Anybody using WD2H+?

[Michael R]

That’s puzzling to me. While an image formed with some dye and silver should have a smaller Callier effect when projected than an image formed entirely with silver (all other things being equal), to begin with the Callier effect (or Q factors > 1) is the scattering of collimated incident light by silver the silver particles. If the light incident on the negative is already scattered/diffuse, there should be no significant Callier effect whether the image is formed by silver or dye.

This all appears to be the opposite of what you observed, so I’m not sure what to make of it. Maybe something else is going on. Not sure how you did the comparisons etc. It’s a tedious thing to test. It seems to me you need to begin with a stained and non-stained negative that contact-print to produce equal contrast prints (or enlarging with a diffused source might serve well enough). That becomes your “control” setup. Then you projection-print the same two negatives with a collimated light source and see if they still print with the same contrast or not.

Of course there’s also the possibility I’m mixing things up as it’s been a long time since I’ve read about the Callier effect, so hopefully I haven’t derailed things.

[Mark J]

I'll try & explain what I mean. It may take a couple or three postings. I can't offer much in terms of proof or quantitative measured, this is qualitative from negs and prints, however I do have some opinions about what is going on in terms of the science. My concept of what happens was clarified a little recently, when re-printing existing negs on a condenser, rather than diffuser enlarger head.

Gordon Hutchings discusses the 'reduction of the Callier effect' on page 49 of his Pyro book, but oddly he gets the pros/cons of diffuser vs. condenser head the wrong way round.

First, I want to show an optical model of condenser illumination, just to comment on your use of the term 'collimated incident light'. The model below is one I put together on CodeV from measurements of the components and spacings on the Durst 138S. The source set-up here is my own case of a 100x100 opal diffuser placed about 20mm from the rectangular 85x75mm aperture of the lamp box.
I wanted to analyse what was the cone angle of light reaching each point on the neg.
The glass carrier and neg plane is close and underneath the big condenser lenses.

First, one point is that the condenser set-up angles the light around as you go further out in the neg - this is so that the light is 'aimed' at the enlarger lens - and why you need to swap condenser lenses sometimes if printing at a very low or high magnification.
In this set-up, you can see that the cone of light going to each point on the neg is relatively narrow. It is between 20° and 22.5° total angle.
Some might say this is semi-diffuse, but the light from a diffuser box would probably expose each point on the neg to a cone of at least 150° , so there is still a big difference.

A smaller bulb, or a point-source, would create a cone much narrower than 20°, but by doing this it would also highlight dust and scratches on the upper neg glass much more.

[Mark J]

If the light incident on the negative is already scattered/diffuse, there should be no significant Callier effect whether the image is formed by silver or dye.

I should probably stop using the term 'Callier effect' in further explanations, because I am likely to be mis-using it. I think that Callier effect strictly refers to the loss of overall contrast in going from a contact print to an enlarged print. It is cause by scatter, so I will just use the word scatter.

Just to continue explaining this from the standpoint of different types of neg illumination, the view up from the negative towards the enlarger source is similar in some ways in the case of a condenser head and a diffuser head ; in that, there is a uniform bright patch of light that is incoherent ( in physics terms ). It's just that in a condenser head, this patch of light is some distance away, and small in angular terms , whereas from a diffuser head, the light fills nearly a hemisphere. It's not a case that one is 'already diffused' while the other is not. It's just that they subtend different angles.

On the other side of the system, we can say that there's no argument about how the light from the underside of the emulsion proceeds to the paper - whatever emerges from the neg and is heading towards the entrance pupil of the enlarger lens will be focused and appear on the paper.

So the bit that interests me, and I think can explain the difference we see in prints from stained and non-stained negs, and between condenser and diffuser-printed pictures, is the bit in the middle - what happens to the light in the film base and particularly in the emulsion.

I really need to draw a couple of pictures to do this properly, but that will take some time, so i will put down some words first.

I believe that what distinguishes the above cases, and how the prints look, is the amount of lateral scatter of the light in the emulsion.

[Michael R]

Mark, thanks for the detailed follow-up. I’m going to need to go through it and think some more to make sure I understand it.

One quick thing I wanted to clarify. The Callier effect is the increase in contrast when going from contact to projection with a collimated (to varying degrees) light source. When a negative is projection-printed, if the emulsion scatters light (ie silver vs dye), the more collimated the light incident on the film, the greater the overall difference in attenuation between low and high densities (ie the greater the printing contrast). For example given a silver negative, printing contrast is higher using a point source head than a diffusion head.

I should probably stop using the term 'Callier effect' in further explanations, because I am likely to be mis-using it. I think that Callier effect strictly refers to the loss of overall contrast in going from a contact print to an enlarged print. It is cause by scatter, so I will just use the word scatter.

Just to continue explaining this from the standpoint of different types of neg illumination, the view up from the negative towards the enlarger source is similar in some ways in the case of a condenser head and a diffuser head ; in that, there is a uniform bright patch of light that is incoherent ( in physics terms ). It's just that in a condenser head, this patch of light is some distance away, and small in angular terms , whereas from a diffuser head, the light fills nearly a hemisphere. It's not a case that one is 'already diffused' while the other is not. It's just that they subtend different angles.

