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



> I find this conversation fascinating and hope you will stay the
> course with all
> my (possibly naiive responses.) I'm no expert at all but I have
> ideas that I
> thought were true and I'd like to work them through with you whatever the
> outcome...

Hi Tony,

I'll do my best ;-)

> On Sat, Jun 08, 2002 at 01:18:17PM -0400, Austin Franklin wrote:
> >
> > > Hi Tony,
> > >
> > > > 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.
> > >
> > > Not true.  You can represent ANY density by any number of bits.  I can
> > > represent an entire density range with two bits:
> > >
> > > 00 - at or below dMin
> > > 10 or 01 - between dMin and dMax
> > > 10 - at or below dMax
> >
> > BTW, that should be "at or ABOVE dMax"...sorry.
> >
> > > This is a completely valid encoding of density range.
>
> Sure it is.

Great start ;-)

> Is that what happens in a scanner though?

The major point is, though they MAY be somewhat "similar", they are not the
same.  They "they" is density values and the numbers you get out of the
scanner.  IF the scanner were calibrated, as densitometers are (but more
complexly), one "could" make them close enough to be considered "the same",
but no commercially available scanner I am aware of does this.

> I thought that the
> CCD behaved in a linear manner?

Yes, but relative only unto it self!

> Does that not dictate the encoding method
> somewhat?

Well, the linearity doesn't, but, the sensitivity does.  If the CCD outputs
+-1V and has a noise level of .001V, then you would have 2/.001 or 2000
discrete values you needed to discern, and would need 10 bits...but that's
dynamic range, NOT density range.  The density range of the sensor is what
ever it is...again, part of the sensor's spec.

>
> > >
> > > I have to run now, so I'll comment on the rest of your post later.
> > >
> > > Regards,
> > >
> > > Austin
> >
>
> > And...to add further to this, as I have a few moments...
> Density values are
> > absolute values, just like one foot is an absolute value.  They
> have meaning
> > in and of them selves.  Someone decided what the exact length
> of one foot is
> > (within a tolerance of course), as well as density values.
> They mean the
> > same thing everywhere...a density value of 1.6 means exactly that in La
> > Jolla CA, as well as in Pascagoula, MS.  Same with one foot.
> But, the tonal
> > values out of a scanner are not the same everywhere, nor are
> they the same
> > even between scans!  The value 136 in La Jolla, CA...does not
> have the same
> > tonality as the value 136 in Pascagoula, MS.
> >
>
> Yep I agree with you here. That density values are absolute values. But
> relative density is also commonly used in film sensitometry is it not?

Yes...but I don't see how that ties into us here.  Again, the scanner isn't
calibrated, and dMax and dMin values ARE calibrated/absolute values...

> > Scanners are not calibrated TO anything, except themselves.
> That is why the
> > data values you get from the scanner are not the same "density values".
> > There is no direct correlation between them, unless you were to
> calibrate
> > your scanner in the same way densitometers are calibrated (even that is
> > insufficient, as the sensors used in film scanners have different
> > characteristics than the sensors used in densitometers).
> >
>
> OK I have just bought a densitometer which lets me calibrate so that a 0
> reading can be set to film base, then readings taken from there.
> Cannot the
> same be done for a scanner?

Yes, but that has nothing to do with dMax and dMin...with respect to any
other scanner, and comparing scanners.  You can't say that film has a dMax
of X when measuring in that mode, you can only say it has a relative density
value of Y compared to the film base.

> If I scan a piece of film and the film base
> gives me a value of 248 (assume 8-bit here just for argument's
> sake) cannot
> I set that as my point from which all my other density values can be
> measured?

Yes, but again, that doesn't relate back to absolute density values...

> I may not know the absolute density
> reading but I would know the density range of the film I am scanning.

But comparing that to another scanner would be meaningless...  You would
have "a" density range, sort of...simply because a density range of 1 to 2
is the same relatively as 2 to 4, but obviously, 4 is one heck of a lot
denser than 2.

> If I
> were scanning a very dense piece of film (say Velvia) then I would need
more
> than 8 bits to see well into those shadows would I not?

No.  I can design a scanner (and have) that only uses 4 bits to encode the
entire range from 0 to 4.  It just has no resolution (dynamic range).  Think
about line art.  It has high density range, but no tonality...and you don't
need more than 1 bit to encode it ;-)

> Surely one is capable of determining relative
> density with one's own scanner?

Not accurately, unless, as I said, you calibrated it.  You need to calibrate
for offset, gain and linearity.  Though the sensors are somewhat "linear" in
the sense that you are talking about, they still aren't perfect.

> I've made some film charateristic curves
> using this method that seem reasonably good.

Great!  But...they are relative unto themselves, right?

Regards,

Austin

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