A Theoretical Quandary - Printing Out Paper

[holmburgers]

Hello good people,

I have a theoretical question that is perplexing us at George Eastman House, regarding the contrast effect obtained with printing out paper. In this case, collodio chloride POP.

Here is the mystery: By slowing down the printing time, higher contrast is achieved.

A low contrast negative is printed in open shade or under diffusion, whereas a high contrast negative is printed under direct sunlight (historically). However, the diffusion aspect is not the culprit (and besides, wouldn't this tend towards lower contrast anyways?), because the same effect is obtained with a yellowed glass. The key here is slower printing, with light of less intensity.

This has been known for a long time, and is considered a standard control in the process. The reason for WHY however, has alluded us.

Some things to know about POP paper. It is pure silver chloride with an excess of silver. As the image prints, it is "self-masking" (that is, metallic silver is formed and thus blocks light from penetrating further).

Now why on Earth is this the case? If the negative has a fixed set of densities from high to low; why would only printing time, or rather the intensity of light, affect the outcome. Could this be due to some kind of reciprocity phenomenon? Could this be due to particle size, and in some way affected by the self masking quality of POP?

I'm interested to hear your ideas, and intelligent thoughts on the matter.

Cheers!

 

[Ian Grant]

This is the same with Albumen prints as well. You use tissue to reduce exposure and increase the contrast (I went on a course along with another APUG member this summer, the tutor is one of Mark Osterman's former wet plate students - small world)

It must have something to do with the self masking effects of the emulsion during exposue and the threshold inertia needed, so the fast that mask builds up the lower the contrast. So yes akin to reciprocity.

It happens with colour papers, the owner of my local Pro lab was an ex Durst technician and once told me that on the old roll head printers the lenses were set to give identical exposures regardless of print size (enlargement is done by using different focal lenght lenses rather than distance), this prevent contrast shifts and also colour shifts due to reciprocity.

Ian

 

[Bill Burk]

Is it the Callier effect operating in reverse? In direct sun a high-contrast neg will block the shadows quickly reaching their dMax. But In open shade, the Callier scattering, will allow some scattered light to slip in under the metallic printed silver to make them denser.

 

[Bill Burk]

But I don't know how the yellowed glass would play with that...

 

[ic-racer]

Doesn't seem any different than using B&W negative film. The dim light needed for longer exposures doesn't produce density in the dimmest areas of the image (reciprocity failure).

 

[Photo Engineer]

Might it be the contrast reciprocity effect?

PE

 

[Klainmeister]

I have seen this phenomenon before and thus tested an image when high intensity was used briefly, waited, then did another burst of high intensity. No change vs doing a single high intensity. If you do a gradual increase in intensity, it acts exactly the same as if you used diffused light throughout, but the reverse produces the same as high intensity.

I think the answer lies in the self-masking effects of a POP system, where that first burst of light creates the mask and you end up with your given contrast based thereon.

 

[holmburgers]

Hey guys, thanks for weighing in on this.

The more I've thought about it the more I think the whole(?) phenomenon can be explained with reciprocity failure. Each successively more dense part of a negative is going to experience more 'reciprocity failure', and if the overall intensity of light is reduced this effect will be exaggerated throughout the scale of densities (thus, more contrast).

The masking explanation doesn't make sense to me... and that's not to say it's wrong, but I just can't reason it out myself. My thinking is that the printed out silver which constitutes a mask is going to be formed at the exact moment the silver-chloride achieves enough exposure and that's that. You can't sneak in any silver ahead of, or behind exposure. The mask is built up gradually just like the exposure is; in fact it's created in direct proportion to exposure, it is after all our only evidence of exposure. This doesn't belie the reciprocity failure explanation (the disproportionate relationship of light to exposure). It's impossible however, for exposure and deposited silver to diverge from one another, they're 1 in the same, and if that's the case, how can the masking have an effect on contrast between 2 different exposures of the same negative?

Though... Klanmeister... that's an interesting caveat... it does kind of make sense how an initial intense exposure could set you up for a given contrast, because this would lay down a mask which doesn't show the effects of reciprocity, and that would undoubtedly have an effect on the subsequent printing. Hmmmm...

