What Makes Middle Gray, Middle Gray?

<i>(Moderator's note: I just spent at least two hours rereading and editing this thread in an attempt to salvage the useful portions from the petty bickering. If a particularly cogent and pertinent reply of yours was lost in the editing process, I apologize. However if I snipped off baiting, sniping or retorts to perceived slights committed by others... tough luck. Deal with it.<p>

 

Frankly, I'm shocked nearly speechless that some of you would react so furiously against a simple rhetorical device. And Stephens seems perfectly clear to me, despite the fact that I've gone 5 days with a severe bout with the flu and can't even remember what day it is. If you're not interested in such mental exercises as these threads provide then don't participate.<p>

 

Either discuss the topic in a reasonable manner or I'll delete your posts. Keep it up and you'll be temporarily suspended from the forum.</i>

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I�m working on an article about middle gray, and I was hoping to get a

general consensus on a concept. The question of 12% and 18% aside (I

don�t want to get into that), where do you think the idea of middle

gray comes from. In other words, what makes middle gray middle gray?

Why use middle gray as a reference, metering point, or printing

target and not the shadow or highlight? I have my own theory so this

is just a survey, and would appreciate any thoughts.<p>

All that middle gray means to me is that is what my meter reads, and it is a point of departure for making compensations based on a scene's divergence from middle gray. Why are light meters calibrated to a middle value? I suspect it has to do with averaging and latitude, and meters' cells' sensitivities, but that's just a guess.

It really doesn't matter at all, a long as you're consistant. You can adjust your meter to give accurate readingd based on any tone or color by simply resetting the ISO. It's all just a matter of needing some standard we can all use. 18% grey was chosen. We adapt.

I always thought it had to do with being halfway between black and white, but hey, who knows...

 

:D

It's just a fixed reference point that a commitee agreed upon. Nothing more, nothing less.

Middle gray (18% or 12%, lets not argue for now) came from what we perceive to be the middle of the [perceptual] gray scale. As you no doubt notice, our vision is by no means linear. :)

 

Most likely they just surveyed a bunch of people and averaged their responses for what the "middle" looked like and said "that's it." Kind of like the standard observers assumed in a lot of CIE measurements. They surveyed a bunch of folks and compiled their data and had this magical "standard observer." There's no reason middle gray had to be chosen, they could have used something else, but I imagine it was just convenient. Let the light reading of the average reflectance be off what we perceive to be in the middle of the scale of tones. The only average reflectance we're concerned with is the one that seems average to us anyway.

Let's not forget the important part of the question - why?

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Jon says, <i>I always thought it had to do with being halfway between black and white</i>?.

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Black and white what? Perceptual? Print? Scene? Negative? Could there be a different and seperate middle gray for the perceptual, print, scene, and negative? Why not use the shadow as a reference point if shadow detail is key to print quality?

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Frank says, <i>It's just a fixed reference point that a commitee agreed upon</i>.

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Why did they agree upon that point? Is it an arbitrary point? Isn't knowing why something is as important or more important than just what it is?

Dear Stephen,

 

You know a great deal more about this than I, and probably a great deal more than anyone else on the forum too. The 'psychophysical' choice of an 18 per cent grey as a mid tone is one of those things we take on trust -- I've never repeated the experiment, have you? -- but as a metering reference my suspicion is that it's quite handy as a way of improving tonality in subjects with a short brightness range, given more or less average development.

 

If you base your exposures on the darkest tone in which you want texture and detail, then on a flat, overcast day you get very thin negs unless you increase development mercilessly. Base it on a mid grey, and while you may under-expose the shadows in contrasty light, you'll get a more pleasing tonality. Of course, if you do use a mid grey as your metering point, and still want shadow detail on bright sunny days, you can end up re-rating the film a stop or so slow just to compensate for this, before you start worrying about a lower C.I. and the attendant loss of speed.

