12% Gray card

Started Aug 3, 2011 | Discussions thread
Jack Hogan Veteran Member • Posts: 7,420
18% card

stctbkb wrote:

Leo, I struggled for a long time because I read from misinformed photographers on forums that light meters were "all" calibrated at 18% gray. Turns out that's widely believed, but just not true. See Sekonic's web site where the specifications for the venerable L-358 is clearly indicated to show calibration at 12.7 K factor (see A.Adams, The Negative, page 34).


See also: http://www.bythom.com/graycards.htm

Also see: http://dpanswers.com/content/tech_kfactor.php

Well intentioned, yet incorrect information is still wrong.

Both articles above are full of unclear and at times incorrect information. This is my understanding:

Ignoring artistic intent, maximum 'proper' exposure in modern DSLRs is one that just avoids saturating the sensor, which would result in a clipped or bloomed camera output. It should be produced by luminance from the brightest diffused white surface in the scene reflecting 100% of the light that is illuminating it - no other reflected light in the field of view (other than direct light sources or 'specular' highlights) should be brighter than this lest it be blown. This is 'proper' exposure because any exposure higher than this will result in blown reflected highlights and any exposure lower than this will result in a noisier image. It works because the human visual system works relatively and not absolutely.

How do we establish maximum exposure? An easy way is to place a neutral object, say a card, of known reflectance in the scene and then calculate saturation exposure accordingly. For instance, if we placed a 100% reflecting white object in the field of view and chose shutter speed and aperture so that it was just below clipping, we would have achieved 'proper' DSLR exposure. A slightly better way, and one chosen by the various metering standards of our day, would be to use a neutral object of perceived reflectance half way between white and black - so-called mid-grey. From the perceptually relevant L*ab color space, which has luminance L* values from 100 (white) to 0 (black), mid-grey is represented by L*=50, which is approximately 18% of 100% reflected white in linear luminance terms. So our neutral reference mid-grey card should have a reflectance of 18% to conform to the known standard. (Any other shade of grey/percentage would have worked, as long as it was agreed upon - but 18% became the standard in part because it is a decent approximation of the average luminance from a natural scene).

However, given digital's hard cut-off at sensor saturation, and because 100% diffused reflectance is hard to peg down, to make sure that the brightest whites have some leeway and are not accidentally clipped the standard mandates that the recommended exposure should leave 1/2 a stop of headroom in the highlights. That would mean that 'maximum' reflected diffused white should appear at 70.7% [=100%/sqrt(2)] instead of 100% of saturation and mid-grey therefore at 12.7% [70.7*18.7%] of saturation. Another way of saying this is that a spot meter when aimed at a standard 18% mid-grey card should produce values for shutter speed and aperture that would result in raw values captured for the card 3EV below the top of the histogram [log2(12.7/100)= -3].

However we typically see histograms out of 255 and in our chosen color space, not out of 100% in linear raw space. 12.7% in linear space corresponds to 100 out of 255 in sRGB color space , for instance - 18% would have corresponded to 118 out of 255 in sRGB. Therefore, in an ideal world, if you spot meter off an 18% neutral card and take a picture of it you should see a spike a bit to the left of center of your sRGB 0-255 histogram, around value 100.

In practice, though, your camera/raw converter will apply white balance and several 'image enhancing' curves during rendering, so you will most likely see a slightly different value.


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