Getting a good white balance is critical to getting a great image, especially if you are not going to be color correcting or grading your footage. When shooting traditionally ie – not with log or raw – A common way to set the cameras white balance is to use the one push auto white balance combined with a white target. You point the camera at the white target, then press the WB button (normally found at the front of the camera just under the lens).
The white target needs to occupy a good portion of the shot but it doesn’t have to completely fill the shot. It can be a pretty small area, 30% is normally enough. The key is to make sure that the white or middle grey target is obvious enough and at the right brightness that the camera uses the right part of the image for the white balance. For example, you could have a white card filling 50% of the screen, but there might be a large white car filling the rest of the shot. The camera could be confused by the car if the brightness of the car is closer to the brightness the camera wants than the white/grey card.
The way it normally works is that the camera looks for a uniformly bright part of the image with very little saturation (color) somewhere between 45 and 90IRE. The camera will then assume this area to be the white balance target. The camera then adjusts the gain of the red and blue channels so that the saturation in that part of the image becomes zero and as a result there is no color over the white or grey target.
If you fill the frame with your white/grey card then there can be no confusion. But that isn’t always possible or practical as the card needs to be in the scene and under the light you are balancing for rather than just directly in front of the lens. The larger your white or grey card is the more likely it is that you will get a successful and accurate white balance – provided it’s correctly exposed and in the right place.
The white target needs to be in reasonable focus as if it is out of focus this will create a blurred edge with color from any background objects blending into the blur. This could upset the white balance as the camera uses an average value for the whole of white area, so any color bleed at the edges due to defocussing may result in a small color offset.
You can use a white card or grey card (white paper at a push, but most paper is bleached slightly blue to make it look whiter to our eyes and this will offset the white balance). The best white balance is normally achieved by using a good quality photography grey card. As the grey card will be lower down in the brightness range, if there is any color, it will be more saturated. So when the camera offsets the R and B gain to eliminate the color it will be more accurate.
The shiny white plastic cards often sold as white balance cards are often not good choices for white balance. They are too bright and shiny. Any reflections off a glossy white card will seriously degrade the cameras ability to perform an accurate white balance as the highlights will be in the cameras knee or roll-off and as a result have much reduced saturation and also reduced R and B gain, making it harder for the camera to get a good white balance. In addition the plastics used tend to yellow with age, so if you do use a plastic white balance card make sure it isn’t past it’s best.
Don’t try to white balance off clouds or white cars, they tend to introduce offsets into the white balance.
Don’t read too much into the Kelvin reading the camera might give. These values are only a guide, different lenses and many other factors will introduce inaccuracies. It is not at all unusual to have two identical cameras give two different Kelvin values even though both are perfectly white balance matched. If you are not sure that your white balance is correct, repeat the process. If you keep getting the same kelvin number it’s likely you are doing it correctly.