With today's brilliant camera technology, you might think that the use of a grey card is no longer necessary at all. But in truth, it can still be an invaluable tool for professional photographers and keen amateurs alike.
Originally, a grey card was used with a film camera as an exposure tool to help guarantee a natural looking photo. They were also used by more advanced photographers as an accurate colour balance. Photographers would include the card in the scene so the lab could adjust filtration and prevent any minor colour casts.
Position a grey card in front of the camera and take an exposure reading. Then, using exposure lock or manual, set this reading on the camera, remove the card and take your photo. This exposure reading from the card represents a mid tone of any scene / subject so any other darker or lighter tones in the scene / subject will record with correct brightness. The card needs to be in the same lighting condition as your scene / subject, otherwise your card's exposure won't relate to your subject.
So what use can these cards possibly be today, with all our advanced DSLR setting and functions? Surely, cameras are now advanced enough to recognise the correct white balance and exposure details. Sometimes, though, depending on the condition, this is not the case. In bright conditions or snow, for example, the camera can still be tricked into making the image too dull.
Grey cards can be used with modern DSLRs to determine a custom white balance for a shot. Take a picture with the grey card being held in the shot (if you're going to use it as a reference for photo editing) or you can fill the frame with the grey card. Then, select manual white balance and take a reading. Your shots should then be perfectly balanced as the card reflects a constant 18% of any colour of light, meaning it is colour neutral.
So although most photographers rely on their cameras' sophisticated exposure metering systems, a grey card can still come in handy with trickier lighting that can fool the camera. They are relatively cheap and can help you find the perfect colour balance for your shot. Why not have a go with one!