 

As for the grey value equivalent to the integration of an 'average' scene, this increasingly strikes me as being of somewhat dubious value, as does the brightness range of the 'average' subject. Yes, well done Kodak: a superb statistical survey. But I don't believe that many people can judge the brightness range of a scene without a spot meter (and even then, you can argue about what to read), any more than I believe that people can choose a 'mid tone' to meter in a scene, to an accuracy of any more than a stop or two. The notorious effect of adjacency of bright and dark areas means that a mid tone (however defined, whether as corresponding to the integrated grey or to 18 per cent) is often difficult or impossible to choose with accuracy.

 

You are no doubt aware that the Japanese representatives of the ISO standards committee have lobbied for a lower C.I. for ISO speed determination (0.55, from memory, though it may have been 0.56). This would be more suitable for Southern Japan than for Rochester, New York. And in the light of your comments on the coincidence of the speed point in the fixed density and fractional gradient systems at ISO contrast (roughly 0.62), it might be that a lower C.I. would lead to even more anomalous speed results.

 

Finally, I very strongly suspect that an awful lot of people are saved by the inherent flexibility of the pos/neg system. This includes very many who *think* they know what they are doing, but cling to certainties that are just plain wrong.

 

I realize that there may be little point in making such modest contributions as the above, but equally, I always enjoy learning from you and (like most people who know a good deal about their subject) you are always generous and flexible in your views.

 

Cheers,

 

Roger

I always have been explained that middele gray is when you have a wheel

painted 50% deepest black and 50% brightest white, when turning fast

around you obtain middle gray corresponding to 18% reflection.

This is a simple rule and easy understandable why it was taken as a standard

for measuring light values. Systems like the Ansel Adams Zone System is also

based on this middle gray. Incident light meters are measuring middle gray

etc

Middle gray as a reference is a concept from the printing industry, on which I am no expert but it appears to me that it has been useful because it is reproducible. As pointed out above, pure white is virtually unobtainable (the whitest paper is maybe 95%) and black is just as difficult because of surface reflectance. Middle gray, on the other hand, is close to something found in nature and therefore easier to duplicate. It is useful in photography because, again, it is easily reproduced -- but not because it produces a "proper" exposure by itself. Just as one can make a one-stop correction to the reading off one's palm, one can make a half-stop correction from a gray card reading. But there is no ISO-standard palm; to be certain one would need to calibrate their palm. And what would you use for the calibration? A gray card, most likely. All this aside, in most cases I feel more comfortable making a half-stop correction from the middle of the film's range, than making two or more stops adjustment from what may be beyond the capability of the film to reproduce.

Italics off, </i>I hope.

 

It's a pretty easy task to repeat if you want actually. Assuming you have a reliable way to measure density. Make a bunch of chips of varying darkness going from paper white to as dark as your B&W paper will get. Choose chips and arrange them so you have what looks to you as though its a progression from light to dark with an equal change between each step. Use 11 steps since that's a pretty standard amount for gray scale targets. Your "middle" gray will probably fall between around 15% and 20% or near that range.

So Carl, you are saying that middle gray is perceptual. Do you think you will get identical results with your test if the background is black, or white, or gray?

It's been a couple years since I made a grayscale like this, but the results are generally similar. I've seen them on computer screens and against black paper. It's best to look at something like this with a relatively neutral tone all around it, just like when you're juding color. If you have access to it my suggestion would be to make your own grayscale and measure it somehow. You can pretty easily make one in photoshop by using the gradient tool and posterizing it to 11 levels.

 

The problem here is I don't think photoshop gives you a way to measure/simulate reflectance, every "measure" of tone in photoshop is based on the perceptual scale, so the middle tone will always fall in the middle of the scale.

 

If I could find one of my books I could probably give a better technical explanation on it, but I'm a little rusty.

 

 

I'm rambling, and now I'm confused. Just got over a cold so I'm a little spacey still. :)

The orgin of the concept is disputed. Some say it is based on the geometric average luminance of an typical photographic scene. Some say it is from the graphic arts industry based on the geometric middle of print density range. Some say it is based on perception. Some say it is based on the geometric average in a typical studio scene.

