Yea, I got that. But is there anything wrong with the ExpoDisk as a WB target? Other than the additional cost?
Hi Robert,
Suppose you want to take an image of a red tomato on a red backdrop. What would the Expodisc (which is intended to average the scene
Luminance) see? How would an Expodisc (or similar) be of use to set the White balance in a way that Red stays red, but also potentially other colors as they should? All it would do is create a blurry red dominated impression of reflected subject colors, which will get color corrected by a shift to Cyan, in order to neutralize the color 'cast'.
The goal of a White balance tool is to get an impression of the spectral qualities of the illuminant(s) of the scene, the lightsource(s) and ambient reflections, that will determine how the subject color reflections will look.
Remember, illumination spectrum, minus subject spectral absorption, is reflected color spectrum. A spectrally neutral subject (e.g. a graycard) will allow to determine the illumination spectrum (an approximation due to the limitations of the tri-chromatic measurement and assumption of a continuous (blackbody like) spectrum), which is the input required for White balancing. We do not want to measure the subject colors, but we want to measure the illumination spectrum/color in order to approximate correct rendering of any color reflection.
Also remember that the scene illumination mostly comes from illumination sources that are behind the camera and to it's sides. Measuring (averaging color
reflection) of what's in front of the camera only makes sense if it is of a subject with known spectrally neutral reflection (of that illumination behind the camera), or the same color as the illumination.
Cheers,
Bart