Mystery of 18 percent
Industrial eksponometricheskie device, as is known, calibrated to 18 per cent level of gray. Often, the imaging literature, this level is called “sredneserym”, “standard gray” or “neutral gray.” But why the average level is 18%? Where did this figure?
Sometimes you can hear that sredneseromu color corresponds to no 18, and 13 per cent [1]. Rumor has it that the American ANSI standard is based precisely on such a magnitude. Perhaps this is true. The author, alas, does not have such a standard text … It also wants to ask the reader about something else: Do not you think the number 13% of the more surprising to characterize sredneserogo color?
If not – then this article is not for you. A novice photographers and just curious readers by now will try to explain where did these strange (at first glance) numbers: 13 and 18 per cent.
Fig. 1. A gray card containing a gray, black and white field
Display Scale
Look at the scale of the camera shutter. It looks like (in fractions of a second) like this:
… 1 1 / 2 1 / 4 1 / 8 1 / 15 1 / 30 1 / 60 1 / 125 1 / 250 1 / 500 …
A characteristic feature of such a scale is that on its two adjacent numbers do not differ by a constant (as in the normal range), and in a number of times. In this case, each subsequent number is greater than the previous two times. The peculiarity of photographic materials is such that, for them it is this scale is the most natural. The difference between the two values on this scale corresponds to the same level of exposure.
We need to mention another important property of this scale: it is not zero. Indeed, moving the scale to the right, we will get more and more short excerpts, but did not reach the ground than ever.
How to interpret such a scale average? Obviously, the average value between the two quantities will defend to the same number of degrees of these same values. Thus, 1 / 60 will average between 1 / 500 (three steps down: 500/23 is approximately equal to 60) and 1 / 8 (three steps up: 60/23 is approximately equal to 8).
Now back to the level of the standard gray color. It is natural to define it as focusing on the scale, the two neighboring numbers which differ by two times. As mentioned above, zero on a scale no. But to choose a certain number (which characterizes the brightness, reflectivity, or anything else in the same way) as 100%, we can not prevent anything.
Since the zero on such a scale is not to determine the number of interest relevant sredneseromu level, we need to determine the total number of steps on the scale. To address this problem, we now turn.
The number of degrees of brightness on a scale that characterizes the image
To determine the level sredneserogo should use the number of levels of brightness that is typical for the final image obtained by printing in the darkroom or printing.
In his book “The color and contrast” Zheleznyakov B. writes: “Many theorists of painting came to the conclusion that the artist specially transposed rows of the object brightness, adjusting the actual balance to the possibilities of his palette, and its prospects, as we know, are such that all colors arranged in brightness in the interval between whiting and soot. This is the brightness ratio is approximately 1:40 – 1:60. In the field of printing and prints a range of possible brightness values is usually even less: 1:30 – 1:40. In terms of levels of exposure, this means that is reasonable to talk about the range of reproducible brightness 5 – 6 degrees (25 = 32 26 = 64).
It is important to emphasize that it is necessary to proceed from the properties of the final image, rather than from the latitude of the photographic emulsion. Indeed, whatever fotoshirota, we always have in obtaining the final print in one way or another to lay the entire range of brightness in 5 – 6 degrees. Additional same fotoshirota provides us with just insurance against errors in setting the correct exposure.
On the other hand, Zheleznyakov would take “the best visual contrast (HVAC) ratio of 1:40. In support of this figure, he wrote: “In the process of improving and adapting to the conditions of validity of our vision has worked more precisely, this interval of brightness as the most appropriate in terms of survival and good legibility of the objects of the world. … It seems that our eye adapts to the brightest portion, it is more appropriate in terms of biological protection of the organ of vision overload. Thus, as it is fixed white. A black automatically and unconsciously “counted” from the white to an amount equal to the optimal visual contrast, ie approximately 1:40.
Thus, based on the concept of HVAC and building on the data on the possibilities of the printing process, it is reasonable to talk about the reach of the eye range of brightness equal to 5 – 6 steps.
So what is the srednesery color?
If we take the full range of brightness (100%) equal to five steps, then, obviously, sredneseromu (5 / 2 = 2,5) the color will match:
100 / 22.5 = 18% (more precisely: 17.68%).
In other words, 18% separated from 100% two and a half step.
If, however, based on the full range of six steps, then sredneserogo (6 / 2 = 3) we obtain:
100 / 23 = 13% (more precisely: 12,5%).
Which of these numbers more accurate? Strictly speaking, they are both correct. In real situation, should be based on additional considerations. For example, discussing the photographic prints is reasonable to say no more than about five levels of brightness and an average of 18%. And, for example, when analyzing the slides can talk about the six steps with an average value equal to 13%.
