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The middle gray area question, 10-18%

randy smith , Jan 26, 2010; 02:46 a.m.

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?


Center square is Photoshop 50%

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Steven F , Jan 26, 2010; 08:19 a.m.

What Ansel Adams wrote decades ago may or may not 100% accurate for cameras today. Camera design has changed significantly since he died. Additionally not all exposure meters are made the same. Many older cameras out there don't care about color. Some of the newer ones do take color into account. When Ansel Adams started out in photography light meters were not available. Digital sees light differently than film and Ansel Adams never worked with digital. What may be true for Nikon may not be true for Canon or Pentax.

Additionally what shows up as gray on the camera can look significantly different on the monitor. Before making comparisons between your monitor and printers they should be calibrated so that they all render colors the same.

For most people "where is middle gray really?" is not as important as using the camera and getting experience with how your particular camera on how it handles various lighting, color, and white ballance situations. Most photographer are more concerned with the final look of the image rather than if the exposure exactly matched the value from a gray card. In fact many people under or overexpose a pictures deliberaely to get the look they want.

John A , Jan 26, 2010; 10:15 a.m.

Actually, the 50% PS gray is not 18% gray in any case--I don't remember the readings-rgb-that are 18% gray, but it is not 127.5. That will be a bit lighter than the 18% gray. And, I would be surprised if the K factor is not still built into meters, as a pro will figure out how to compensate for it and the amateur will want to be sure that Aunt Martha's face is clear, even if backlit! Slightly overexposed images, except with slide film, have always been ok but not so much the other way around.

The real test, if you are wanting to know how your camera works, would be to shoot a towel, with texture, evenly lit and meter, with the spot mode, and then shoot off frames at the meter reading and then one stop increments in both directions for 4 or 5 stops. Then, put the images in your raw processor and output them with no modification. See where the texture is no longer visible. Then, take the images where the texture disappears and adjust the exposure, in raw, and see again, which one doesn't allow you to recover texture. This will give you your camera's dynamic range at the ISO you shot at. (The digital camera will retain detail longer than it will color, so there is actually a different dynamic range for color than B/W as I have tested it)

Anyway, once you know that, you start to appreciate what your camera can do or can't.

Stephen Lewis , Jan 26, 2010; 10:34 a.m.

I guess the question is do you want to know all the technicalities of why one system which really isn't directly compatible with digital works or not under certain circumstances, or do you want your pictures to be predictable and manageable time and time again? I suggest spending more time familiarizing yourself with and mastering the nuances of your particular gear...having an explanation for all the stuff will just drive one nuts.

Bill Tuthill , Jan 26, 2010; 01:29 p.m.

Finally, an interesting question! (Where did they all go?) Your Photoshop 50% gray in RGB is hex #808080, in other words 128 decimal. The 18% gray is 117 decimal [citation, Bart van der Wolf] assuming gamma 2.2 Whereas the Kodak gray card is 160 decimal [same citation, Tony Spadaro] as in the attachment below. This is why many threads say 18% gray is really 12% gray! J.

Bill Tuthill , Jan 26, 2010; 01:32 p.m.

Attachment.


Middle square 160 instead of 127

randy smith , Jan 26, 2010; 01:56 p.m.

Vary true Steve,
Ansel Adams was vary aware though of the K factor though even in the fifties. Adams also well supported placing exposure were you want for expression, as does any photographer in not too much time in their growth.
The monitor issue is not relative i think, Photoshop produces the 50% gray value from equal amount of RGB. To make sure I had as little color bias as possible I used 55K light sources and balance for the light source. When the image move to photoshop, I used the Eye Dropper to for any minor corrections for RGB, the Gray card was only off by two points on one color, before correction to RGB.
It is odd that my meters with calibrations around 12.5 yielded middle gray, I suspect that these differences in statement of middle gray are still intended to be middle gray, but that the value come from different log scales with different number steps, or log bases.
i just found this statement on Widipedia ( http://en.wikipedia.org/wiki/Middle_gray ) : "As early as 1903, middle gray was defined as the geometric mean intensity between a white and a black intensity that are in a ratio of 60:1.[5] That is equivalent to 12.9% of the white intensity"
So I am wondering if people aren't making more of this than is 12-18% issue than is needed, half way between white and black is middle gray no matter how many steps you use to divide it in (as long as it is an odd number).
The reason why it's useful for me is, I can look at a scene and if I can count on my exposure for a mid tone exposure, I can see the 18% card in my mind fairly well, I then can place exposure compensation while looking through the viewfinder, placing a tone where it should be fairly close, rather than shooting and then inspect the histogram. It allows me to get much better results if my time with the subject is time limited.
thanx

randy smith , Jan 26, 2010; 01:56 p.m.

Vary true Steve,
Ansel Adams was vary aware though of the K factor though even in the fifties. Adams also well supported placing exposure were you want for expression, as does any photographer in not too much time in their growth.
The monitor issue is not relative i think, Photoshop produces the 50% gray value from equal amount of RGB. To make sure I had as little color bias as possible I used 55K light sources and balance for the light source. When the image move to photoshop, I used the Eye Dropper to for any minor corrections for RGB, the Gray card was only off by two points on one color, before correction to RGB.
It is odd that my meters with calibrations around 12.5 yielded middle gray, I suspect that these differences in statement of middle gray are still intended to be middle gray, but that the value come from different log scales with different number steps, or log bases.
i just found this statement on Widipedia ( http://en.wikipedia.org/wiki/Middle_gray ) : "As early as 1903, middle gray was defined as the geometric mean intensity between a white and a black intensity that are in a ratio of 60:1.[5] That is equivalent to 12.9% of the white intensity"
So I am wondering if people aren't making more of this than is 12-18% issue than is needed, half way between white and black is middle gray no matter how many steps you use to divide it in (as long as it is an odd number).
The reason why it's useful for me is, I can look at a scene and if I can count on my exposure for a mid tone exposure, I can see the 18% card in my mind fairly well, I then can place exposure compensation while looking through the viewfinder, placing a tone where it should be fairly close, rather than shooting and then inspect the histogram. It allows me to get much better results if my time with the subject is time limited.
thanx

randy smith , Jan 26, 2010; 02:12 p.m.

Thanks John, I actually was thinking about that as a process on my own just yesterday, how some people say 11 stops and some say 7 stops is the dynamic range of the digital image. I know the answer for the larger value was in the RAW file data. So I did that in 1/3 stop increments.

Bill Tuthill , Jan 26, 2010; 05:46 p.m.

Randy, thanks for the Wikipedia quote. Now it all makes sense! Previously it had been one of those never-ending discussions like DPI on the digital darkroom forum (yawn).

Other participants, if applicable, which gray square best matches your Kodak gray card?


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