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WB test target reflectance - for Michael Tapes


Doug Kerr

Hi, Michael,

It is great to see a forum area devoted to this topic.

You have often emphasized the desirability of utilizing, as a target for white balance "calibration", a test target that was not only reflective-chromaticity neutral but which had a reflectance in an appropriate range. (Read, "well higher than 18%".)

I wonder if you could expound a little on the optimum reflectance issue. As I understand it, the premise is that an exposure result for the target that lies in some upper region of the camera's response will best allow the camera (or WB analysis capabilities in a raw converter) to most accurately determine the chromaticity of the reflected light from the target (and thus the chromaticity of the illumination, which is what we really seek to detemine in this exercise).

If the image of the target does not fill the entirel frame (or at least the area of metering sensitivity), then in a metered test shot, this "fairly high" exposure result for the target of course would come from a target of relatively high reflectance (essentially, a reflectance that is high compared to the actual average reflectance of the whole scene).

Of course, if one has taken a metered "frame-filling" shot of the target for WB calibration, it would not seem that the reflectance of the target would matter - the exposure result will be whatever the camera's metering calibration produces for a uniform-luminance scene. But perhaps that is not the practice of most interest.

Am I at all on the right track here?


Best regards,


Michael Tapes

OPF Administrator/Moderator
Yes. Exactly. Since a WhiBal or similar product is not shot full frame, the exposure of the WB gray area will be the bright gray that it is in real life. Obviously if you spot meter on gray, white or black they will all come up at ~18% gray. So........if you use a certified neutral 18% gray card (that is really neutral) and overexposed it by about 2 stops, you would have the equiv of a WhiBal. But in most cases you will simply want to throw the card to the subject and take a Reference shot in which case the ref card has to BE the light gray and not the 18%.

That is why when shooting a Custom Whiter Balance regardless of device, you really want to use a LIGHT gray, rather than a White or 18% gray, because the camera will be able to make a better Custom White balance. it might be that some cameras (not Canon) automatically take this into account and that is why one MUST use an 18% gray for that specific camera. Same with White. But one would have to KNOW the innards of the camera to make this assumption. Only Canon (AFAIK) actually uses a developed picture to set the CWB. Nikon and others use a virtual capture to set the WB and we never get to see it. We just get to see OK or NG.

Hope that this answered what you were asking.

Thanks for being the first poster!

Doug Kerr

Hi, Michael,

Thanks for filling all that in. It jibes well with my presumptions (always nice to have that happen!).