How to get a correct exposure in digital photography - Part III

Part III Using the 18% gray card to get a correct exposure.
Reflected light meters are calibrated to give an accurate exposure when pointed at subject with reflectivity somewhere near 18%; the exact value varies and the details are complex. By placing a gray card in the scene to be photographed and taking a reading off of it with a reflected light meter, the photographer can be assured of consistent exposures across their photographs. This technique is similar to using an incident meter, as it depends on the illuminance but not the reflectivity of the subject.
Using the gray card to determine exposure
For artificial light place the card to and in front of the subject, aimed halfway between the main light and camera.


When using daylight place the card in front of the subject pointing at the camera.

You can take your your readings with the card near your camera if you place the card so it is in the same relationship to the light source as the subject.
Exposure compesation:
- if the subject is very dark -> increase your calculated exposure by 1/2 or 1 stop
- if the subject is very light -> decrease your calculated exposure by 1/2 or 1 stop
- if you are unsure of the best exposure -> use bracketing technique to be sure you have the appropiate exposure.

Using the gray card to determine lighting ratios
Lighting ratios are very important in studio portraiture and product photography because they reflect how much contrast there will be between important light and dark areas in a picture.
Some contrast is essential to give a three-dimensional quality to a photograph, however too great a difference will exceed the ability of the film to capture both the highlights and shadow detail. For best quality prints, the lighting ratio should not exceed 3:1 for color film and 5:1 for black and white film.
In calculating a lighting ratio, the combined illumination from the main light and the fill light are compared against the illumination provided by the fill light alone. This ratio is expressed as two numbers, as in 2:1. The first number represents the combined strength of the main light and fill light, while the second number represents the fill light alone.
Lighting Ratio Technique
You can use an incident light meter to determine the lighting ratio, or a gray card and your camera light meter can do the same. Let's see how this is done.

Placing the gray card close to the subject, first read the main light plus the fill light, positioning the card for the highest reading on your light meter. Usually this means aiming the gray card at the main light. (Position 1 in the diagram.) Note the reading. Turn off the main light, aim the gray card toward the camera lens (position 2) and read the fill light illumination. Note the reading and calculate the difference in f. stops.