[filmscanners] Re: Dynamic range

["Todd Flashner" <tflash@earthlink.net>]

David,

Thanks for your reply.

I think one of us is missing the others point. Since my post was all over
the place I'll take responsibility. ;-)

My primary point was that with DyR defined as Dmax - Dmin, as it is by the
ISO, it is the range between the minimum discernable signal (which is what
the ISO calls Dmax) and the maximum signal before clipping (which is what
the ISO calls Dmin). Period.

These values can be obtained from testing, and the bit-depth/resolution
within that range is immaterial to the DEFINITION of DyR. It may be material
to the values you will measure in testing, but it is immaterial to the
definition/formula.

As to your point that scanners report density ranges as values...

You say a 1-bit scanner may assign a value of zero to any tone from black to
mid gray... Okay, but do you realize that the range that Austin is using as
his Dmin for the ISO formula is the ENTIRE density range of the scanner? If
a 1-bit scanner can assign any range a value that is 50% of its density
range, what bit depth scanner is it that will assign a signal the entire
scanners density range?

With all due respect, I believe the type of ranges you are speaking to
(quite small for highbit scanners) is totally disproportionate to the
discussion at hand. But I may have misinterpreted your points, other than
those about noise, which are well taken.

Todd Flashner

>
> "Todd Flashner" <tflash@earthlink.net> wrote:
>>>>>>>>>>>>>>>
> Well I guess this really breaks down to the simple difference as to whether
> DyR is a resolution vs a range. I think your opponents agree that IF it were
> a resolution then your approach would be correct, they just believe it is a
> range, as its name would indicate.
> <<<<<<<<<<<<<<
>
> My latest theory as to what's the problem is that people don't understand
> what the values reported by a scanner mean. What a value n means is "the
> range of actual densities the scanner reports as n."
>
> Consider a 1-bit scanner. The value "0" means "I saw a level somewhere
> between true black and the first value I can recognize as "1", and the value
> "0" means "I saw a level somewhere between true white and the first value I
> can recognize as "1". Thus, if we view the value as the center of the range
> it corresponds to, "0" does _not_ mean "black", it means a somewhat dark
> gray, and "1" does _not_ mean "white", it means a somewhat light gray. So
> dMax for this scanner is dark gray and dMin for this scanner is light gray.
>
> When we view or print, we (incorrectly) interpret 0 to mean perfect black
> and 1 to mean perfect white. But that's not what the scanner means. The
> scanner means "I saw a value somewhere in a range."
>
> Note that this range for a 1-bit scanner is incredibly wide: it reports the
> same value for fully half the possible density ranges on the film. This is
> called quantization error.
>
>>>>>>>>>>>>>>>
> Okay so in my example above, the dmax of the scanner coincides with the
> densest patch on my target film that the scanner can distinguish over noise.
> I said it to be a density of 3.7.
> <<<<<<<<<<
>
> And as the noise (including quantization noise) goes down, this value goes
> up.
>
>>>>>>>>>>>>
> This corresponds to my film base+fog. Let's assume it could theoretically be
> zero density (maybe the film doesn't fill the film holder).
>
> So what don't I get?
> <<<<<<<<<<<<<<<<
>
> Similarly, the noise (including quantization noise) defines this level as
> well. Note that this isn't film base + fog. It's film base + fog + noise.
>
>>>>>>>>>>>>
> If the density of the film base+fog were 0, and the readable dmax  occurred
> at a film density of 3.7, using THIS spec, how could the DyR be anything
> other than 3.7 - 0 = 3.7 for BOTH scanners?
> <<<<<<<<<<
>
> (1) The density of the film base+fog+noise will never be 0.
> (2) The dynamic range is the _ratio_ of these values.
> (3) An important noise component is _quantization error_. If you have enough
> bits, it can be ignored, but if you don't, it can't.
>
> David J. Littleboy
> davidjl@gol.com
> Tokyo, Japan
>
>
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