[filmscanners] RE: Dynamic range

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

Hi Todd,

> I thought what determined the DyR of a scanner was its ability to
> accurately
> read into dense film. IOW, to discern low voltages (readings through high
> density film) from noise. IOW, how far from clear film into density it can
> distinguish -- not how finely the image gets chopped.

Basically, correct.

> I understand that if you don't have enough "chops" (bits) your
> scanner will
> run out of encoding room, so to speak, and won't be able to continue
> assigning values to the sequentially lower voltages. Thus insufficient bit
> depth can LIMIT DyR.

Correct.

> But this is just incidental to how scanners work, not
> what DyR is. Thus, simply adding more bits to a noisy scanner may
> do nothing
> to increase DyR.

Correct.

> Therefore, isn't a scanner's DyR in fact a function of
> being able to discern shadow detail above noise?

Not only the shadow detail over noise, but it is based also on the overall
range it can scan.

> IOW, the
> "distance" it can
> record from one endpoint (clear film base) to the other (film dMax)?

"Distance" is only one part of the equation, how finely one can discern over
that distance is the other part.

---------------------------

> If you designed a scanner that could only handle film with a DyR
> of 2.2, but
> it could do so by spreading that density across 16-bits, would it have a
> higher DyR than a scanner that uses 14-bits to accurately record
> film with a
> DyR of 3.9? I think not, not by any conventional use I've seen of the term
> DyR.

A scanner that had 16 "accurate" bits, compared to a scanner that had 14
"accurate" bits would have a higher dynamic range, no matter what the
overall density being read was.

The dynamic range changes based on the minimum discernable signal (noise)
and/or the overall signal range.  If one changes, the dynamic range changes.

The dynamic range of 16 bits = 10log2**16 or 48dB
The dynamic range of 14 bits = 10log2**14 or 42db

A sort of "counter" example...Now, if you designed two scanners that had the
same noise for both a density range of 2.2 and then a density range of 3.9,
the scanner that had the density range of 3.9 would have the higher dynamic
range...remember, the dynamic range is based on the OVERALL range AND the
noise, so if you increase the overall range, and have the same noise, you
increase the dynamic range.

Note you would need a different number of bits for the A/D for each scanner
design, since the noise is the same, but the overall range changed...the
scanner that scanned 3.9 would require more bits than the scanner that
scanned only 2.2.

> If I'm correct, here is an analogy. We have two rulers, one a 12"
> marked in
> 1/16" increments, the other 24" marked in 1/4" increments. If we
> understand
> DyR to be the ability to measure further from one endpoint to another, the
> longer ruler has the capacity to record the greater DyR (24"), not the one
> with the greater resolution (12").

Let's look at the dynamic range of each:

24 (overall range) divided by 1/4 (minimum discernable) = 96
12 (overall range) divided by 1/16 (minimum discernable) = 192

The 12" ruler with 1/16" markings has a higher dynamic range than the 24"
ruler with 1/4" markings...  That's why I said that the actual values of the
data aren't relevant to the dynamic range...

Austin

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