The general understanding that reflective light meters are calibrated to 18% is not quite correct. Canon and Nikon meters are set for 12.5%. Light meter calibration is based on formulas that include a K or C value which is from a large statistical sampling of images shot having better over all exposure from average angles of view of images shot outdoors “under natural disposition of light and shadow” (Ansel Adams, The Negative, Pg 43), a fudge factor if you will. My under standing was that they increase density in the shadow areas, with 12% being about the middle gray calibration center. If you shoot an image of a 18% gray card and examine the histogram, sure enough the 12.5% exposure results in a curve to the left of center, about 1/2 stop. One would assume that I would need to add 1/2 stop exposure to reproduce my medium gray 18% back to it’s middle gray value. And the K or C factor was intended to add density, which means more exposure. Seems like our K factor adjustments should result in a middle gray of maybe 22% or so, Higher, not lower than 18%. But I did not make any exposure adjustment in my test (again my camera is calibrated for 12.5%), I wanted to see how this exposure of the gray card would look against the middle gray value of Photoshop’s 50% middle gray. Now it seems like the Photoshops 50% middle gray value should be truly middle gray, half way between full white and full black. In fact I shot the Canon 5D mk II, a Gossen Incident light meter, and Minolta spot meter against the same lighting and gray card and compiled the images into a Photoshop image file without exposure adjustment, and compared them all against each other. They were all scary close to each other, less than even a 1/3 stop difference from the 50% middle gray value of Photoshops middle gray value, and all the light meters were slightly darker. The only really thing that is clear is, is this really isn’t vary clear. Ansel Adams suggested that without the K factor used the middle gray value would be 18%. Is 12 or 13% of one log scale’s middle gray reflectance value able to equal and different scale of printing inks of 18% reflectance, and that maybe we should not be using the word reflectance for one scale or the other? Please mister Wizard! where is middle Gray Really?