Digital BW, The Print

Steve Kale wrote:


> Yes I guess this is a bit off this topic but there is an interesting
> discussion here from which I conclude we want greater bit depth and lower
> noise 

Amen to that.


> All
> else being equal (including noise), greater bit depth increases dynamic
> range.

No, that's just not correct. Sorry, it is not my intention to be merely 
contentious; I think this is an important user issue with respect to 
digital cameras. All greater bit depth gives you is better (finer) 
sampling of the dynamic range the sensor can sense.

If you take a Kodak test strip - one with "black" at one end, "white" at 
the other, and a bunch of grays in between - you could slice that up 
into 8 equal pieces with an x-acto knife, or you could slice it up into 
12 equal pieces. Although the size of the pieces gets smaller, your top 
and bottom ends remain the same no matter which you do.

This is analogous to what is happening in a digital camera - well 
potential is being amplified and sent through an analog-to-digital 
converter. You could have that converter output in eight bits, ten, 
twelve, or fifty, but changing ADCs would *never* change the potential 
in the well. The only thing you change with bit depth is the tonal 
"distance" between steps. Remember these sensors are linear, unlike film!

It is more correct to say that all other things being equal, greater 
photosite size increases dynamic range. The rubber meets the road for 
photographers when choosing a camera; there isn't much relevance to this 
at exposure time when you are already committed. Paul's 8 megapixel XT 
is a great camera. But it and its brother the 20D both have less - 
considerably less - dynamic range than Canon's 1D mark II. The 1D II 
does not read out more bits, nor does it have more pixels, nor is it 
lower in intrinsic noise. What it has are larger photosites.

OTOH when shooting a scene with a large dynamic range, then you want as 
many bits as you can come by - I don't dispute this. This isn't because 
it lets you record more range before blowing the highlights or burying 
the low end in mud; it is because you want all the flexibility you can 
get when you pull that range to the gamut of an output device and start 
to put tree trunks in the histogram. This is a separate issue.

All this is measurable. Don't take my word for it; if dynamic range is 
important to you, it is a pretty easy job to compare the dynamic range 
of different cameras to an adopted level of the signal's statistical 
significance.


 > This is a big "sales point" for the digital backs that are true 16
> bit.

The digital backs have greater dynamic range, but this is not a result 
of their bit depth. Rather the reverse is true - they *need* to sample 
with more bits due to the greater dynamic range; if they did not, the 
flux differences between adjacent ADU's would eventually grow large 
enough to appear posterized even in the unmanipulated, linear image. 
Somewhere around here I've got images read out from engineering grade 
sensors that show just this effect, using 35mu photosites and 8 bit ADCs.

--
Jeff Medkeff
Eagle River, Alaska