[filmscanners] RE: Dynamic range

["Paul D. DeRocco" <pderocco@ix.netcom.com>]

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

> From: Austin Franklin
>
> "Distance" is only one part of the equation, how finely one can
> discern over that distance is the other part.

I don't think that's an issue, because it's trivially easy to use enough
bits in the A/D converter that you're limited by noise _throughout_ the
range, not just at the dark end.

> 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.

You can't really base dynamic range specifications on the numbers, because
there's no guarantee that the numbers bear any particular relationship
(linear, log, gamma) to the light power. Dynamic range really should be
measured as the (log of) the ratio between the strongest light power that
the scanner can read without clipping, and the weak light power that is
equivalent to the electrical noise in the sensor and subsequent electronics.

If you then took the log of the signal level, and converted that to a
digital value, the number of bits you have would no longer have any
relationship to the dynamic range, only to the resolution. That is, if the
electrical noise corresponds to a light level that's one millionth of the
light level that clips the sensor and converter, you've got 60db of dynamic
range, regardless of whether you use eight bits (60/256 db per step) or
sixteen bits (60/65536 db per step).

All the stuff about rulers is meaningless, because dynamic range is only
meaningful when measuring the ratio of power levels, not linear quantities
like length.

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