What's a Gray Card and Why You Should Own One
by Bob Shell

All light meters including those in your cameras and hand held meters must be calibrated to assume a certain percentage of light being reflected from your subject. Basically there are two types of light meters, those designed to measure the light reflected from the subject and those designed to measure the light falling on the meter itself.

Each type can be used successfully to determine correct exposure so long as the photographer knows the angle of measurement and knows how to isolate what is being measured. In addition to knowing the area from which the reading is taken, it is also important to know the approximate reflectance of that area. This is the part that can make using a reflected meter difficult, since the meter can't determine subject reflectance for you and you must mentally calculate it.

You can simplify the mental gymnastics by using a standardized surrogate subject such as the common Kodak Gray Card, which has a stated reflectance of 18%. However you must use the Gray Card correctly, which means to angle the card toward the light source about 1/3 of the angle between the camera lens axis and the light source, both horizontally and vertically, rather than holding it flat in front of the subject as many amateurs do. If you have done reading about or studied photography, you may have heard that 18% reflectance is a photographic mid tone. This has been stated in many books and taught in photography courses for some time Unfortunately, it is wrong. There has never been an ISO standard created for mid tone reflectance, so by default the older ANSI standard applies. The ANSI standard specifies a reflectance of about 12.5% for a photographic mid tone. This is exactly one half stop less reflectance than 18%.

What happened to that missing half stop? I found it. In 1999 Eastman Kodak decided to revise the Kodak Gray Card since the old text and packaging had become dated, and they came to me and asked me to write new text for them. This gave me the incentive to delve into just why the Gray Card was made with 18% reflectance (actually it is 17.7XXX) when 12.5% was the standard and made much more sense. It came out after a lot of digging that the last revision of the Gray Card had been in 1979, and at that time a mistake was made in the instructions and this mistake was not noticed in twenty years. The previous instructions had included the statement that you should open up 1/2 stop from the reading obtained from the Gray Card, which would compensate for the difference perfectly, but this was somehow left out of the 1979 instructions.

To make a long story short, I put it back into the revised instructions in 1999, so if you have an older Gray Card you should remember this correction, and apply it, and if you buy a Kodak Gray Card you should make sure it is the newer version. For a lot of photography an error of half a stop will not be a problem, such as many types of black and white film and color negative films. But when shooting color slide film a half stop error can ruin an image.

I’ll talk more about how to use your light meters properly in future columns.