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Dynamic Range in Digital Images Explained
Digital images are made up of three primary colors, Red, Green, and Blue. Each of these colors is assigned a numeric value between 0 and 255 (hexadecimal value from 00 to FF). These are assigned to each pixel in the image. The value of each represents the intensity of that particular color in the image. There are 16,777,216 possible combinations of colors in a digital image. No value less than 0 nor greater than 255 can be properly rendered in a digital image. Nothing darker than black nor greater than white can exist. Any attempt to process an image requiring these values will insert a "0" for black or "255" for white resulting in an underexposed or overexposed image.
The Columns show the Primary colors. Each row shows the resulting color based on the numeric value assigned to that particular color. Two values are given; the first is the actual numeric value, the second its hexadecimal value. The first row is a gray scale. All colors are present, only the intensity is altered, zero % in the first row, 50% in the second row and full intensity in the third. The fourth row shows the individual colors at 50% intensity. The fifth row is the primary colors at full (100%) intensity. These colors may also be blended together in various degrees to provide additional coloration. That is the method used to provide the millions of possible color combinations seen in a digital image.
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Red |
Green |
Blue |
| Numeric value (Hex value) |
0 ( 000000 ) |
0 ( 000000 ) |
0 ( 000000 ) |
| Numeric value (Hex value) |
127 ( 7F7F7F ) |
127 ( 7F7F7F ) |
127 ( 7F7F7F ) |
| Numeric value (Hex value) |
255 ( FFFFFF ) |
255 ( FFFFFF ) |
255 ( FFFFFF ) |
| Numeric value (Hex value) |
127 ( 7F0000 ) |
127 ( 007F00 ) |
127 ( 00007F ) |
| Numeric value (Hex value) |
255 ( FF0000 ) |
255 ( 00FF00 ) |
255 ( 0000FF ) |
| Numeric value (Hex value) |
0 ( FFFF00 ) |
0 ( 00FFFF ) |
0 ( FF00FF ) |
The chart above has full dynamic range. The numeric value shows colors present over the entire range of 0 to 255. But what happens if we limit the range? The chart below is the same except that numeric values only extend from 70 to 130. The entire range of colors has been limited to values within that range. Instead of 256 possibilities for each color we now have only 60. When we are confined to only this range instead of 16 million possible colors we have only 200,000. It becomes much more difficult to see the differences between each color displayed. This is a case of Low Dynamic Range. If we were looking at a picture of evidence it is likely we would miss anything not extremely noteworthy. Evidence obtained under these conditions becomes much less credible.
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Red |
Green |
Blue |
| Numeric value (Hex value) |
70 ( 454545 ) |
70 ( 454545 ) |
70 ( 454545 ) |
| Numeric value (Hex value) |
100 ( 636363 ) |
100 ( 636363 ) |
100 ( 636363 ) |
| Numeric value (Hex value) |
130 ( 818181 ) |
130 ( 818181 ) |
130 ( 818181 ) |
| Numeric value (Hex value) |
100 ( 634545) |
100 ( 456345 ) |
100 ( 454563 ) |
| Numeric value (Hex value) |
130 ( 814545 ) |
130 ( 458145 ) |
130 ( 454581 ) |
| Numeric value (Hex value) |
0 ( 818145 ) |
0 ( 458181 ) |
0 ( 814581 ) |
Some people take pictures with this condition and attempt to lighten or increase contrast. This does not improve the resolution since the lower range of pixel values simply requires each value to cover a greater range of color intensities. That means a single color now must accept a group of pixel colors that are near, but not the precise value, of the original. An image becomes pixelated and grainy in appearance. The proper way to correct this problem is to adjust your exposure or light conditions when you take the picture. Once reduced, there is no reliable way to correct the image or improve the dynamic range.
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