QUOTE (james_elliot @ Nov 9 2006, 01:37 AM)
I just had a look at the wonderful review of the M8. However, I don't exactly understand what Michael says about the 8/16 bits issue of the Kodak sensor.
The M8 has a Kodak KAF-10500 sensor.
The spec sheet of the KAF-10500 says that the linear dynamic range is 71.5Db, with a note indicating that 71.5 = 20 log(Vsat/VN)
This means that log(Vsat/VN)=3.575
In log2 form, we hav log2(Vsat/VN)=11.88
Thus the sensor seems to be a 12 bits sensor, much like the sensors usually found on Nikon and Canon DSLR.
Am I wrong somewhere?
James,
Per my limited understanding of these complex matters, DR has basically nothing to do with the bit depth.
1. DR can be expressed as a ratio in db, but it could also be expressed in decimal format as the Vsat/Vn ratio.
It just expresses the ratio between the brighest recordable illumination not resulting in blown highlights (saturated output current of the sensor), and the lowest illumination not producing a signal to noise ratio higher than a given value (impossible to distinguish between current resulting from actual image information and background current of the sensor).
It is usually expressed in log 2 form mostly because photographers are used to working with stops, and because a one stop increase corresponds to a doubling of the incoming light.
2. Completely independantly from this, you can decide to code the electric current coming out of the sensor photosites as a result of the incoming light any way you like.
Things in the digital world being discrete and based on binary arithmetics, the values of current are usually expressed as a binary number. The longer the number (bit depth), the more values you can use to describe the range of electric current correponding to the values between blown highlights and deep black, and the finer the division of the available range becomes.
You could very well decide to code using 16 bits a DR range corresponding to one stop only. Conversely, you could decide to only use 4 bits to express the same range.
Think of it as a ladder. The DR is how high the ladder is, and the bit depth is how many steps there are in the ladder.
Practically speaking, there seems to be little value in using very high bit depth to code a small DR, and using too few bits to express a large range would not do justice to the sensor's ability to output subtely different currents when small variation of illumination occur (especially when those changes of illumination occur in highlights or shadows).
Anyone else, please feel free to correct me.

Cheers,
Bernard