Re: (Gum) Tonal scale

From: Tom Sobota ^lt;tsobota@teleline.es>
Date: 12/07/05-07:01:33 AM Z
Message-id: <7.0.0.16.0.20051207122349.020f3b10@teleline.es>

Katherine,

Thanks for your answer and for your time. These discussions help me not only to
understand better the gum process, but also to see how others understand it.
Sometimes there is more to be learned from the discrepancies than
from the agreements :-)
I hope that what is said is also useful to others.

There's still one point that I do not understand in what you say, but
before coming to that:

1. I agree that the thickness of the layer is still not enough
understood and that for the time
being we can leave it out.

2. No discussion about the pigment not being involved in the
reaction. Yes, you contributed
somewhat to the confusion posting your examples of dichromated gum
exposed without any
pigment, but never mind, I now understand the distinction. Also,
those tests were useful to
see the effect of stain and the several attempts to remove it.

3. What still puzzles me, however, is your use of the term 'density'.

You say that you use it as Ansel Adams did. Well yes, but for him and
the rest of the folks
working with silver halide emulsions, the term clearly meant 'optical
density' and it was
just a shortcut for the longer term. It certainly did not mean some
generic 'amount of reaction
product distributed in a particular area of the hardened colloid
film', as you call it. How would
they have measured this reaction product if not by optical density?

All 'The Negative' and all the Zone System, and I might say all of
traditional BW
photography is about optical density, after all, be it in the
negative or in the positive.

Of course Adams himself did not invent the term, it was in common use
well before his time.

Clerc, in his 'Photography, Theory and Practice' (I have the second
edition, translated to
English in 1930) speaks about the 'Law of Densities':

"... This fact is generally expressed by saying that the quantity of
absorbing material (the
mass of reduced silver per unit area in the case of a photographic
negative) is proportional
to the logarithm of the opacity. The name 'Optical Density' is given
to this value ...'

Agreed. And Phil Davis, in his 'Beyond The Zone System' clarifies the
matter even more (I am
back-translating from my Spanish version):

'The term "density" has a specific meaning in photography. By
definition, the density
is the logarithm of the opacity, which is the inverse of transmittancy'

But the 'logarithm of the opacity' is the optical density, again.
See? 'density' and 'optical density'
are one and the same for Phil Davis too, as they certainly were for Adams.

But in your mail to Yves, you say:

'And perhaps I should emphasize here that when I say "density of
crosslinked gum" I'm not
talking about tone, I'm not talking about optical "density,..."'

And _there_ lies the difficulty for me. If you are _not_ talking
about optical density, you
are not using the accepted use of 'density' in photography. Or in
Ansel Adams, for that matter.

Mind you, I understand what you're talking about, or at least I
_think_ I understand, but Adams
or not Adams, I still think that you should be using some other term
for what you call 'density'.
Don't be afraid to coin one :-)

As I see it, for Adams and the rest, the reaction product was indeed
the blackening of the
silver which was immediately seen as having an optical density. That
is why they simply called
it 'density', meaning always 'optical density'. But this concept is
not easily translatable to
dichromated colloids, since in these the reaction product is
essentially transparent. Even worse,
there is a byproduct of the reaction, the dichromate stain, which
sometimes has a very quantifiable
optical density but is an undesirable result.

As we all know, the situation is even more complex, since in normal
use an extraneous
ingredient is added in wildly different concentrations and/or
covering power: the pigment, which
without intervening in the reaction, is finally the (hopefully only)
visible result. It is almost
impossible not to mistake conceptually this pigment with the
'density' you speak of.

I think that the correct use of terminology is important. How else
different people, coming from
different countries and speaking English with varying levels of
accuracy can even hope to
understand each other?

I hope not to have muddled the issue even more ...

