Start with a gray card which is known to be color neutral and 12% reflectance (not 18%) which is the ANSI standard for meters. Older pre-2005 Kodak grey cards were 18% and not color neutral. They became color neutral when upgraded around then but Kodak stayed with the 18% reflectance rather than changing to the newer 12% ASNI standard. The reason for that apparently was lobbying by Ansel Adams who had based his perceptual based Zone System on the 18% reflectance.
The difference in reflectance of the card does not affect setting WB with it, only how exposure indicated in the histogram. With a digital camera the spike from a 12% will be centered when highlights are correctly exposed because modern cameras use the 12% standard for exposure. The more reflective 18% card will put the card spike right of center in the histogram and setting exposure by centering it will result in clipped highlights.
If your camera has a custom wb function you shoot the card then use that frame for setting custom WB. I put the WB in daylight mode when shooting the card then shoot it again after using the first image to set custom WB. That way by scrolling between the two frames in the playback the difference between the color of the light and daylight can be visualized.
When I switched to digital I started using a white towel to set exposure, cognizant that 255 values need to be reserved for specular highlights as in the top edge of this shot created with the back rim light to provide separation with the white background and textured whites of the towel which get recorded in the 240-250 range.
The challenge in exposure is matching scene range to sensor. Here’s how I suggest doing it:
1) Select the aperture you desire for DOF, e.g. f/8 to record an entire face in focus, then put fill directly above camera lens because there it will not create any shadows the camera sees. Raise intensity of fill alone until the camera is recording the desired amount of detail in the shadows. I used the knob on the stand but a black towel works better as a visual gauge.
2) Turn on the off axis key light. Placing it 45° to side of nose and about 30- 45° above produces realistic rendering of 3D objects in 2D images. Keep raising the intensity until clipping of highlights in the white towel appear in the playback.
With that approach the lighting ratio is controlled with step one, the setting of the fill light for the shadows. Other than any spill fill from key light off the walls and ceiling it will not affect shadow tone. In a small studio space with white walls you might consider covering the walls with black King size sheets for better control of lighting ratio. Thats what I did in my basement studio.
As seen in the gray scale of the color checker target on the test card setting lights based on detail in textured black and white towels matches scene range to sensor perfectly. Any object placed in that lighting will appear as perceived by eye with modeling similar to the direction of 10 am / 2pm sunlight which creates a flattering “mask” of highlights on the front of the face which helps pull and hold veiwer attention there.
Most beginners put key and fill on opposite sides but that results in unfilled crossed shadows in deeper areas like smile lines. Try both approaches and compare results paying addition to the tone of the darkest shadows on the face and whether or not they are getting any fill other than the spill fill off the walls and ceiling of the studio space. Centered fill will create a second catchlight in the center of the pupil but it is easily eliminated in post processing.
