[filmscanners] Re: scanner dmax discussion

["Karl Schulmeisters" <karlsch@earthlink.net>]

As a computer nerd geek with a background in electronics, your analysis of
the mapping of bit depth to DMax is correct.  In fact it goes further than
that, there is no requirement in either electronics or the conversion from
Analog signal to Digital data (A/D converter) for there to be a linear
conversion.  It turns out that a linear conversion makes the electronics a
touch simpler, but that in turn assumes a perfectly linear detector -
something that does not exist.

So in part, the 'wizardry' of the electronics inside the scanner A/D
converters is to try and compensate for the non-linearities of the detectors
themselves.

Note also, most of the detectors have photo response curves not that
dissimilar from film, so by changing their detection threshold, you can
drive them into different parts of the response curve.  Not all scanners
allow you to do this, but in theory it is possible.

So answering your question about the Epson is a bit difficult without
looking at the data stream coming out of it.  There are scanners that allow
you to scale (or autoscale) the image range to maximize the data extracted -
but they are pretty pricey. That said, I would speculate to say that if you
are scanning a negative on most affordable scanners, that has a dMax less
than the theoretical dMax of the scanner, the scanner will simply throw away
the high order bits.


George DeWolf is wrong in his analysis.

----- Original Message -----
From: "Tim Atherton" <tim@KairosPhoto.com>
To: <karlsch@earthlink.net>
Sent: Tuesday, July 08, 2003 9:42 PM
Subject: [filmscanners] RE: scanner dmax discussion


well I've got to tell you, I couldn't make head or tail of the first post,
never mind the follow up - now I'm going to have to wait for my copy of View
Camera to arrive and see what the article actually says...


"I thought I understood the relation of bit depth to dynamic range, but
now I'm not sure.  In the latest issue of View Camera, George DeWolfe
says that the maximum possible dmax for an 8 bit gray scale is 2.4 while
  the maximum possible dmax of a 16 bit gray scale is 4.8.  I understand
mathematically where these figures come from.

log_10(2^8) = 2.41   and  log_10(2^16) = 4.82.

But I don't see what this has to do with dmax.  The light detecting
devices in scanners are physical devices capable of responding to some
range of light intensities.  Below a certain level, say I_min, nothing
will be recorded.  Above a certain level, say I_max, additional light
won't produce any more output.   So it seems to me the total range of
densities the device can handle should be log_10(I_max/I_min).  It seems
to me that the bit depth just determines how finely that range is
subdivided.  For 8 bit, it will be subdivided into 256 distinct levels,
while for 16 bit, it will be subdivided into 65536 distinct levels.  Of
course, if there is some minimal ratio of intensities which is
detectable and we assume the scanner is keys to seperating values
reflected by that minimal ratio, then the two calculations above would
be relevant.   But why can't a scanner with 8 bit depth just use a
larger step size.  After all, the theory behind all this is that if you
take the human visual system as a standard, then when viewing a single
gray scale from deepest black to whitest white, a discrete set of 256
values separating that range will appear continuous.

I would appreciate any comments from experts about just what is going on.

Let me also ask a related question.  Just what does my Epson 3200
scanner and its software do when I use it to scan a b/w negative whose
maximum density is about 1.4  at 16 bit depth?   Does it try to divide
up the range from 0 to 1.4 into 65536 values, or does it just use
1.4/3.4 of that range for about 27000 distinct values.  What I get using
Vuescan is values ranging from 0 to 255, and although those may be
further split up, when I read them into my photoeditor, they also stay
in that range."

> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of LAURIE SOLOMON
> Sent: Tuesday, July 08, 2003 9:29 PM
> To: tim@KairosPhoto.com
> Subject: [filmscanners] RE: scanner dmax discussion
>
>
> Austin,
> We have been through this discussion several times over; I am not about to
> embark on it again, much less start it. :-)  That topic has
> always provoked
> heated discussions involving misused terminology and
> misunderstood concepts
> with everyone claiming theirown use to be the proper and correct usage
> unlike that of others whom they are debating.  Rather than
> solving vagueness
> and ambiguity by trying to settle on a common set of terms have a commonly
> understood denoted concepts, arguments erupt over language and
> whose notion
> is the real one.
>
>
> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Austin Franklin
> Sent: Tuesday, July 08, 2003 9:31 PM
> To: laurie@advancenet.net
> Subject: [filmscanners] RE: scanner dmax discussion
>
>
> > Interesting discussion at rec.photo.equipment.large-format and
> > comp.periphs.scanners
> >
> > Relation of bit depth to dynamic range
>
> That subject, for some reason, seems to be a very heated topic.
> Unfortunately, due to marketing misinformation, it's a very misunderstood
> concept...
>
> Regards,
>
> Austin
>
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