on 9/2/02 12:51 AM, Austin Franklin at darkroom@ix.netcom.com wrote:
> Roy,
>
Gee, Austin, here again you snip totally out of context. I DIDN'T
say the following quote. Vincent said it and I explicitly cited
him as the author.
>> "Dynamic range is the ability to distinguish tonal differences."
>> If you take that as a given, naturally you're going to directly lead
>> to the idea of Dynamic Range being equivalent to number of levels and
>> that a 12-bit file has more DyR than an 8-bit file. Simple logic.
>
> That statement IS true, if the number of bits was not the limiting factor.
> If you are digitizing 4 bits of noise into 12 bits, then they will digitize
> the same dynamic range of the input signal. A more accurate statement would
> be that a 12 bit file can HOLD more levels than an 8 bit file.
>
>> The numbers in the 8-bit file go from 0 to 255, in the 12-bit file they go
>> from 0 to 4095. These numbers are NOT signal values, in no way is the
>> signal represented by the value 100 twice the size of the signal
>> represented
>> by the value 50.
>
> Well, yes it is. It's a direct reading of voltage from the CCD. It has
> nothing to do with density values, the human eye, logs or anything. If your
> A/D measure, say, 1V/count...then 100 IS twice as many volts as 50.
Again, this is the THIRD time I've explicitly stated that the data in
the 8-bit file I'm talking about has been altered by Levels or Curves.
The numbers no longer resemble the output of the A/D of the scanner.
>
>> They just plain AREN'T signal
>> values.
>
> Well, they are when they come from the A/D. If you manipulate them in PS,
> then, of course, their relativeness has changed.
>
>> The ONLY numbers you can put into the DyR ratio are the
>> measurements of real
>> signal values.
>
> That's true for the dynamic range of the scanner, but it's simply a fact,
> that N bits are required to hold a particular dynamic range. That in no way
> assures that the data that's in those bits occupies the entire range of
> values mind you.
>
>> In imagining, a piece of paper with the maximum amount of
>> black ink, outputs the least amount of light possible. Again that amount
>> of light is the MDS for that paper/ink combination.
>
> That's not necessarily true. You may ONLY be able to print a black of, say,
> 1.8D, but you can print tones in an increment of .01D. Your max black in
> this case is not the minimum discernable signal, it's merely a limit of the
> ink darkness.
>
>> And, Austin, yes I know in this case I'm saying density range is exactly
>> the same range as dynamic range with note that the max density corresponds
>> to the min light and the min density corresponds to the max light.
>
> But I still hold that that is not correct. I understand why you believe
> that, but see what I said above about that. You CAN have a piece of film
> that has a DENSITY range of .4 to 1.8, and the minimum discernable signal
> could be FAR less than your density measurement, in this case, the MDS would
> be simply noise. Again, density range says nothing about your ability to
> discern within that density range, that, in THIS DISCUSSION is typically
> limited by noise.
>
> Austin
>
I really feel like pulling an Austin, here. How about this out of context
snip from the above paragraph:
THIS DISCUSSION is typically
> limited by noise.
>
> Austin
Sorry, couldn't resist.
I've got to go away for a couple of days so I won't be able to respond
right away.
Roy
Roy Harrington
roy@harrington.com
Black & White Photography Gallery
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