Understanding white balance/gray cards????

[mattyd]

Hi all,


I work primarily as a 3d designer and im trying to understand how to correctly measure white balance and apply WB correction in photoshop.

3D renders typicaly render with no WB applyed, its my understanding that old style cameras has no built in WB and you could mesure the WB by using white or gray cards. Im not 100% sure but this same princible can be applyed to 3d renders. But unfrtunalty i dont really understand how the gray cards are used......so, can somone give me an idea how to use them!?

cheers!

 

[Garbz]

Old style cameras (i assume you mean film) it depends on the film. Most film is balanced to around 5100K (daylight) but you can get tungsten balanced film as well as others.

With digital cameras we have the benefit of choice both when taking the picture and, if you shoot in raw also when editing the picture.

A grey card is a card which is true grey. All colour components are the same brightness. The standard card is the equivalent of 128 128 128 RGB values. Although others exist and you could even use a pure white card providing it's underexposed slightly.

In software you can then autowhite balance to this card. Providing the value of the pixel on the card is not 255 for any of the 3 colours and assuming your card really is white the software scales the colours proportionally so that the result is averaged so all colour values for that pixel are again the same brightness.

For example if you light a grey card with a halogen light and photograph it with the white balance on the camera set to daylight the RGB value may read more like 160 150 80 (yellowish), set the white balance to halogen and the entire colour balance moves to the blue side of the scale and you may again end up with something close to 128 128 128.