[filmscanners] RE: scanner dmax discussion

["Austin Franklin" <austin@darkroom.com>]

Karl,

> >"> and "1", means "0.5 to 1.0", so if you see the center of
> >> that range as the "meaning" of the reported value, a 1-bit
> system is only
> >> representing the range 0.25 to 0.75.
> >
> >I think that is what's incorrect, as the meaning IS the range, not the
> >center of the range"
>
> Except Austin, that's not how these devices are calibrated or tested.

Not so, Karl.  There is nothing TO calibrate with the A/D as far as how many
steps it has and the range that each step represents.  This is a design
property of the A/D.

> The
> universal (I have yet to see another one used but I am willing to be
> corrected) approach is to use a step tablet with density increments of 1/3
> of a stop and the 'dynamic range' is measured by seeing where the scanner
> ceases to differentiate between two adjacent steps.

I don't see how that relates to what I said.  CCD film scanners, in general,
are self calibrating, and simply calibrate for intensity when powered up.
An offset table is created, one for each position of the sensor.  This
offset table is basically a LUT (Look-Up Table) that is then applied to the
data from each sensor position.  The A/D "values" (as in what value out of
the A/D represents what analog voltage) aren't calibrated at any time, they
are simply a property of the design.

> Assuming that is step 'n',  and that maps to 'all bits on', then it is
> ambiguous as to whether 'All but LSD bit on' means
> a) the range from 'all bits on -2' to 'all bits on -1'
> or
> b) the range from 'all bits on -1' to 'all bits on'
> or
> c) the range of density values that map to 1/2 way between 'all
> bits on -1'
> to 1/2 way between 'all bits on'.

Sorry, I have no idea what you're trying to say there...

> Since it is ambiguous, the most consistent way to do this is to
> assume that
> each discrete measurement represents the center of a value range,
> +/-(0.5 x
> the full value range).

What is the difference between:

1) each discrete measurement represents the center of a value range +- .5 x
of the range

and

2) each discrete measurement represents a range of values +- .5 of the value
given

The first says that a value of 3 represents the center of a range from 2.5
to 3.5, and the second says the value 3 represents a range from 2.5 to
3.5...that is the EXACT same thing.  In both cases, an input value of 2.5 to
3.5 will give an output value of 3.  Is that not so?

Of course there some overlap in input values that can give a 2, 3 or 4
depending on how high/low the value is, as there is no "hard" thresholding,
again, everything has a range, including the resolving of the values...it
just resolves to one of two values out of the A/D.

Austin

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