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 a thermometer stays 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 the 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 body temperature. Kind of like a reference point or calibration point if you like. Similarly a camera’s light-meter is calibrated for, surfaces that reflect 18% light or surfaces with 18% reflectance, or in other words, the cameras calibration point. Any deflection from this point will infer an under or an over exposure. 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) in such a way that the frame is filled with all black background, the famous example is when you shoot a black cat resting on a black couch. Place a black cat on a black leather couch and then compose the frame filled with the black cat and the black couch.
The results will be surprising! you will see that the camera will fail to distinguish 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 composing a frame will all white background and a white object in the foreground the results are more or less similar.
So, how do we fix this and 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 as a reference point to determine the exposure readings”.
The light meter inside your camera is usually 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 to the subject. Most SLR’s today implement the TTL (Through the lens) metering for increased accuracy in determining exposures.
So, under any given light conditions if you read the light from a 18% gray surface then your readings are calibrated for a good exposure.
Place an 18% gray card or any surface that has 18% reflectance (human palm apparently has 18% reflectance), next to the subject of interest. Zoom in with your lens (or go closer if you are zoom impaired) and now focus on 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 reproduce in the picture as opposed to the one the light-meter has suggested you to have.
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.