The plot thickens!

What do you guys think?

 

[Klainmeister]

I'd love to hear what you find out--this is from a simple experiment performed about a year ago because I had found the same phenomenon and was trying to pin down contrast. That said, I never ruled out reciprocity, but also didn't take the time to experiment further.

While you are right about the masking not making sense as a response to light intensity....there are many things that we have experimented with recently that haven't made too much sense. We're currently working on a new Carbon tissue and it's the opposite of what physics is telling us. Likewise, try adding more PT to a PT print and you get less contrast, less dynamic range, weaker blacks...but there's more metal!

Anyways, please let us know what you find. Hope you haven't froze your jiblets off already up there in the great white east.

 

[Bill Burk]

Yes, the failure of the Bunsen Roscoe reciprocity law.

I bet you could measure this effect and use the results of the study to devise a set of variable-contrast "yellow" filters you could use in direct sun (or a UV printer)...

 

[Vaughn]

The way I understand the formation of the PO image in pt/pd, cyanotype, and even carbon (though you can't see it for the pigment -- but can be seen if you leave the pigment out...info from Sandy King), is that the printing out image does not form instantly, but takes awhile to form (info from Terry King). Thus taking a longer time to reach a certain level of exposure allows for more of the PO image to form during the exposure. A brief intense exposure, but at the same level of exposure, there will be less PO image formed during the exposure (but I am guessing the same strength of PO image will form, but mostly after the exposure is over...as the PO image formation has a little lag time).

The PO image holds back the shadow exposure a little, and also holds back the mid-tones, but not as much as the shadows -- and the highlights not at all. This can give a smoother gradation between the shadows and the highlights.

To improve the tonality of some small PT/PD prints that only required a 6 minute exposure, I have given the print two to three minutes exposure, removed the print from the light to allow the PO image to form, then returned the print to the light to finish the rest of the exposure. It seemed to help...but this is a subjective test, not quantitative by any means.

I have read that for smoother tonality in salt prints to start the exposure in the sun (perhaps to kick-start the PO image and build up good shadow density), then finish the exposure in open shade.

 

[cliveh]

Could this be part of a Becquerel effect?

 

[Klainmeister]

The way I understand the formation of the PO image in pt/pd, cyanotype, and even carbon (though you can't see it for the pigment -- but can be seen if you leave the pigment out...info from Sandy King), is that the printing out image does not form instantly, but takes awhile to form (info from Terry King). Thus taking a longer time to reach a certain level of exposure allows for more of the PO image to form during the exposure. A brief intense exposure, but at the same level of exposure, there will be less PO image formed during the exposure (but I am guessing the same strength of PO image will form, but mostly after the exposure is over...as the PO image formation has a little lag time).

The PO image holds back the shadow exposure a little, and also holds back the mid-tones, but not as much as the shadows -- and the highlights not at all. This can give a smoother gradation between the shadows and the highlights.

To improve the tonality of some small PT/PD prints that only required a 6 minute exposure, I have given the print two to three minutes exposure, removed the print from the light to allow the PO image to form, then returned the print to the light to finish the rest of the exposure. It seemed to help...but this is a subjective test, not quantitative by any means.

I have read that for smoother tonality in salt prints to start the exposure in the sun (perhaps to kick-start the PO image and build up good shadow density), then finish the exposure in open shade.

This also reflects of what Sullivan has shown me and said. Interesting about carbon...didn't know that.

 

[holmburgers]

If there is a lag time between exposure and PO image formation, that would be very interesting.

However, mask formation inherently leads to less contrast, not more... right? If you think about these two mechanisms, reciprocity failure and self-masking, they're actually working against each other.

Furthermore, if indeed there is some lag time between exposure and PO image formation, by slowing down the printing time the effect would again be less contrast, not more. This lag could give us more contrast only if we get in a lot of exposure before the PO mask begins forming, which would only occur with a faster printing time (assuming we're beating the lag to some degree). So slower printing time should equal a more direct relationship of exposure to PO image formation (less effect of lag), and thus stronger masking, i.e. less contrast. However, the quandary here is that we're getting more contrast with longer exposure times.