Tom

At 02:39 07/12/2005, you wrote:
>Hi All,
>I'm responding here to Tom Soboda's post, which I didn't actually
>receive but found on the live mirror when I was checking on
>something else. Since I've had to cut and paste it over from
>there, it won't be designated as quoted text in the usual way, so
>I'll separate my comments from his by preceding them with my initials.
>
>
>Tom: Katherine, Will you allow an 'independent' opinion in this matter?
>
>KT: Certainly, that's what a forum is for, for everyone to weigh in
>on everything; the more the merrier.
>
>Tom: After reading your inconclusive interchange of opinions with Yves, I
> must say that I agree with what you say, but not necessarily with
> your terminology. There seems to be a measurement units discrepancy here.
>
>
>Tom: Yves speaks correctly of an "optical density of this tone". Now
> 'optical density' is an unequivocal concept related to opacity.
> Actually, the optical density is the logarithm (in base ten) of the
> opacity. Even a completely transparent layer of pure gum has an
> optical density, namely zero. The tonal scale, then, is the gamut of
> optical densities.
>
>KT: No argument there, but as I said in a post earlier today, which
>you may or not have received, I didn't understand until today that
>"optical density" was what Yves was talking about; I thought he was
>talking about density of reaction product.
>
>
>Tom: Yves, as far as I can see, does not make a distinction between gum
> with pigment and gum without pigment. He assumes presence of pigment
> and this is reasonable, since it is the normal practice.
>
>
>Tom: You, on the other hand, make such a distinction. But you say that the
> gum layer without pigment "it's colorless and transparent; there's no
> tone to read the optical density of". This is not so: there's always
> an optical density, even if it is zero, as is the case in a perfectly
> transparent material. Or in a very transparent gum coat without a
> pigment load.
>
>KT: Actually I think you don't quite understand the distinction I'm
>making. I'm not making a distinction between gum with pigment and
>gum without pigment, but between the part of the emulsion that
>participates in the reaction (gum and dichromate) and the part that
>doesn't (pigment). I seem to have confused everyone by saying that
>printing gum without pigment helped me understand that distinction,
>but the distinction is simply about what is the reaction product and
>what is not. The reaction to exposure, the crosslinking of the gum,
>proceeds equally well whether the pigment is in the emulsion or not,
>and when I printed without pigment, I saw with my own eyes that that
>is the case. That's the whole point there.
>
>KT: I do concede your point that I should have said that the
>hardened gum has zero optical density, rather than that it has no
>tone, although I'm not sure I see the practical difference between the two.
>
>
>
>
>Tom: However later on you make clear that you are speaking of 'density',
> not 'optical density'. You say "I'm talking about the actual density
> of the crosslinked gum, the proportion of the layer that's crosslinked."
>
>
>Tom: This is somewhat puzzling since 'density' is also a well determined
> physical magnitude! Put simply, for a given substance it is the
> weight divided by the volume. A very common example: if something has
> more density than water, it falls to the bottom.
>
>
>Tom: Now I seriously doubt that you are interested in the actual density
> of the gum coat which, in any case, crosslinked or not, is higher
> than water! The 'density' of the gum is irrelevant in this context,
> as far as I can see.
>
>KT: But that's not the meaning of "density" that's usually used in
>photography, and that I am using here. I mean "density" to mean
>the distriibution of a product across an area, in the same way that
>Ansel Adams means when HE defines density: "Portions of the film
>which have been exposed to great amounts of light yield a
>considerable deposit of reduced silver upon development, referred to
>as a higher *density*; areas of film exposed to less light yield
>less silver, or lower *density*.
>
>
>Tom: Correct me if I'm wrong, but your use of "proportion of the layer
> that's crosslinked" makes me think that you are speaking, not of
> density but of 'thickness'. Indeed, what we are interested in is the
> thickness of the crosslinked layer.
>
>KT: No, I'm speaking of *density* in the exact sense that Ansel
>Adams meant it: the amount of reaction product distributed in a
>particular area of the hardened colloid film. I suppose I mean the
>number of molecules crosslinked per area. Whether that translates
>into "thickness" of a layer is a controversial question and I don't
>care at this point what the right answer is; at any rate no one knows.
>
>
>Tom: "the proportion of the layer that's crosslinked" that you speak about
> is then the thickness of the hardened layer divided by the thickness
> of the brushed on layer. When this ratio is 1, all the gum has been
> hardened and nothing has been washed away.
>
>KT: No, I don't find this a useful distinction at all, sorry.
>
>
>Tom: The relation between thickness and opacity? Well we know that. A
> thick crosslinked layer will carry more pigment and will be more
> opaque, that is, will have more optical density for any given pigment.
>
>
>Tom: I agree with you that in the case of gum the hardening reaction is
> independent of any additive. The layer will be thicker or thinner but
> always transparent.
>
>
>Tom: But in normal operation, there will be a pigment present, and then we
> can speak of varying levels of optical density (or transparency, or
> opacity) with varying concentrations of pigment. And this can be
> calibrated and so on and so on.
>
>KT: Of course, and no argument there, and I think I've said the same
>thing about half a million times in the last few days.
>
>Tom: There is also the possibility that I have misinterpreted _both_ of
> you, in which case I hope to be forgiven :-)
>
>KT: You're forgiven.
>Katharine
Received on Wed Dec 7 07:02:48 2005

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