 

Here are some things we can say for sure:

 

1) 18% or 12% middle gray is the geometric mean of a luminance range that is log 1.3 or 1.5 respectively (18% = log[1.3/2], 12% = log[1.5/2]). This could be reflected density range from a print or reflected illumination from a scene. For what it's worth, a density range of log 1.3 to 1.5 is about what you see in an ink-based print. In a photographic scene, we'd probably describe the range in terms of stops rather than denisty: 4.33 and 5.0 stops respectively (1.3/0.3 and 1.5/0.3, one stop = 0.3).

 

2) Perhaps most importantly, either 18% or 12% gray represents an easily reproduced reference.

I'm probably making matters worse right now...

Don't use the computer monitor thing, because like I said the scales are not linear. That and 18% gray is for reflectance values.

 

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<a href="http://www.apogeephoto.com/mag2-6/mag2-9st_1.shtml" target="_blank"> this page has a bit about it a few paragraphs down</a>

<br><a href="http://www.bythom.com/graycards.htm" target="_blank">And more fun</a>. This gets in to the 12%/18% issue. 18% is from the printing world more or less, and it is correct in the sense that it was probably experimentally determined by surveying a group of people.

D Poinsett is right. Being the geometric mean of the log scale closely approximates our eyes, because they are logarithmic in nature.

 

I'm punching out though, I can tell I still can't think clearly. As general advice to all you, it's probably a good idea to avoid technical discussions such as this when getting over a cold. :)

 

Good luck on your article Stephen.

<i>18% is from the printing world more or less</i>

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Then why do they sell 18% gray cards in photo stores?

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D., Isn't 1.5/2 = .74 and an antilog of .74 is 18%? That would make 12% 1.86/2 = .93. The antilog of .93 is 12%.

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<i>like I said the scales are not linear</i>

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Are we talking about Flechner's Law here?

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I'm reading a lot of hearsay about what "someone said", or "I've heard." Is this an indication that many concepts of middle gray are photographic myths?

The reason that 18% gray cards are sold in stores (rather than 12%), is that Ansel Adams convinced Kodak to use that value (I believe Kodak was the first to make them). 18% falls on Zone V of Ansel's Zone System, whereas 12% falls on about Zone 4.5, so 18% was more convenient for Adams.

 

If one calibrates their meter, camera, and printing processes to the Zone System, it is not important whether you use 12% or 18%, because the difference will show up as an adjustment to film speed (EI). If you don't calibrate your process (especially film EI) and use the manufacturer's film speed rating, then 12% yields better (and more realistic) shadow detail. Some meters (especially Sekonic) have adjustable calibration in the range of 12% - 18%.

Thanks to Mr. Benskin for the correction. The formula for converting percent reflectance to log density or luminance is:

 

d = log10(100/r) where r is percent reflection.

 

12% reflection is middle of a log 1.85 range (6+ stops).

18% reflection is the middle of a log 1.5 range (5 stops)

<i>I always have been explained that middele gray is when you have a wheel painted 50% deepest black and 50% brightest white, when turning fast around you obtain middle gray corresponding to 18% reflection.

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Maybe in some universe where the conservation of energy doesn't hold. But in this one, you'd get the average of the two reflectances. If they were perfect white and black you'd get 50% reflectance.

Whether Adams' had much influence at Kodak is questionable especially in 1948 when the cards were first available. At that time Kodak still had Mees, Jones, and Condit working for them. It would be similar to listening to a college physics professor over Einstein on a question of relativity. The concept of "recalibrating" a meter is also questionable, but that is a subject for another thread.

 

A more important inquiry is why did Adams chose the value for middle gray over all the possible other values? It's easy to say that Zone V is 18%, but why is it?

1. Although there is a mathematical basis for the gray card reflectance, it does depend on the number of zones used in the Zone System, and the density of a Zone I value (.1 over B+F). In that sense, what Adams suggested was not scientifically incorrect, and there should be no objection by Mees, Jones, and Condit. But Adams is the one that made 18% gray as Zone V in his Zone System.

