[filmscanners] RE: Density vs Dynamic range - was: RE: opinions? Reviews? of Primefilm 1800 ?

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

Tony,

> The number of scanner bits is a necessary but not sufficient
> requirement for
> seeing densities at a scanner's theoretical dmax (i.e. log 2^bit-depth).
>
> Number of bits have two main roles. They do indeed represent the
> theoretical
> maximum density that a scanner could have if the electrical
> components were
> up to scratch and could use those bits to their full.

I've discussed my disagreement with this in another post...

> More importantly
> though the number of bits determine with what resolution the
> scanner can see
> the density levels that it is capable of scanning.

That is correct, and the only thing number of bits is.  It's called "dynamic
range".

> The number of bits represents the POTENTIAL density range that the scanner
> is capable of.

And that's not right...

> Scanner CCDs are linear devices - it is important you
> understand this concept because it determines how well scanners can
> potentially see into the shadows.

What is done in some scanners, is the analog output from the CCD to the A/D
is "adjusted" (like a tonal curve) such that it boosts the ability of the
scanner to "see" into the shadows, or any other parts for that matter.

> With a linear device each doubling or
> halving of a scanned linear data value represents a doubling or halving of
> light, or 1 stop, or 0.3 density units.

But, as a note...don't confuse relative density units with absolute density
values.  Yes, two values from the scanner can be 2:1 relatively...but that
does not say what the absolute density value of either of them is.

Also, because the CCD is linear, does not mean that that the system doesn't
have gain and offset...

> Let us take our 8-bit scanner then - 8-bits gives us a total of 256 values
> that the scanner can use to scan the image. Let us look at the numbers to
> see how these data values can be spread out across a density range.

As I've said, there is no direct calibration/correlation between "density
range" and data values.  I can take the same density range, and spread it
across ANY number of bits, or position it anywhere within any number of
bits...it solely depends on the output characteristics of the CCD, as well
as the analog front end between the CCD and the A/D.

It is up to the CCDs sensitivity that determines what density range is
presented to the analog front end, and then what the analog front end does
with it before it gets to the A/D that determines what density range the A/D
sees.  I can take the exact same CCD that is used in a 14 bit scanner and
put an 8 bit A/D in, and still get the exact same density range (less the
lower 6 bits).  I could also take your 8 bit scanner example, and simply put
a 4 bit A/D, yet get the exact same density range out of it...

I'm not speculating on this.  I design digital imaging systems, and have
been for 20 years.  I've designed quite a few film scanners, and I know how
they are designed, or at least how I've designed them ;-)

One easy example to show that what you say is inaccurate, is that the
density range of a 16 bit file, converted to an 8 bit file, is the same
(except minus the lower 8 bits)...and the converse is true, you can take 8
bit data and convert it to 16 bit data, and it still represents the same
density range.

Regards,

Austin

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