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  Previous versions of mpmendenhall's message #11452988 « More Mpix or perfect per-pixel quality? »

  

mpmendenhall
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Re: More Mpix or perfect per-pixel quality?


RustyBug wrote:


Are the multiple (connotative/denotative) definitions for DR?

When I think of DR, I think about range of recordable information @ dark - light and the number of stops that range between these two extremes. Yet, much of the discussion is concerning noise and degrees of refinement/distinction/tonal separation within the range of that captured.


The two issues are closely tied together: sensitivity to tiny amounts of light compared to to the maximum saturation point (well depth) of the sensor both determines the dynamic range (range between the largest signal before clipping and smallest above noise), and also the finest distinction you can make between nearby brighter values. Dividing the scale between the darkest and brightest measurable point into finer intervals than the noise level in the darkest regions doesn't help tonal separation, because the same noise that limits DR into the shadows is present at the same or larger magnitude in mid-tones. A system with 12 stops of dynamic range will not benefit from dividing that range into finer intervals described by a 14-bit readout.

Note, however, a system with good tonal separation at mid and bright values doesn't necessarily have better dynamic range than one with worse tonal separation.

Numerical examples:
a) a sensor with a "perfect" readout (no noise besides statistical shot noise) and a well depth of 2^12 = 4096 electrons has a dynamic range of 12 stops (from 1 to 4096 electrons). At "mid grey" 25% of the max signal, the noise as a fraction of the signal is sqrt(0.25*2^12)/(0.25*2^12) = 2^-5.

b) a sensor with 2^4 = 16 electrons readout noise and a well depth of 2^16: this sensor has the same 12 bit dynamic range (2^16/2^4 = 2^12). However, the mid tonal separation is better: the fractional noise at 25% max is sqrt(0.25*2^16 + 2^4)/(0.25*2^16) ~ 2^-6 (twice as good as above).

So, deep well depth (a necessary factor for dynamic range, combined with low readout noise) also improves tonal separation.



Mar 30, 2013 at 10:06 PM



  Previous versions of mpmendenhall's message #11452988 « More Mpix or perfect per-pixel quality? »