With masking, the shadows will build up density first, and this corresponding density will reduce the rate at which more density is formed there. The upper mid-tones will print behind the shadows with regards to time, and the masking effect less extreme here (that is, exposure will be a truer indicator of density, since less masking takes place). The result: darks are held back, upper tones print darker faster = less contrast.

With reciprocity failure, the shadows build density in a relationship that's nearest to Light X Time = Exposure. Under the stronger densities (mid-tones, high-lights) in the negative, we get further & further from this reciprocal relationship because our light value is effectively diminishing. The result is a breakdown of this expression and more light (or time) is required to get the same exposure. The result with regards to contrast is that darks print "normal", and high-lights print proportionally slower = more contrast.

 

[Bill Burk]

This also reflects of what Sullivan has shown me and said. Interesting about carbon...didn't know that.

For some reason I thought you were talking about Timothy O'Sullivan... He would have known about this effect.

 

[Bill Burk]

If there is a lag time between exposure and PO image formation, that would be very interesting.

However, mask formation inherently leads to less contrast, not more... right? If you think about these two mechanisms, reciprocity failure and self-masking, they're actually working against each other.

That's what I kept sketching as I tried to figure this out. Self masking, as image appears slowly, would be more complete at longer times. So the impact in the shadows if it is due to time lag of image formation must be trivial...

I am betting on reciprocity law failure in the highlights, suppose for sake of example that the reciprocity law failure makes it necessary to give 400% exposure to reach the threshold highlight...


In daylight without reciprocity failure and a normal contrast negative:
10 Light x 10 Time = Exposure = 100%

Under yellow filter causes reciprocity failure and a flat negative compensated adequately for the highlights to match:
4 Light x 100 Time = Exposure = 400%

The shadow in the 400% exposure would have less reciprocity failure, so it would be as if exposed 4 times as much, and it will be blacker. Higher contrast at lower intensity...

 

[Klainmeister]

For some reason I thought you were talking about Timothy O'Sullivan... He would have known about this effect.

Sorry, Richard Sullivan from Bostick and Sullivan. He's been teaching me these processes and their controls for the last year or so...

 

[Vaughn]

The affect that I (think) see is a more even tonality, not a contrast change. But then, one adjusts the contrast as needed for whatever one's methods are.

I tend to keep the printing process/method consistent and then select a scene, expose and develop a camera negative to match the process/method. I make pt/pd prints with no contrast agents, so my negs have a lot of contrast with healthy shadow areas, which means long exposure times. Whatever the longer exposure times has done to the contrast is taken care of automatically by the rest of the process.

The images that used only a 6 minute exposure were much thinner 120 roll film negs not processed originally for alt processes -- using pd and Na2.

 

[nworth]

Thinking about it, with no experimental evidence, it seems like both reciprocity and masking are at work here. Reciprocity gets worse with less exposure. There may be a crossover point somewhere that the slowly forming mask reduces exposure enough for reciprocity to increase beyond the mask's contrast reducing effects.

 

[Vaughn]

What about picking a light source, paper and process...and going for it? Make new negatives for the new process, rather than use old images processed for other processes. Print until one is happy with the prints, give oneself at least a year or two or three to reach this point, then print more. The differences in tonality between short and long exposures are subtle. One has to print a lot and study the results before seeing many of these subtle differences caused by this and other techniques!

Enjoy the Ride!

Vaughn

 

[holmburgers]

I appreciate Vaughn's outlook about just printing! But this kind of discussion should exist purely for it's own sake too; an intellectual discussion that is an end in itself.

We've yet to figure the answer out, but getting here has been informative. Lots to think about...

Well at any rate, tomorrow we are teaching this collodio-chloride POP workshop and Mark has warned me about getting into a discussion on this point. I'm going to have to bite my tongue...

 

[Prof_Pixel]

Mark has warned me about getting into a discussion on this point. I'm going to have to bite my tongue...

Chris,

I'm sure Ron and I will be glad to discuss it further with you over lunch.

Fred

 

[holmburgers]

That'd be great Fred!

 

[Klainmeister]

Lemme know the results please! Wrong side of the country this time.