Dynamic Range and bit depth
Ξ January 14th, 2008 | → | ∇ Tutorials |
Dynamic Range and bit depth
Bit depth and dynamic range are indeed separate concepts and there is no direct one to one relationship between them.
The bit depth of a capturing or displaying device gives you an indication of its dynamic range capacity, i.e. the highest dynamic range that the device would be capable of reproducing if all other constraints are eliminated. For instance, a bit-depth of 12 for a CCD tells you that the maximum dynamic range of the sensor is 4096:1, but the captured dynamic range is likely to be much less once noise is taken into account (most 12-bit sensors have on average a dynamic range around 1,000:1 only).
In the case of an image file, the bit-depth in itself does not tell much about the dynamic range captured or reproduced by the file.
First, the bit depth of an image file is not a reliable indicator of the dynamic range of a reproducing device. For instance, when a RAW file is converted into a 16-bit TIFF file in linear space, the real bit-depth -and thus maximum dynamic range- of the captured data is most probably 12-bit only, which is the bit-depth of standard digital cameras. It is just because 12 bits are not convenient for computers that the file will be stored in 16 bits, but of course it does not change the dynamic range of the information stored.
Second, the bit-depth of an image file is even less a reliable indicator of the dynamic range of the scene reproduced. When a 32-bit HDR image has been properly tone mapped, it will show the original dynamic range captured, even when it is saved in an 8-bit image format. This is why a tone mapped image is often confused with an HDR image. A tone mapped image is not an HDR image as it does not represent the original values of light captured anymore. It just reproduces the dynamic range captured on standard monitors or prints.
Shouldn’t 24-bit be higher than 16-bit?
There are two ways to “count” bits for an image — either the number of bits per color channel or the number of bits per pixel.
A bit is the smallest unit of data stored in a computer. For a grayscale image, 8-bit means that each pixel can be one of 256 levels of gray (256 is 2 to the power 8).
For an RGB color image, 8-bit means that each one of the three color channels can be one of 256 levels of color. Since each pixel is represented by 3 colors in this case, 8-bit per color channel actually means 24-bit per pixel. Similarly, 16-bit for an RGB image means 65,536 levels per color channel and 48-bit per pixel.
To complicate matters, when an image is classified as 16-bit, it just means that it can store a maximum 65,535 values. It does not necessarily mean that it actually spans that range. If the camera sensors can not capture more than 12 bits of tonal values, the actual bit depth of the image will be at best 12-bit and probably less because of noise.
The following table attempts to summarize the above for the case of an RGB color image.
| Type of digital support | Bit depth per color channel | Bit depth per pixel | Theoretical maximum Dynamic Range | Reality |
| 12-bit CCD | 12 | 36 | 4,096:1 | real maximum limited by noise |
| 14-bit CCD | 14 | 42 | 16,384:1 | real maximum limited by noise |
| 16-bit TIFF | 16 | 48 | 65,536:1 | bit-depth in this case is not directly related to the dynamic range captured |
| HDR image (e.g. Radiance format) | 32 | 96 | infinite | real maximum limited by the captured dynamic range |
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