Digital BW, The Print

Roy,

Thank you for your well thought out post on Imaging. I refrained from
responding earlier in hopes that someone new would add to your thoughts. As
you mention at the end of your post people are probably a more than a bit
reluctant of even mentioning some of these terms at this point.

I spent some time going through what books I have on photography from a
technical imaging point of view and the concepts that are of concern were
much as you have described them. The spatial component of the system is so
intuitive it is not mentioned except in an artistic manner. What is
discussed at length is the relationship between the SBR (Scene Brightness
Range or Scene Luminosity Range), Density Range and Average Gamma (the
average slope of the line from minimum to maximum which represents
contrast). From these three they map SBR to the film and then to the paper
showing characteristic film and paper curves.

Here in the area of digital printing we still need to define that path from
mapping film density to the print and digital capture opens up similar
questions. With the new technology added to these mappings different
variables may have greater importance at different stages but your 3D
concept holds a great deal of merit in keeping us focused on what is
important.

The idea of distinguishing between tones and the number of tones are
concepts that I do not ever recall hearing in discussions of traditional
photography. If not new concepts, they seem to be new to the discussion of
photography. I believe that they were not discussed in traditional
photography because they simply were of no interest. The technology of
traditional photography generally exceeded the ability of our eyes to
discriminate tones. With the advent of digital imaging, which started out
with quite poor quality, these ideas became vitally important. Once the
quality of the equipment, software and materials of digital imaging
consistently exceeds our eye's ability to discriminate tones the concepts
will once again hold little interest.

Thanks,
Martin Wesley

----- Original Message -----
From: "royvharrington" <roy@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Monday, April 01, 2002 3:02 PM
Subject: [Digital BW] Thoughts about Imaging


