What does a fractional color bit depth like 23.5 bits mean on camera comparison sites?
Asked 10/6/2016
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Some camera comparison sites list sensor "color depth" values such as 23.5 bits or 24 bits. Since image bit depth is usually described in whole bits per channel (for example, 8 bits per RGB channel in 24-bit color), what does a fractional value like 23.5 bits actually represent? Is it a real hardware or file-format bit depth, or just a calculated rating such as the log2 of the number of distinguishable colors?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
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Snapsort is using DxOMark for its sensor data, and DxOMark explain here what each of their scores mean. Specifically for Color Depth, they say:
Maximum color sensitivity reports, in bits, the number of colors that the sensor is able to distinguish.
i.e. DxOMark claim that the 5D Mark III can distinguish 2^24 = 16.8 million colours, while the D7000 can distinguish 2^23.5 = 11.9 million colours. Don't worry too much about the absolute numbers, all it means is that the 5D Mark III can distinguish about √2 times as many colours as the D7000, at least by DxOMark's metrics (which they don't make public).
Originally by user11371. Source · Licensed CC BY-SA 4.0
user11371
9y ago
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Generated from our catalog & community — verify before relying on it.
A fractional value like 23.5 bits is not a literal image encoding depth or sensor ADC setting. On comparison sites, it usually comes from a sensor test metric such as DxOMark’s "color depth," which they define as the number of colors a sensor can distinguish, expressed in bits.
So 24 bits means roughly 2^24 distinguishable colors, while 23.5 bits means about 2^23.5 distinguishable colors. In that example, 24 bits is about 16.8 million colors and 23.5 bits is about 11.9 million.
The fractional part is meaningful as a measurement scale, not as indivisible hardware bits. It represents a logarithmic score of effective color discrimination, not “8 bits per channel plus half a bit.”
In practical terms, a 0.5-bit difference means one sensor is rated to distinguish about √2 (around 1.4×) as many colors as the other by that testing method. It’s best interpreted as a comparative sensor-performance metric, not a file format or native pixel bit depth spec.
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