Any SLR user (advanced) who shoots in “M” mode must have surely come across the term “18% gray”. So what exactly is this 18% gray and how does it affect exposure?
Mostly all cameras have an in-built light meter, which are calibrated for 18% gray i.e. the reference point for a camera's light-meter to determine exposure are the surfaces that reflect 18% of light. It is similar to a thermometer: At room temperature the thermometer stay at 94-97 deg. As the temperatures rise above this point the mercury starts reacting and shoots up the meter depending on the temperature it has been subjected to. So a reference point for a thermometer are the points at which it stays stable. Depending on this you can say if a person has a fever or if there is a drop in temperature. Similarly a camera’s light-meter is calibrated for, surfaces that reflect 18% light, or in other words, surfaces with 18% reflectance. The light meter in the camera measures light and produce a medium 18% gray tone. So when you see through the view finder the camera's light meter is actually suggesting you to adjust your shutter speed for 18% gray tones.
For instance if you compose a picture with your SLR camera in such a way that the frame is filled with any black background, the famous example is when you shooting a black cat on a black background. Place a black cat on a black leather couch and then compose the picture with the whole frame filled with the black cat and the black couch.
The results will be surprising you will see that the camera will fail to understand the shades of black and your picture will have a noticeable layer of gray on it. This is the work of the light meter inside your camera.
Similarly try shooting just the snow in your frame and the results are more or less the same.
So, how do we expose correctly? or how do we record the blacks as black and whites and white and not some 18% gray?
The answer is very simple: “Use an 18% gray card/surface to determine the exposure”.
The light meter inside your camera is a reflective light-meter i.e. the camera reads the light that is bouncing off the subject as opposed to the incident light-meter which reads the light that is falling on the subject. Most SLR’s today implement the TTL (Through the lens) metering for increased accuracy in determining exposures.
So, under a given light conditions if you read the light from a 18% gray surface then your readings are calibrated for a good exposure.
Place the 18% gray card or any surface that has 18% reflectance, next to the subject of interest. Zoom in with your lens (or go closer) and now focus the gray card, adjust your shutter speed such that the camera’s light meter indicates perfect exposure. Now remove the gray card, zoom out and recompose your frame with your cat or whatever it is, DO NOT CHANGE YOUR EXPOSURE SETTINGS (let the reading be the one that you took off from the gray card) and then shoot et voila you now have a better and most realistic colors and shades of the picture as opposed to the one that the stupid light-meter has decided for you.
So now where do I get a gray card?
You can pick them at any of your local store for a few dollars.
What If I don't have one or If you forgot one at home?
Most of the camera bag manufacturers (LowePro) provide the adjustable partition strips inside the bags with gray color. you can just strip off these and use it as your gray card. Alternatively an average human palm is supposed to reflect 18% gray so you are never out of options.
Tricky situations can be encountered when you are shooting a landscape with hills, clouds, flowing water, rocks and the green valley. Obviously, one cannot walk up to the hill place your gray card, come back place it on the water and then on the greens, take readings and shoot. The solution for this is quite simple and also complex