>
> Hi All,
>
> The ongoing discussion and debate about dynamic ranges and
> numbers of tones, etc. seems to just go on and on endlessly
> without getting anywhere.  All of us in this group are
> interested in "Imaging" in some form and at some level. I've
> written the following with the idea defining Imaging, seeing
> where the ideas come from, how they relate, how we talk
> and communicate amongst ourselves and others about Imaging.
> Its a little long winded but I felt I need to get my thoughts
> down.
>
> IMAGING
>
> What is Imaging?  I'm talking about anything and everything
> we SEE with our eyes.  Thus X-Ray Imaging, Sonic Imaging are
> all out of the discussion.  Imaging is really DEFINED and
> orginates by and because of human sight.  Our eyes focus
> light from the outside world projecting a 2-dimensional
> imaging on the retina.  The rods and cones of the retina are
> sensitive to light and collect light intensity information
> over a two dimensional space.  The hardware of the eye in
> conjunction with the software of the brain work together to
> give us the perception we call "sight".
>
> An Image is a varying intensity of light spread over a
> two-dimensional space.  Everything in Imaging is related to
> the creation, detection, processing, storage or reproduction
> of Images i.e. varying light over a two-dimension space.
> Certainly, there are many abstractions or intermediate
> representations of Images but they all relate back to the
> original concept of "varying light over a two-dimensional
> space".
>
> Obviously, since "sight" is one of the most basic sense,
> the English language (and I have no doubt all languages) have
> many words relating to "light" and "varying light".  We talk
> about light intensity, brightness, dark/light, black/white.
> A very key issue is that the light intensity varys; we
> are very aware that there is a range of light intensity
> that is "useful".  We can't stare into the bright sun
> because its too intense.  Likewise in a totally dark cave
> we can't see anything.  There is a range of light intensity
> that is humanly useful.  Light intensity can go above or
> below the humanly useful range, but this "useful range" is
> what is interesting.
>
> Putting this all together we have a very good model of
> what an image is.  All told its a 3-dimension object with
> two dimensions that are spacial and one dimension that is
> light intensity.  Another key aspect is that all 3 dimensions
> are "bounded" -- nothing extends out to infinity.  The
> space dimensions are obviously bounded by left and right
> bounds in the horizontal direction and bounded by top and
> bottom in the vertical direction.  The light intensity is
> bounded by "darkest" and "lightest".
>
> Thus, an Image is a 3-dimensional bounded object with two
> spacial dimensions and one light intensity dimension.
>
> As I said earlier, Imaging is everything is related to
> the creation, detection, processing, storage or reproduction
> of Images.  All Imaging operations take an image (3-dim object)
> and at best keep as much information as possible.  In
> reality most will tighten the boundaries some how chopping
> off some of the information.
>
> For a simple example, lets take some photos.  We see a nice image
> and point the camera and look thru the viewfinder.  Immediately
> we've trimmed the two spacial dimensions.  We now have
> much less in view than with our eyes. (Artistically, that's
> probably exactly what we want.)  Next of course we trim
> the light intensity dimension by putting the image on
> film.  The lightest and darkest we can see with our eyes
> is a much wider range than the film in the camera can
> handle.  The film can only record a much narrower range
> of light intensities.  It has a smaller "useful" range.
> Things aren't so bad because although our eyes can handle
> very large ranges of light, often the scene we are
> photographing actually has a much narrower range.  We
> adjust the exposure settings on the camera aligning the
> narrower scene range to narrower film range.  Later on
> when we print the negative we again have the opportunity
> to trim the spacial dimensions (cropping) and of course the
> more challenging task of taking the range of light intensities
> on the film and mapping or transferring them to a piece of
> paper.  The mapping can involve just shifting darker or lighter,
> but also can involve compressing or expanding the range to
> better fit the physical characteristics of different media,
>
> In general, all imaging steps put constraints of some kind
> on the image information.  For an image to go thru many steps
> we've got to be careful at every step of the way.  The end
> result can never be any better than the worst step.  Trimming
> in the spacial dimensions is so obvious that it probably
> needs no discussion, but dealing of the light intensity
> data is probably the main effort in all imaging endeavors.
>
> The set of technologies and industries that an image may
> pass thru from start to finish is truely amazing.  From
> light meters, to exposure, to film, to developing, to
> photo paper, to another developer, toners, etc.  If there's
> digital involved there's CCD's, scanners, A/D and D/A
> converters, lots and lots of computer software, printer
> drivers, printer hardware, paper technology, ink technology.
> Get the idea!
>
> Anyway, every single step of the way deals with the range
> of light intensity.  It's represented in many, many ways,
> but there are several universal characteristics.  First,
> they all have to represent a range of values.  They have
> to have dark values thru light values.  It may be a continuum
> of dark to light or it may be discreet values, but there
> are always endpoints i.e. max and min.  The endpoints
> may be fixed obvious points like white paper and paper
> with the maximum amount of black ink.  Some endpoints are
> a little less obvious -- like film.  Film itself goes from
> fb+f (filmbase+fog) to very, very dense (take film expose
> it to lots of light and develop it a lot), but there is
> definitely a sweet spot -- its linear between the toe and
> the shoulder.  This is the useful range for film.
>
> So in moving the image from one step to the next we need
> to keep moving the range of light values along the road.
> Essentially we want to map the darkest value (and lightest
> value) of one step into the darkest/lightest value of the
> next step.  We are trimming info if that mapping moves
> a value outside the next step's useful range and conversely
> if we don't use the whole range of the new step we are
> using that range to the fullest extent.  One very important
> characteristic is that given two values (darker and lighter)
> they can get closer or farther away tonally. They can even
> merge together so they are no longer distinguishable --
> lost info of course.  But they can never change places.
> This guarantees that mapping max to max and min to min
> keep the range monotonically intact.  We can lose info
> due to merging of tones but sometimes that's unavoidable.
> What is not guaranteed is that the mappings in the middle
> not at all the linear.  Its the linearity and the
> distinguishability in going from one step to the next that
> we spend all our time and effort.
>
> The image with its range of light values moves from step
> to step being subjected to the range of possible values
> for each step.  Each step can represent completely different
> technologies.  In each technology there are whole sets of
> terminology, descriptions and measuring standards for its
> dealing with the range of light values.  The range of
> useful values is called the Dynamic Range.  The Dynamic
> Range describes the range of useful values, but of course
> since the values may be represented in different ways the
> units of expressing the range may vary.  Some technologies
> have very commonly used units and the Range is expressed
> in those units.  Some use the term just descriptively
> to refer to the range but there really isn't a measure
> or quoted number for the dynamic range.
>
> Here's a few examples to show some of the variation.
>
> Dynamic Range of film can be expressed as density range
> of film itself from the minimum to the maximum useful
> density.
> Alternatively, it could be quoted in the number of stops
> of exposure variation that leads to the above densities.
>
> Dynamic Range of a scanner is typically quoted as the
> maximum density of the film that the scanner can  get
> detail from.  Here, minimum density is assumed to be
> zero, so it is a range.
>
> For a final print Dynamic Range is just the white of
> the paper to the max black ink we can get.
>
> A particularly curious case is the data file inside the
> computer.  If its a 8-bit grayscale, by convention 0 is
> black and 255 is white, if it was a bitmap (1-bit) there's
> just 0 and 1.  In a sense they both have the same
> dynamic range but being unitless and totally arbitrary
> it probably not very informative.
>
> The point here it that the term "Dynamic Range" is used
> in many different contexts both as a calculatable measure
> and as a descriptive label for the range.  The key is that
> all the contexts are tied together by the mapping of
> the range of one step onto the range of the next step.
> And the ranges are all ranges of light values from
> dark to light.
>
> --------------------------------------
>
> Obviously, there are other viewpoints about using this
> term "Dynamic Range" (Hi Austin).  But I've looked at quite
> a few specs and discussions using this term, each and
> every one in Imaging uses it in this context.  The range
> of light values from dark to light is unquestionably the
> most basic property of sight.  Austin's Range of Contrasts
> from min contrast to max contrast, leaves me and I believe
> others with a "What the hell is it?" feeling.  I've
> seen several people innocently ask about Dynamic Range
> in a context (I believe) much like my description and barrage
> of discussion that gets nowhere ensues.
> The regulars here now avoid saying the term like they
> avoid the plague.
>
> For me it boils down to two issues:
> 1) Range of Contrasts doesn't help my understanding of
>     Imaging i.e. it doesn't go with a perceptual property
> 2) Nobody else in Imaging uses the term that way.
>
> ---------------------------------------
>
> Term usage aside, I found the exercise of thinking thru
> Imaging to be helpful to myself.  Comments?
>
> Hopefully someone else will find this useful.
>
> Roy
>
> --
> Roy Harrington
> roy@...
> Black & White Photography Gallery
> http://www.harrington.com
>
>
>
>
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