Digital BW, The Print

>> I've
>> heard it said over and again, including from Andrew Rodney, that
>> DR and bit
>> depth are totally unrelated. When people say that, do they mean in all
>> instances other than at point of capture?
 
> I don't know why they say this, because it's simply wrong!  Yes, you CAN
> represent any dynamic range with any number of bits 2 or more...BUT you
> don't get the intermediate values (tones).

Right, but doesn't that just show you CAN have one independent of the other?
That IS the point, no? You've always maintained that you can't get to a high
DR w/o sufficient bit depth, but what you offer above, to my mind, violates
that. I understand high DR with low BD won't give us imagers what we want or
need, but on a fundamental level.....

>  Can you show me where Andrew
> Rodney states this.  I would like to read exactly what he as to say.  Ed
> Hamrick agrees with me on this, BTW.

Alright, I've asked similar questions on other lists in the past. But most
people who've responded aren't engineers, so they probably look at it
differently. I really think the step by step approach of: voltage to CCD, to
front end, to A/D converter, to tones assigned, etc., which you've taken, is
the only way to approach this subject in a meaningful way. The other
answers, while illuminating, are more of a random thought approach which
leaves room for too many gaps. ;-)

I will try to collect a few responses to show you a sampling of some of
those answers. Did you see the one I posted a couple of days ago from Dan
Margulis (he spoke of the extra digits on the thermometer, and related it to
noise)? If not I'll send it to you off list -
not that it is terribly relevant to this.

But, in short, most people are of the mind that bit depth and dynamic range
are independent qualities, or that's how I've interpreted them. I'll venture
they may be right when considering the attributes of a file already before
us, but not realize it's a little different when we are discussing the
elements of a capture device.

Your comments are welcome.

********

From Andrew Rodney:


Todd asks:
> First off, when in 16 bit mode in PS does the histogram indicate how the
> file maps into 16 bit space, or an 8 bit space?

Andrew answers:
In a way both. The numbers in the Histogram (0 to 255) remain the same.
Technically that¹s not really correct as there are a whole lot more numbers
in a high but file. But consider that Photoshop wisely treats 8 bit files as
one thing, ANY file more than 8 bits (10, 12, 14, 16) bits as another. The
Histogram would be a mess if we had numbers for 12 bit verses 14 bit verses
16 bit. What Adobe did was just leave everything the same as far as numbers
are concerned. The Histogram you see is accurate to the bit depth you are
currently in. Turn off cache for Histogram in preferences.

> More specifically, when I capture a raw unmanipulated HDR file from my Leaf
> scanner, which I believe scans in 14 bit, and bring it into 16 bit space in
> Photoshop, the tones in the histogram are bunched up.
> 
That¹s because Leaf provides you untonede (linear) raw data. Newer scanners
can provide high bit files that are toned. The Leaf is pretty old stuff so
at the time, it was about the only scanner that even provided you high bit
data but they just gave it to you untoned. You can pull the histogram and
make it look nice and lovely and you¹ve just toned it. Since it¹s high bit,
no damage done. Once you convert to 8 bits, you¹ll see a lovely resulting
Histogram with no data loss.

> Would it look any different if the same film were captured on a similar device
> that captures at a true 16 bit level?
> 
If said scanner also provided you untoned data, no. If it toned the data, it
could look a whole lot different. With my Imacon Flextight, I can tone and
capture 16 bit data. The software allows me to pull curves and set endpoints
and then provides me 16 bit scans. Your Leaf doesn¹t provide that option so
you get that dark looking file with the Histogram pushed to one side.

> Or, Does it indicate that the dynamic range of the film scanned was just not
> as large as that possible even in an 8 bit space?
> 
Dynamic range has nothing to do with bit depth! They are completely
different spec¹s. You can have a scanner with 16 bits per color and a
dynamic range of 3.3 and you can have a scanner with 12 bits and a dynamic
range of 3.8. Bit depth is the number of steps. Dynamic range is the height
of the star case. You can have a staircase that¹s 20 feet high and have 40
steps. You can have a staircase that¹s 30 feet high and have 30 steps.

> Is it a function of the
> dynamic range of film itself, the dynamic range of the capture device, or
> the bit depth of the capture device relative to the space?
> 
Film has a dynamic range, so do scanners and digital cameras. It¹s the
device that plays a role here.

> Basically, I am confused overall as to how the histogram works in 16 bit
> mode, in that anything over 8 bits is handled as 16 bit. Thus, does it
> display 10 bit data differently relative to say, 14 bit, or any other
> possible comparison of that sort?
> 
No, Photoshop deals with 8 bits pre color in one fashion and then everything
higher (what I like to call ³High Bit²) as 16 bits pre color. At least
that¹s what you see in the mode  menu. There is simply no reason to break
down a high bit file into categories.