That’s the whole secret.
Delusions
Sometimes you may encounter with the assertion that the level of 18% was selected, based on the value of the standard reflectance of human skin (or, say, the typical reflectance of leaves of trees).
Such assertions are not really out right, but everything in them, as it turned upside down, that is turned on its head.
If we talk about skin Europeans, on average, it is lighter gray card on 0,5 – 1 degree.However, when taking a measurement often make it to human skin, wishing it was this tie to the brightness level sredneserogo. The same can be said about the leaves. No matter what its actual ability to reflect in a given situation. But the important thing: if the foliage is an important element of the plot, then, of course, it is reasonable to adhere to its brightness sredneseromu.
When shooting with digital cameras, which are characterized by a small matrix of range of the brightness constraint “five steps” appears at the moment of shooting, that is, until the press.
Thus, often (but certainly not always) it is expedient to make measurement on an important part of the plot. In this case, it is this element of the image will be tied at the level of sredneserogo. At the same time on the print (a matrix of digital cameras) will also work out all the brightness of the objects of which two and a half step more, and for two and a half level compared with the selected.
And one more remark. Earlier photographers often had to choose, what to donate for the fingerprint: the shadows or light. This was due to the fact that the range of brightness, reproduced on photographic paper substantially less than the range typical of films. To exit from this situation had to use sophisticated printing techniques, different exposing some parts of the frame. Now the problem is quite simple to solve computer compression range of brightness before printing. As a result, the image will be visible and the light and shadows. But nature does not cheat. On the print as it was five degrees of brightness, and left. What is the matter? Paying for the preservation of details and in the highlights and shadows will reduce the number of distinguishable colors.
About measuring scale
Sometimes you can find the assertion that the level of sredneserogo color corresponds to the average reflectance of any measurement scale. In this case as an argument to say that such a scale – similar to the average subject in photography, whether it be landscape or portrait. However, to acknowledge that the truth is difficult.
Indeed, consider the frequently referred to in such cases, the measuring scale, containing eight equal size fields with coefficients of reflection:
0.8 0.4 0.2 0.1 0.05 0.025 0.012 0.006
First, it argued that such a scale corresponds to the average plot can only be one who carefully to her not watched. Part of the fault – printed images of a scale, adapted to the possibility of printing (see picture on the left in Fig. 2).
Fig. 2
The real scale (picture right) at 50% filled with deep shadows. Surely the average shooting scenes are exactly? Of course not. Typical intervals of brightness for different subjects can be found in the book of Zheljeznyakova “The color and contrast.Brightness changes, submitted by 8-quadrupole scale, although it is difficult to be attributed to a rare, but typical of his name is quite impossible. Besides, if we talk just about photography, not about the filming, it should be borne in mind that the photographer must always focus on the real possibilities of the printing process. A pass in all the fields that scale when you print almost impossible (unless, of course, do not use digital compression methods for dynamic range, or similar technological tricks).
Second, refer to the calculation of the average of the above scale is not appropriate and from the standpoint of common sense. Indeed, if you make a half-scale nonreflective:
0,8 0,4 0,2 0,1 0 0 0 0,
then the average of the reflection coefficient remains practically the same: 18.75% against 19,9%. For high-precision optical test facility for more than a strange result: it turned out that half of it can be without prejudice to gloss over the thick soot!
No, by no means argues that the scale is bad in principle. For the purposes for which it was created, it is very important and necessary. And in certain situations it actually can be used as an analog of the model test object. Smeared it on 50% of soot or not – but the scale is really an average of approximately reflects the very same 18%, referred to in this article. That’s just cause and explanation for those 18%, it certainly would not be.
Generally speaking, the root causes and explanations in the area generate natural sciences should first be sought in the properties of objective reality and not in the parameters of artificial objects, no matter how useful and appropriate these objects are.
How to calculate the average gray formula
Is there a mathematical formula that allows us to calculate srednesery level for a given difference between black and white? Of course, yes. Since in this case we are dealing with the scale, adjacent grades which differ in two times, then the calculations should use the formula for the geometric mean:
Make sure that the application of this simple formula is found in the rational, inquisitive reader can himself, considered the geometric progression with ratio equal to 2. Application of this formula to the previously valid ranges 64:1 and 32:1 gives the following results:
Understanding that the average level of gray to determine the geometric mean between black and white, appeared almost simultaneously with the appearance of pictures (photographs), that is, long before the advent of the modern scales and standards. Is flirting with the wording can be found, for example, in the New International Encyclopedia, published in 1903. (Daniel Coit Gilman, Harry Thurston Peck, and Frank Moore Colby. The New International Encyclopædia.) In it for the basis of agreed ratio of 60:1, which gives: