a corresponding medium density (density 0.70). Digital technology must register a gray value for this brightness in analog form.

If you are involved in photography, you will likely discover or hear firsthand that the calibration standard for all light meters corresponds to a reflection value of 18%. The photography community has firmly anchored this idea in their minds for decades, which presents a significant challenge. Because: It is wrong!

Exposure meters are calibrated according to ANSI (American National Standards Institute) standards, and they provide for a luminance that corresponds to a reflectance of 12%. Note the difference between luminance and reflectance: luminance, or luminance measured in candela per m², corresponds to light energy that can be accurately and directly measured. Reflection, on the other hand, refers to the perceived brightness of light after it has been reflected from an object.

The 18% value seems to come from the printing area, because there it corresponds to the average value between white and black, i.e., a neutral, medium gray. Gray cards, as we use them in the photo area, are adjusted to the 18% value for two possible reasons: because they are easy to produce at a constant quality under this specification, or because their manufacturers do not know what the exposure meters are calibrated to. That there is a great deal of disagreement in this area can be seen, for example, when Sekonic states a calibration value of 14% on its website and points out that Minolta calibrates to a higher value. Minolta, on the other hand, publishes a different value.

That the 12 % value is actually correct can be deduced from the addition that Kodak has been printing on its gray cards since 1999 and keeping in mind that the difference between 12% and 18% reflection corresponds pretty much exactly to ½ exposure stops:

„Meter readings of the gray card should be adjusted as follows: 1) For subjects of normal reflectance, increase the indicated exposure by 1/2 stop. 2) For light subjects, use the indicated exposure; for very light subjects, decrease exposure by 1/2 stop. 3) If the subject is dark to very dark, increase the indicated exposure by 1 to 1.5 stops.“

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