Why is white balancing in video always with a white card, wouldn't a mid-grey card work just as well?

it depends on what you want your picture to look like... every different color is going to give you a different white balance. i mostly use a really light blue card.. if the grayish card gives you the colors you're after then stick with it

Why is white balancing in video always with a white card, wouldn't a mid-grey card work just as well?

FWIW, I have an 18% grey card that I used to w/b outdoors the other day just to see how it would work. It matched the colors in the viewfinder with what my eyes were seeing perfectly.

-gb-

Grey card will work fine.
The white balance operation is matching the RBG channels values to each other assuming it's viewing a reference that is reflecting equal levels of these colors to the camera.

What will "trick" white balance is off-white colors that contain some tinting towards red, green and/or blue.

so why always the white card for WB in the DV world? A bright white card can blow out easily...

White cards do not blow out. The iris needs to be adjusted to your source. I use www.warmcards.com on almost all of my interviews and exteriors.

Gray card is fine. White card is more likely to blow out than a gray card. I always argue that balance to pure white or black is hitting the extremes of the imaging systems, where they are more prone to error. Balancing to a properly exposed gray card hits the system in the middle, where it will be most accurate.

these warmcards seem cool, i'd never realised what you use to white balance could make such a difference.
one thing i've always noticed. should you hold the card exactly where your subjects face will be, or will it be just as effective if you just hold it just infront of the lens?
Thanks

Ideally the card should be in the same light as your subject. That is the appropriate way to do it for accurate whtie balance. The light directly in front of the lens probably differs from the light your are casting on your subject. Of course, you can break the rules for creative effect.

Ideally the card should be in the same light as your subject. That is the appropriate way to do it for accurate whtie balance. The light directly in front of the lens probably differs from the light your are casting on your subject. Of course, you can break the rules for creative effect.

thanks. thats what ive always done up to now, i just wondered if for example i needed to zoom in massively to ensure the card fills the viewfinder, this would effect the whitebalance?