 

2. You may think that calibrating a meter is questionable, however as I stated, for many Sekonic meters �calibration compensation can be made in precise 1/10 step increments in a +/- 1.0 EV.� This is addition to a separate exposure compensation adjustment.

 

3. As previously stated, when you use Adams� Zone System, with Zone I at .1 over B+F on the film, then a Zone V print value has an 18% reflectance value. So the 18% is a combination of mathematical formulas (as explained by others) and the precise use of Adams� Zone system where Zone 0 is pure black, and Zone X is pure white and Zone V is right in the middle.

Let's not gloss over the proof. Let's go deeper.

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<i>In that sense, what Adams suggested was not scientifically incorrect</i>

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How is it scientifically correct?

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<i>Zone I at .1 over B+F on the film, then a Zone V print value has an 18% reflectance value.</i>

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Is this independent of the film and paper curves? Wouldn't a long toed curve place the Zone V value at a different density than a more linear curve, and wouldn't this plus the variation in the paper curve influence where Zone V falls in regard to reflection density?

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<i>Although there is a mathematical basis for the gray card reflectance, it does depend on the number of zones used in the Zone System</i>

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What is the basis and how does it depend? Does that mean the reflectance of Zone V, or middle gray is variable?

Interesting stuff here.<BR><BR> Maybe old Ansel read the pre WW2 Kodak literature; on copying; and learned something from Kodak reading the Kodak literature?<BR><BR>In a Kodak guide from before World War 2 ; a copying paragraph reads "By placing a neutral card in front of the subject; and scanning it with a meter; any unevenness in the illumination can be shown by variations in the needle deflection." It also mentions "using the neutral card for exposure measurements; of line work"<BR><BR>It is abit funny that Kodak references the usage of neutral cards prior to WW2; in bookletes on copying; and that folks think that somehow Ansel "educated Kodak" about exposure methods after WW2.<BR><BR>This is very bizarre.<BR><BR>White and neutral grey cards were used in the copying industry well prior to WW2; for exposure and illumination uniformity control. With lith and high contrast materials; exposure is a hell of alot more tight than artsy fartsy scenic images; done with grey scale materials.

1. <I>In that sense, what Adams suggested was not scientifically incorrect </i> </p>

How is it scientifically correct? </p>

<b>As noted above: �12% reflection is middle of a log 1.85 range (6+ stops). 18% reflection is the middle of a log 1.5 range (5 stops).� Perhaps I should have said mathematically correct.</b> </p>

 

2. <I>Zone I at .1 over B+F on the film, then a Zone V print value has an 18% reflectance value. </i> </p>

Is this independent of the film and paper curves? Wouldn't a long toed curve place the Zone V value at a different density than a more linear curve, and wouldn't this plus the variation in the paper curve influence where Zone V falls in regard to reflection density? </p>

<b>The Zone System depends on the mathematical relationship of reflection values on the print, not the film. That is why film development needs to be adjusted to compensate for the differences. Zone V is the middle of Adams� Zone system based on the assumption of Zone I print value using a film density of .1 over B+F (printed for the minimum time for maximum black). That is not a completely objective determination, due to variations in print reflectance and the number of Zones used, but for Adams (and perhaps his predecessors) it was convenient. </b></p>

 

3. <I>Although there is a mathematical basis for the gray card reflectance, it does depend on the number of zones used in the Zone System </i> </p>

What is the basis and how does it depend? Does that mean the reflectance of Zone V, or middle gray is variable? </p>

<b>If Zone I is not exactly .1 over B+F, or the number of zones varies, and the properties of the paper are slightly different, then the reflective value of middle gray could be different. So in some respects it is arbitrary, but it is based on print reflective values of average materials using the assumptions of Adams� Zone System. As previously stated (but conveniently not quoted) The actual middle gray is closer to 12%, but that corresponds to about Zone 4.5, and Adams thought it would be more useful to make it exactly Zone V and adjust the film speed accordingly.</b>