There are several things about our craft of photography that may not be known to new photographers or those who have not studied it in school. Optical science is involved, as well as a LOT of math as it relates to the law of the inverse square, and how apertures and shutter speeds work mathematically. But one of the most important aspects and most useful in all of photography is how 18% gray is a part of the DNA in photography and now that we have left film behind for digital, it still reigns supreme and dictates much of our world in terms of exposure controls.
By definition 18% gray is the “mid-point between black and white on a logarithmic or exponential curve.” Think of it this way; it is simply halfway between black and white. It is the average in terms of scene brightness and has for many years been the one constant thing that photographers use on which to base their exposures on.
18% gray cards have been used for decades as a tool onto which photographers could aim a spot meter and use the brightness as the exposure of the scene. Interestingly enough, 18% gray cards will reflect the equal amount of light that they receive. In other words, this mid tone is the only tone value there is where you can use an incident meter reading from a hand-held incident meter (to measure light falling on or striking your subject) OR a reflective spot meter reading (designed to yield a brightness relative to 18% gray) and get the same reading with either meter. Keep in mind that a spot reading off of a white bridal gown will render an exposure that is too dark a because the meter will try to make it read middle gray. The same situation applies if there is a very dark or black tone you are using for a spot reading. It will render brighter than it should truly appear. Oh, and if you are working in Photoshop or Lightroom, An image of an 18% gray card should reveal a histogram with one pixel width in the very center of the histogram.
In addition to using a true 18% gray card for taking spot meter readings in order to get a good exposure in any given scene, there are other tones that are of the same relative value as 18% gray. For example, the color red is a good choice to use for a spot meter reading and will render a middle gray tone. The average green grass that you might find in a city park will be the same. As long as the amount of light that is striking either of these tones is the same amount of light that is striking your subject, the exposures will be great.
One final thing to discuss is what we call the true range of contrast in a scene IF it will be printed as it’s final usage. Your eye has the ability to “see” an extremely wide range of dynamic contrast and the cameras today also can yield a great result in scenes with a very high degree of contrast. However, most photographic paper can only hold full detail in approx. five stops of light. Therefore, it is our responsibility to know the range of contrast on each scene we photograph so we can be assured that it will give a full range image that is easy to print, has all of the important subject matter exposed correctly, and will give predictable results for the final print.