Andrew Rodney 

******

>At 05:15 PM 7/26/01 -0400, Todd Flash wrote:
>>I'm rather confused about the interaction/relationship of these concepts:
>>Dynamic Range, Bit Depth, and the Histogram.
>>
>>First off, when in 16 bit mode in PS does the histogram indicate how the
>>file maps into 16 bit space, or an 8 bit space?
>>
>>More specifically, when I capture a raw unmanipulated HDR file from my Leaf
>>scanner, which I believe scans in 14 bit, and bring it into 16 bit space in
>>Photoshop, the tones in the histogram are bunched up. I know, I haven't set
>>endpoints or gamma yet, but is this showing that the dynamic range of the 14
>>bit data is not as large as that which is allowable in a 16 bit space? Would
>>it look any different if the same film were captured on a similar device
>>that captures at a true 16 bit level?


Todd, as I recall, our buddy Austin claims that the
Leaf can do perfectly fine scans over a wide range
of exposures.  This seemed odd to me.  But I did say,
at the time, that this could only be true if the A/D
was using only a small portion of its available input
range.  So in a way -- your observation about bunched-
up histograms makes some sense.

Rafe Bustin

********

On Fri, 27 Jul 2001, tflash wrote:

> Thank you Andrew and Maris. You guys seem to be in accord, and what you
> expressed was in agreement with my assumptions.
> 
> Rafe, you seem to feel other wise, so I would like to address this to you.
>  
> >>> More specifically, when I capture a raw unmanipulated HDR file from my
Leaf
> >>> scanner, which I believe scans in 14 bit, and bring it into 16 bit space
in
> >>> Photoshop, the tones in the histogram are bunched up. I know, I haven't
set
> >>> endpoints or gamma yet, but is this showing that the dynamic range of the
14
> >>> bit data is not as large as that which is allowable in a 16 bit space?
Would
> >>> it look any different if the same film were captured on a similar device
> >>> that captures at a true 16 bit level?
> > 
> > 
> > Todd, as I recall, our buddy Austin claims that the
> > Leaf can do perfectly fine scans over a wide range
> > of exposures.  This seemed odd to me.  But I did say,
> > at the time, that this could only be true if the A/D
> > was using only a small portion of its available input
> > range.  So in a way -- your observation about bunched-
> > up histograms makes some sense.
> 
> Could you say more about what makes sense?
> 
> This question does in fact stem from a difference of opinion between myself
> and Austin. He is of the mind that the histogram of the raw (linear) data
> appears the way it does because it is 14-bit capture mapped into/displayed
> in 16-bit space. I am of the opinion that it is due to the dynamic range of
> the data, whether it be due to the DR of the film, or the scanner. He
> believes the DR of the data is in part a function of the bit depth. He
> referenced the way higher bit A/D converters allows scanner manufacturers to
> make extended DR claims.
> 
> At least that's the way I interpreted his statements.


No, I'm not trying to take sides in your dispute
with Austin.  As you recall, Austin claims (and
cites the Leaf manual as proof) that the Leaf
can render perfectly fine scans over a wide
range of exposures at the scanner-driver level
(on the Leaf, these exposures would be measured
in milliseconds per scan line, I presume.)  This
didn't make a lot of sense to me.

As a (former) designer of instrumentation
circuitry, my goal was generally to set
analog gains so that one used as much of
the A/D input-voltage range as possible,
while also ensuring that over-range and
under-range conditions would not occur.

To may way of thinking, the only way
that Austin's claim could be true was
if the Leaf was only using a small part
of the available A/D input voltage
range on any given scan.  And, voila --
this is what your "narrow" histograms
seem to show.

I'm only trying to put "2 and 2 together"
here, that's all.

Rafe Bustin

(Austin, I still never did understand what Rafe was saying here. Too
technical for me. Could you expound on this in more simple terms?)

*********

Dynamic range is a separate concept from bit-depth. Think of bit depth as
the number of data points that can exist between dMin and dMax. In Photoshop
Levels, dMin and dMax will always be 255_0. In 16-bit mode, the Levels
theoretically should be 65,535_0, but since that would be impractical to
display on screen, the distributions are scaled to 255_0.

If the levels in your histogram (14bit-16bit) are bunched up or do not fill
up the entire range (255_0), it is caused by your capture settings and/or
the dynamic range of the captured image.

Nathan