Can I use the ExpoDisc to set a custom white balance if the manufacturer recommends a white card, rather than a gray card?
Yes. The ExpoDisc reference image is usable by those digital cameras whose manufacturers recommend either gray, or white reflective surfaces for setting a custom white balance. An accurate custom white balance requires measuring the exact color temperature of light illuminating the subject. The intensity of that sample light is not relevent, provided the sample is neither severely over, or under exposed. The ExpoDisc is calibrated for 18% light transmission, meaning a properly exposed reference image through the ExpoDisc will produce a usable 18% neutral gray.
