One of the reasons why you are reading this blog post is because you have heard a lot about the 18% gray card, and wanted to know the actual reason why it is used by professional photographers. Why 18% gray, why not 20% or any other number? In this blog post, I would be explaining the reason behind the use of 18% gray card to achieve the perfect exposure.
The digital cameras we use to click pictures are by default set to expose for 18% gray. What it means is that the camera is engineered in such a way that it reads each and every subject as 18% gray. But why?
If we look around, each object reflects different amount of light. For example, snow reflects around 80% of light whereas a dark colored stone would reflect about 10% of light or even less. Basically white objects reflect back more light whereas dark objects reflect very less light.
By analyzing that the average percentage of light reflected by objects around us is 18%, the same number was considered as the standard in all the digital cameras. 0% represents dark black and 100% is used to denote pure white, as 18% is taken as standard and it appears gray in color, thus we call it 18% gray.
The digital camera meter is engineered in such a way that it exposes every object for 18% gray, irrespective of the object’s ability to reflect back the light.
If you photograph a white car, your camera will try to under exposure the white car because it is engineered to see everything as 18% gray, thus converting it into underexposed frame. Similarly, if you point your camera towards a black car, it will try and overexpose the frame because your camera is trying to see the car as 18% gray.
How to set correct exposure using 18% gray card
Place the gray card in front of your subject, under the same lighting condition which you would be using for your final image. Select the spot metering mode, put your camera on manual mode and point your camera towards the gray card.
Now set your camera settings (aperture, shutter speed and ISO) until exposure scale on your camera comes to ‘0’, which denotes that your camera exposure is set correctly. You can now click photos using the camera setting which gave you ‘0’ on the exposure scale and you will get properly exposed photos.
This is not a hard and fast rule that you have to use the exact settings achieved using the gray card, buy in majority of lighting situations using a gray card would give you perfectly exposed photos.
NOTE: The lighting conditions should be the same at the of clicking the test shot using the gray card as well as at the time of shooting the actualy photo of the subject.
Where to buy the 18% gray card
You can easily find a good 18% gray card online at Amazon.in, or you can visit a camera store near your place and grab one. Some websites might suggest to print out the 18% gray colored sheet, but trust me you would not get the exact color tone when you print it.
Image by: Peter Bitic