Today is rare to find a photo showing fenónemo known as posterization. Understand the why of this situation very soon.
In the vast majority of the photographs transciones between different colors that are present are usually very mild. It is true that if a model wearing emerald earrings, the transition between the color and your skin will be very sharp, but not much detail in areas where fenónemo is appreciable. If we observe, however, abrupt transitions over a large area color photography, especially the sky, the inevitable feeling will be contrived. A sky is not "so." Humans are very good at detecting such anomalies. Another very prominent example, but in another area, is the color of the skin. We are very good at detecting unnatural skin colors. Therefore
posterization is characterized by abrupt color transiones in situations that the viewer of the photograph considered unnatural.
How is posterization? To understand this we must remember what the mechanism for registration of light in a picture just capturing it in the histogram. (See article on histograms).
Each color in a photograph is actually a triplet numbers. Each number represents one of the basic colors: Red, Green and Blue (RGB, in English). So if we for example 8 bits to represent each color, tone vary in the Red channel from the Black (1) the purest possible Red (255). The same with the other channels. With these three values \u200b\u200bwill get varying tones, when combined, from the Pure Black (0,0,0) to white as pure as possible (1,1,1). All other panels of three values \u200b\u200brepresent the remaining tones of the photograph.
But the recording light on the sensor is linear. "What does this?
If we focus on a single channel for greater simplicity in the explanation, we found that 8 bits have 256 different shades.
If we use a sensor with a dynamic range of 6 steps ( See article on Dynamic Range ), we might be tempted to think that those 256 propocionalmente tones are divided between the 6 steps. But not true. The first steps only has 4 tones, the second 8 and so on until the sixth to be 128. Set the ISO setting and shutter speed change step is performed by opening a passage (of course) the diaphragm. So if we open
very (very) little diaphragm (and thus greatly underexposed photograph) in each channel there are only 4 different tones (! EYE from 0 to 3, no 56 to 59, for example). Or that if we err
exposure (causing a "slight" underexposure of 1 step), we are! Lose half of TONAL RANGE!.
All this long explanation is needed to understand the mechanism of posterization, or whatever it is, How is it possible to produce these abrupt transitions in tone?
The explanation is as follows:
Suppose we take a photograph in JPEG format (using 8 bits) and due to an error in the exposure we get very underexposed. We lose two such steps. This means that each channel will have, at most, 32 different tones (in the most clear.) The histogram reflect this situation by presenting the right boundary in an area about 2 / 3 of the maximum possible. Using a graphic editor
modify this "widening" the histogram until it approaches the limit of the right side. What happens then? Well
two shades of red with values, such as 12 and 14, go now to assert, for example, 135 and 201. With the original tones of red had a smooth transition. With the new is an abrupt change. If we combine all these transciones steep channels will very often. If you belong to an area of \u200b\u200bphotography, for example the sky, which are expected smooth transitions, the final result will artificial. This is what happens in the picture accompanying the article.
Why posterization is rare to find today?
Because the digital SLR camera users get Raw files and JPEG files no. They have a bit depth of 12 to 14 bits. With 12 bits, for instance, have tonal values \u200b\u200bof 4096. With 8 bits we have only 256. So there are many more shades to the process of "widening" of the histogram is much less critical, because originally it had a sufficient distance between the numerical values \u200b\u200bof the tones.
Under what circumstances can observe the phenomenon of posterization?
In practice, only two.
a) as a creative resource. That is, done on purpose.
b) If you capture a picture generating a JPEG file. If, moreover, this photograph is clearly underexposed. And if you use a graphics editor to expand the histogram.
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