The Grey Card ('The Gray Card' to some)When taking digital photographs, a grey card can be an essential accessory in creating white balance and getting precise representation of colour. A digital camera will record the colour of light very accurately. The eye and brain are extremely adaptable to variations; digital is not, it records what is there. Something might look white to us, but if it has a colour cast, that will show on the digital file. By using a grey card, the camera can be set to a true white balance, resulting in a more accurate recording of all the colours. This is particularly important when there is mixed light from different sources. There might be a mixture of daylight and tungsten and even a bit of fluorescent thrown in.
A grey card is a piece of card which is grey. These days, they are often made of a more durable material than card but they will be a neutral grey. The older photographic grey cards are not suitable because they are not neutral enough. They were used for judging exposure rather than judging a white balance and colour neutrality.Photographic grey cards have a reflective surface of 18%. Digital grey cards usually have a reflective surface of 18% or 25%. That doesn't matter; what does matter is their accuracy, they must be neutral grey. The grey card is put into the image so that the light falling on the subject also falls on the grey card. The white balance reading is taken from the card. The photograph should be taken in Raw and not as a JPEG in camera.
The sample on the right uses a card with a true white, an 18% grey and a black. Setting the image to these tones will give as accurate interpretation of the colours as possible. Without a grey card there is no chance of getting the colours of the paints looking like the originals. Where lighting conditions are completely controllable, such in a studio, a grey card is still an essential gadget. Exposure and colour balance need to be consistent in every image. Relying on the camera's automatic reading of the conditions means that when the subject is changed, the readings will change. With a grey card, the digital photographer can take a specific reading from a single target point and the resulting settings for the white balance will be good for all the images in that lighting, even when the subject matter is changed. Two objects might look the same colour in one type of light, but can look different colours in a different light source. For example, a matching scarf and handbag might look different colours in a second type of light. This is due to the amount of ultra-violet light the objects absorb and reflect. Using the grey card can help minimize the problems here.
With the Cube, the complete range of tones can be measure, together with neutrals along the range. Highlights, mid-tones and shadows can all the set accurately. Again, Raw images should be taken. A photograph would be taken with the Cube in the shot, as well as all necessary pictures without the Cube.
In Camera Raw, all sliders would be set using the Cube version of the image.
Grey cards and colour cards are available from the following sources: http://www.rawworkflow.com/whibal/ The Cube illustrated here comes from: http://spyder.datacolor.com
  
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Short of using a proper copying bench with flash or tungsten lighting, then the best way to photograph a painting is outside. Choose a bright day with diffused light, and be out of direct sunlight. The only way to make an accurate white balance is to use a grey card.
The Cube takes the grey card to a more precise level. A Cube is particularly useful in a studio environment.
