[filmscanners] Re: scanner dmax discussion

["David J. Littleboy" <davidjl@gol.com>]

"Austin Franklin" <austin@darkroom.com> writes:

>>>>>>>>>>>>
Well, hum.  Even if 8 bits wasn't enough, and it really needed 16, does the
density range actually increase?
<<<<<<<<<<<<

Yes, the measurable density
range increases as well, by definition. Since density range is defined as a
dynamic range sort of thing (i.e. <full range>/<step size>), the data that
comes out has a wider/narrower range depending on how many significant bits
got thrown out. That's the confusion in scanner specs: "density range" is
_defined_ to be identical to "dynamic range".

>>>>>>
  Keep in mind, that every value is +- 1/2
the bit value.  So, the bottom and top number still CAN represent the lowest
value (as well as the top number), and therefore, the density range that is
"seen" can very well be identical.
<<<<<<<

This is what I now believe to be incorrect.

Here's what I see as the right way of looking at it: Each value measured by
the scanner corresponds to a range of densities: in a 1-bit system, "0"
means "0 to 0.5", 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.

To bash this nail a bit more: in a 1-bit system, any density in the range 0
to 0.5 will be reported as 0. So when you look at a measured value of zero,
the best you can say about it is that it's "0.25 with an error tolerance of
+/- 0.25". It's quite problematic to look at a measured value of 0 and claim
it means "0" with an error tolerance of +0.5/-0.0", which is what you are
doing in the above.

As you increase the number of bits, the "center values" of the extreme
values (0 and 2^n -1) get closer and closer to 0 and 1 respectively.

Again this makes sense from a measurement standpoint, since a
measurement should be seen as the center value of the range of physical
measurements that could return that value. Arbitrarily declaring two of the
measured values to have different meanings is problematic.

David J. Littleboy
davidjl@gol.com
Tokyo, Japan


----------------------------------------------------------------------------------------
Unsubscribe by mail to listserver@halftone.co.uk, with 'unsubscribe 
filmscanners'
or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or 
body