Digital BW, The Print

Hi Austin,


--- In DigitalBlackandWhiteThePrint@y..., "Austin Franklin" <darkroom@i...> wrote:
> Hi Roy,
> 
> > Now the big question is "What's the signal?".
> 
> Very simply, for a photographic image, it is the density value.

This is precisely what I'm trying to say.   Let's take the original definition
equation and substitute "density value" for "signal".  We have:

Dynamic Range = log10 (largest signal/smallest discernable signal)
            =  log10 (largest density value/smallest discernable density value)

Isn't that precisely the same as: dMax - dMin  as I derived last post?

> 
> > All my own
> > recollections and everything I saw in the book has an audio/video
> > type flavor.  I.e the diagrams show nice sinusoidal signals in
> > the time domain.
> 
> But time has never been a part of the dynamic range equation at all, even in
> the case of audio...it is purely voltage.
> 
> > Austin, you've made a clear choice in this mapping from the time
> > domain to the space domain.
> 
> Not at all...see above.  Neither time or space have anything to do with
> this.  It is purely a (any in fact) measurable quality that is required,
> whether voltage or density or what ever.  It is what ever property YOU want
> to figure out the dynamic range of.

It not that time or space figure into the calculation, its just that they
built into the properties that we are interested in.

You say I can figure the dynamic range of whatever property I want.
Well I want the dynamic range of the density of the print!

It seems that you are insisting that I can only calculate (i.e. have) the
dynamic range of the contrasts of the print.

> 
> > For you "signal" is difference
> > between to tonal densities, i.e:
> >     largest signal = dMax - dMin
> >     smallest discernable signal = smallest density difference
> > or maybe I can paraphrase "signal is basically contrast".
> > (I hope I'm not putting words in your mouth!)
> 
> Sure, I'll fly with that.
> 
> > As far as I can tell your derivations from this and all the
> > discussions are consistant and mathematically sound.  But I've
> > been a little unsure of picking this mapping rather than
> > something else.
> 
> I disagree with your use of the term "mapping".  Again, you pick the
> property you want to figure out the dynamic range of...note the dynamic
> range equation does NOT use any units for largest/smallest...they simply
> have to be on the same scale, of the same unit.

Right.  And I want the property "density".

> 
> > For pictures, what's the basic perception?  It seems you've picked
> > contrast --  so the ratio is (highest contrast/lowest contrast).
> 
> Yes, since contrast (as in delta change) is all that distinguishes tones
> from one another.
> 
> > Why not the more basic perception: darkness versus lightness?
> 
> What, exactly, do you mean by "darkness" and "lightness"?

Can't you go with: darkness = high density, lightness = low density??

> 
> > The ratio would be (darkest/lightest). I guess you can see
> > where this is going->> it reduces the whole thing to
> > Density Range.
> >
> > DynRange = log10 (darkest/lightest)
> >          = log10 (darkest) - log10 (lightest)
> >          = dMax - dMin
> >          = Density Range
> 
> But you have now removed, arbitrarily from what I can tell, the smallest
> discernable signal from the equation, that above, you agreed was part of the
> equation.  Why?  This seems like one of those Gary Larson "and then magic
> happened" on the chalk board...

I figured dMin = smallest discernable density  = smallest discernable signal
I haven't any problem keeping the word "discernable" throughout.

The fact that density, as commonly used, already has a log10 built into it
makes the math manipulation slightly different order.  The idea is to distribute
the log10 function or the ratio.  The log10(largest density) becomes dMax,
log10(smallest discernable density) becomes dMin and finally the divide
becomes a subtract.

> 
> > I doubt that you're ready to switch definitions but intuitively
> > using "dynamic range" to means "number of tones" rather
> > than some "range" of values seems weird.
> 
> Not at all, my guess is it really means dynamic OVER THE particular
> range...but I do not know the history of the term.  It isn't really A range,
> but it describes a property OF the given range.  Density range isn't a range
> either, but it does describe the width OF the range of density.

"Range" as commonly used in math is used to describe the set of possible
values.  Density range given by dMax and dMin entirely describes the
possible density values i.e. they all have to lie between dMax and dMin.

> 
> > And by the prevalence
> > of the "dynamic range" threads that go on regularly many
> > others don't feel comfortable with your definition.
> 
> Yeah, but I didn't make up the definition, as you well know!  Remember, this
> is a very technical issue, being described to and by and used by a lot of

True, but, being technical does NOT exempt it from scrutany and passing the test
of usefulness. 

I'll acknowledge that you didn't make up the definition taken from the Higgins
book, but I've yet to see corroborating info for your interpretation of
"signal" in the equation.   Higgins most definitely does NOT address how to
apply the equation to the photographic prints, so I'm assuming you 
decided how to do that.   As you say below there's lots of info about.
I'd like to see more supporting your  interpretation.

> lay people.  There are a LOT of misperceptions/understandings when this type
> of thing happens...especially these days with the proliferation of the
> Internet.  You can find "compelling" (as in decent presentations, that look
> well thought out) sources for both sides of near any argument!
> 
> > Here's an interesting thought experiment, Say we a printing
> > out gray swatches from an inkjet printer.  Each swatch
> > is a small square of each grayscale value.  Assuming we're
> > talking 8 bit files and printer drivers, we can have only
> > 256 possible gray swatches.  If its all perfectly
> > calibrated ideally we ought to be able to distinguish
> > each swatch.  Now imagine a swatch which is a checkboard
> > with two different adjacent gray values, for instance
> > 137 and 138 mixed together.  What does it look like?  Is
> > it appear as a checkerboard of two different grays or
> > does it merge into a new and different 137.5 gray swatch?
> > I think the answer is "it depends" ... i.e. how close you
> > get to it, got your bifocals on? etc.
> 
> It does depend.  More so on lighting than anything else.  We actually can
> distinguish a lot more than 100 gray tones, but the 100 number is in "a"
> given light...but vary that light, and you increase the number of tones you
> can see...and your standard photographic print may not have tones that
> extend into that range, since most people don't view a photo in the dark or
> extreme light, so they weren't designed TO be seen in those conditions.
> 
> > The point is "number of tones" is kind of nebulous.  This
> > also adds to my dissatisfication with "dynamic range"
> > meaning "number of tones".
> 
> Very interesting point, but I have an answer ;-)  You are right, in your
> example, you will be able to shift the tones all by .5 of a tone, and get a
> whole new set of shifted "discernable" tones...but you can STILL only
> discern the same number, as well as have the same separation!  This does

Well I was figuring that you're adding all these new tones -- i.e. double the
number of tones.  But I do agree we are probably beyond the capability
of human discerning of all these tones.  My real point here was just that its
hard to get a handle on "how many grays?".   Here's a system which is
based on 8 bit values -- i.e. 256 values and there's at least a potential for
actually having more the 256 grays.  On the surface that seems very weird.

> fail at some point, when your number of tones is high enough (where adjacent
> tones aren't discernable).  Does that make sense?
> 
> > Hopefully someone will find this useful.
> 
> Yes.
> 
> Regards,
> 
> Austin

-Roy