The Benefits of Grey Card
Grey cards are even more important to the digital photographer because the digital photographer has a much greater range of tools for manipulating the image to a far greater degree than was hitherto possible. Subtle changes can be made with ease giving the photographer unlimited flexibility in the manipulation of an image.
Before covering the uses of a grey card let us look into their construction.
Grey Card Construction
The colour of the grey side should be as close as possible to the mid point between pure black and pure white. This is usually determined as 18% which equates to Zone V in the Ansell Adams Zone System.
What is important is that it is as matt as possible to avoid reflections. Another fact that should be taken into account is if the card contains artificial brighteners such as are used in detergents and many paper products. There is no doubt that the use of these will affect exposure readings taken from the card, but as to how much depends upon the amount of brighteners used. It is easy to detect if a card is using these materials by the use of a cheap handheld device used for detecting forgeries by Ultra Violet light. If a card is being used for medical photography then it essential that it is washable for reasons of hygiene.
Let us look into the most important uses of a grey card.
As exposure meters can easily affect factors that can distort exposure such as sky, a reading taken from a grey card from the same point of view as the camera and under the same lighting conditions, will give a more accurate reading.
Under standard conditions the average exposure meter does quite a good job, but in conditions such as snow then a grey card becomes essential. Some suggest that exposure in snow be decreased by a couple of stops but a reading from a grey card will be more accurate. In reality an exposure taken with a reading from a grey card is a reliable way to determine exposure.
As we know, the colour of light not only varies with the time of day but with the source of the light.
Digital camera users have an advantage over those using film, as they have the ability to control the colour balance automatically by using Automatic White Balance (AWB).
However, these systems are often fooled, especially if there is a particular colour dominating the scene. In such cases, and provided that the camera has the option to override the AWB setting, we can instruct the camera to use a particular colour temperature so that colours that appear white when viewed also appear white in the image.
However, some digital cameras are more susceptible to this problem than others, so that it is worthwhile making a test to establish how well your camera does.
Firstly take a picture of some colourful subject using the camera’s AWB.
Next, using the card, which should be lit in the same manner as the subject and should fill the frame of the camera with any highlights or reflections being avoided, activate the Manual white balance setting.
The camera will now be set for the prevailing lighting conditions and will retain this setting until another white balance is per-formed or the camera is reset to AWB.
We should now take another picture of the same subject so that we can compare the two.
To achieve the most accurate colour balance include in the scene (perhaps outside the printing area or on a separate frame), a grey card lit in the same manner as the subject.
For the digital photographer a number of imaging software applications such as Paint Shop Pro and Photo Shop have the facility to set the grey point.
It is possible to save this “white balance adjustment”, so that it can be used again with other shots in similar lighting conditions without having to use the card in each and every shot.
Since no two light conditions are the same, the best results will be obtained by taking a grey card shot at the beginning of each shooting session for each lighting condition.
As in all things photographic, experience is the key to success and by experimenting with the card and by knowing its limitations it will soon become a trusted tool that you will take everywhere.