Digital BW, The Print

There are three issue involved. They are sensor sensitivity, noise floor 
and dynamic range. If the sensor is not very sensitive you don't need 
much dynamic range since the peak power to noise floor is not very 
great. Where dynamic range becomes an issue is in a low noise sensitive 
sensor. In this case the dynamic range requirement is determined by the 
minimal signal detectable (sensitivity) vs. the highest signal you need 
to detect.
If you assume the mininal detectable signal is right at the noise floor 
of the sensor the the dynamic range can be estimated by 6*(n-2) + 4.8 or 
6*(n-1) depending on the "type" of dynamic range you are interested in, 
where n is the number of bits.

In any digital system you need extra bits for the very simple reason - 
the numbers above are best case. In general two bits are waisted - under 
exposure. Most people don't meter to the peak signal value in the scene 
- rather to some average over the scene or the average in a "spot." BTW 
the zone system is a wonder method developed to get the maximum dynamic 
range out of a sensor or film, since it starts and the peak signal power 
-places that and works down from there.

How many bits are necessary for optimal quality - I venture to say 14 to 
16 especially if integer arithmetic is used in any point of the 
processing. When digital music was first being developed, many people 
though that since the ear could not discern past 8 bits on a simple tone 
test that 8 bits was sufficient. However, for a complex piece of music 
just about anyone can tell the difference between a 14 or 16 bit ADC and 
an 8 bit ADC. I suspect the same is ture for a complex scene.

Truman

Jeff Medkeff wrote:

>Roy Harrington wrote:
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>>Basically you increase the sensor dynamic range by either: reducing the noise
>>at the low end and/or increasing the clip point at the high end.  The bits of
>>the A/D have to fine enough to take advantage of the reduced noise at the
>>low end and coarse enough to not get clipped at the high end.
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>It sounds to me as though we are approaching this from two different 
>perspectives. When you say the bits have to be coarse enough to take 
>advantage of reduced noise, and fine enough not to clip, it sounds to me 
>as though you are saying something about the digital sampling of the 
>signal being read off the sensor. And if I understand correctly you are 
>dead right - there is no point to making the luminous flux density delta 
>per ADU so coarse that the 100 (or whatever) highest ADU values all clip 
>to white. Not only do you throw away values, but you also increase the 
>chances of posterization in the resulting digital image.
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-- 

"If you want to make an apple pie from scratch, you must first create 
the universe."

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