White balance measurement - "over the shoulder"

There are some (Me, teacher, me!) who hold that the only way we will get a "valid" incident light white balance measurement with a diffuser on our camera, used from the camera position, is if (by good luck or good planning) the chromaticity of the illumination on the diffuser there is similar to that of the illumination on the subject.

Will Thompson, picking up on this, said that (especially in a "clear" outdoor situation), when for practical reasons we have to measure from the camera position, we might expect the best result by aiming the camera-plus-diffuser away from the subject. He calls this the "over the shoulder" technique.

It makes a lot of sense. If, for example, the subject is illuminated by "north" light, then we would want the diffuser to capture the same for measurement.

Naturally, this probably wouldn't work well if there were a substantial building of some bold color just behind the photographer! But otherwise it seems like a pretty good idea. Thanks, Will.

By the way, if you are looking to the exposure metering system to give some desired exposure result for the "reference frame", don't forget to cork the finder eyepiece with your thumb when shooting "over the shoulder".

Of course, none of this has for me the attraction, from a purely colorimetric theory standpoint, taking all the cosines into account, of the model clasping a well-calibrated gray card to her ample, heaving bosom.