[filmscanners] RE: scanner dmax discussion

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

Roy,

> The formulas above also are based on physical values i.e. the formula
> is log10(intensity) = density.   So saying that 8-bits implies a max of
> 255
> and therefore log10(255) = 2.4 implies a dMax of 2.4  ONLY works if
> the 8bit values are directly proportional to the intensity of the light
> passing
> through the film.

Just to add to this...  Density numbers are absolute numbers.  If you have a
calibrated densitometer, and measure a density value of 2.4 on "a" sample,
the same "sample" can be measured on another (calibrated) densitometer as
2.4.  What you are able to represent with 8 bits is a DENSITY RANGE (and
just so happens, dynamic range, because in this case they are, more or less,
"equal") of 2.4.  That in no way at all means that one of the values, much
less 255 is equal to a DENSITY of 2.4, it would be equal to a relative
density of 2.4 ONLY relative to the lowest value, but not necessarily an
absolute density of 2.4.  This is, again, why density values are not the
same as dynamic range.

> You can use a larger step size.  However, it has no effect on the
> density
> range.

Agreed.  And is why adding a higher bit A/D (beyond what is matched to the
CCD/Analog Front End) doesn't increase either the dynamic range or density
range of the scanner...the "extra" bits are basically "in the noise".  So,
when a scanner claims 16 bit A/D, there is no guarantee that all 16 bits are
actually "good" bits.

Regards,

Austin

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