On the other side of the system, we can say that there's no argument about how the light from the underside of the emulsion proceeds to the paper - whatever emerges from the neg and is heading towards the entrance pupil of the enlarger lens will be focused and appear on the paper.

So the bit that interests me, and I think can explain the difference we see in prints from stained and non-stained negs, and between condenser and diffuser-printed pictures, is the bit in the middle - what happens to the light in the film base and particularly in the emulsion.

I really need to draw a couple of pictures to do this properly, but that will take some time, so i will put down some words first.

I believe that what distinguishes the above cases, and how the prints look, is the amount of lateral scatter of the light in the emulsion.

[interneg]

When a negative is projection-printed, if the emulsion scatters light (ie silver vs dye), the more collimated the light incident on the film, the greater the overall difference in attenuation between low and high densities (ie the greater the printing contrast). For example given a silver negative, printing contrast is higher using a point source head than a diffusion head.

Which seems exactly contradictory to the standard claim about staining developers - that they make highlights easier to print - aka less dense (yet seemingly sharper). More intense adjacency or exhaustion effects than D-76 produces will both intensify apparent edge definition and hold highlight density dramatically more under control (DIR couplers do the same thing even more spectacularly so in chromogenic negative films) - which will effectively lower apparent granularity and still make edges seem apparently sharper. The true causes of these effects in B&W seem to have been well known to the major research labs and steadily commercialised for several decades (often quite slowly however) - and has nothing to do with the dye formed, but a lot to do with the other components in these developers. As for the UV effects of the dye formed, I think many people would do well to look at the spectral characteristics of a yellow-green filter (Wratten #11 is a good one) which gives a rather stark illustration of potential UV blocking characteristics of dyes in the region of PMK's colour.

[Mark J]

I can't see how you can increase the contrast ( with a fixed light source colour/grade ) from a contact print situation, by going to any form of enlargement. Contact printing pretty much has the maximum directionality of the light reaching the negative - because it is illuminated by a small distant lens pupil that might be 2 or 3 feet above the neg.

There is a thread here in the UK where I showed how i worked through my thoughts on this.

This is where I joined, but I was late to the party -
http://www.film-and-darkroom-user.or...t=14020&page=6

There are print scans near the end.

[Michael R]

Which seems exactly contradictory to the standard claim about staining developers - that they make highlights easier to print - aka less dense (yet seemingly sharper). More intense adjacency or exhaustion effects than D-76 produces will both intensify apparent edge definition and hold highlight density dramatically more under control (DIR couplers do the same thing even more spectacularly so in chromogenic negative films) - which will effectively lower apparent granularity and still make edges seem apparently sharper. The true causes of these effects in B&W seem to have been well known to the major research labs and steadily commercialised for several decades (often quite slowly however) - and has nothing to do with the dye formed, but a lot to do with the other components in these developers.

All things being equal, dye clouds scatter much less light than silver particles (which is why colour/chromogenic films how show minimal/no Callier effect as far as I’m aware). Since a negative processed in a staining developer has less silver for a given density, in theory the stained negative should exhibit a smaller Callier effect than an all-silver negative. How much less, I don’t know. As I speculated earlier, probably not a huge difference since there is still a lot more silver than dye forming the image, but it would have to be tested. Objective testing could be done but it is tedious work.

[Mark J]

Picking up on what Interneg has just said.
I contend that the key difference in both mids and highlights in staining-developer negs is not about a difference in density , but that they are less scattering. Hence there is more local contrast ( and MTF ) even if they are the same density as a grain-only neg. That's what I see.

I have two near identical negs from 1999 that i will try to selectively print next time I am in the darkroom.

[interneg]

Since a negative processed in a staining developer has less silver for a given density, in theory the stained negative should exhibit a smaller Callier effect than an all-silver negative. How much less, I don’t know. As I speculated earlier, probably not a huge difference since there is still a lot more silver than dye forming the image, but it would have to be tested. Objective testing could be done but it is tedious work.

I think any nominal difference in Callier effect between stained and non stained is probably vanishingly small, and probably has much more to do with inhibition/ exhaustion effects overall. I do wonder how many people will recognise that the comparator has to be the Beutler formula (modified to use Kodalk) and not some other random developer...

[Michael R]

Thanks, Mark. I’ll have a look at this.

Yes on the surface it seems somewhat counterintuitive that scattering effects result in increased overall contrast when the light source is collimated. This is reflected in the formula for the Callier Q factor, which for a given amount of developed silver is the attenuation of collimated light divided by the attenuation of diffuse light, and the Q factor is always >=1. Some diagrams could help here but I’m stuck at work late…

I can't see how you can increase the contrast ( with a fixed light source colour/grade ) from a contact print situation, by going to any form of enlargement. Contact printing pretty much has the maximum directionality of the light reaching the negative - because it is illuminated by a small distant lens pupil that might be 2 or 3 feet above the neg.

There is a thread here in the UK where I showed how i worked through my thoughts on this.

This is where I joined, but I was late to the party -
http://www.film-and-darkroom-user.or...t=14020&page=6

There are print scans near the end.