[Shiraz (Ray)]

The new CMOS chips coming onto the market have on-pixel converters, which limits the available bit depth. The ASI1600 for example has only 12 bit conversion and generates 16 bit data by simply padding the bottom 4 bits of the 16 bits with zeros. Theory says that the bottom 4-5 bits from a typical 16 bit camera are dominated by read noise, so they have no value anyway and that 12 bits is enough for all but the highest dynamic range chips - is this true?

To test, took 10 subs from my genuine full 16bit H694 CCD camera and divided the fixed point pixel values in the subs by 16 to yield 12 bit fixed point data. I then stacked the new 12 bit subs and scaled the floating point stack for comparison with a normal stack of the original 16 bit data. No pixel rejection was used and the calibration was not perfect, so the results are a bit ropey, but good enough.

Compare the 12 bit and full 16 bit stacks below (STF stretched) - individual pixel values may be very slightly different, but there is no real change in the depth, detail or noise. Although the H694 has a 16 bit converter, 12 bits is actually enough for it - which is consistent with its ~11 bit dynamic range. Conclusion, there is no loss of depth or detail in using a 12 bit converter for a chip with the 11-12 bit dynamic range of typical CCD or CMOS chips. CCD or CMOS chips with greater than 12 bit dynamic range will need more than 12 bit conversion, but there is no penalty in the 12 bit conversion of the current ASI1600.

thanks for reading. regards Ray