Setting white Balance with 18% card.

[JX]

A friend told me she sets her custom white balance on a Mark II with an 18% gray card. Is this a valid method for setting a custom white balance? For film I used the gray card for getting a correct exposure. I thought setting the white balance has more to do with the temperature of light. Could someone shed some light on the subject? (Sorry that was bad.)

Thanks

[Jon]

Sure. The 18% grey card is two things - a middle tone, representing an average subject's reflectance for exposure, and colorimetrically neutral. Because it's color-neutral, it can also be used as the subject for setting custom white balance (either side - the 18% grey or the 90% white). It won't impart any off-tones to the effort as might happen if you were, say, using a bride's old ivory wedding dress as the subject.