[filmscanners] RE: Dynamic range

["Austin Franklin" <darkroom@ix.netcom.com>]

Roy,

> A few days ago you and I were discussing scanner output and you noted
> 3db per bit.  You said 24db for the 8-bit file and 33db for the
> 2000 values
> (i.e. 11-bit file).

Merely for simplification, whether you use 10 or 20 for ILLUSTRATIVE
purposes does NOT change the concept.

> Note that ALL the above numbers and info are from YOUR posts.
>
> And finally you've made the following pair of remarks:
> Dynamic range is DIRECTLY based on the number of discernable steps.

It is.  If you believe it is not, than WHAT IS IT BASED ON?  It IS based on
the minimum discernable signal over a range, and that gives you resolution.

> Dynamic range is not based on the actual values the data represents.

That is true.  You can NOT derive the values of the data simply from knowing
the dynamic range, it says absolutely NOTHING about the values of the data,
what it "characterizes" about the data is a relationship.  MANY different
data ranges  could produce the EXACT same dynamic range.

> ----------------------
> So here you have two systems where "the actual values the (digital)
> data represents" are different.  For the same dynamic range the
> number of levels is drastically different.  For audio a 16-bit system
> using 65536 levels is capable of 96db of dynamic range.  To get that
> same dynamic range in imaging you'd need 32 bits using 4 BILLION
> levels.  The difference is entirely because of the encoding of the
> data.

Where's the difference in the "encoding"?  The numbers for both are the same
ratios, and "encode" the same thing...all the values from noise to clipping,
in increments of noise.  Plain and simple.

> Looked at another way, the full 48db dynamic range of a
> 16-bit scanner output can easily be fit into an 8-bit file by
> changing the encoding (i.e. using PS>Adjust>Levels or >Curves) before
> chopping off the bottom 8 bits.

No, that is completely wrong.  You are confusing DENSITY range with DYNAMIC
range.  What you do when you clip the bottom 8 bits from a 16 bit file is
(for the most part, you may lose a small amount) keep the DENSITY range, but
you decrease the dynamic range.

> The dynamic range of the two files
> (16-bit and 8-bit) will be the same.

Absolutely wrong, the DENSITY range will be the same, but the 16 bit file
has a dynamic range of 65535:1 and the 8 bit file 256:1.

> So, Austin, you made all the statements above the dotted line and they
> are self-contradictory.

I see no contradiction at all in my statements, what I see is
misunderstanding in yours.  Claiming a 16 bit image, that has all 16 bits as
good data, converted to 8 bits has the same dynamic range is simply wrong.
That's your own misunderstanding, I have nothing to do with that.

Austin

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