Exposure time calculations - puzzling results


#1

Hello all.

I have spent a few days testing the Standard Exposure time of Pt-Pd printing with my 4000 W Plate burner. I have made two different kinds of tests, by using the Stouffer 31 steps covered with Pictorico and also by making strips of different exposure values of a coated piece of paper with half of it covered with Pictorico. I have found some really puzzling results…

Basically, when I observe the print from above, under reflected light (or I scan), certain areas seem darker than the rest. So far, so good. Then, when I place the print on a big light table, under transmitted light the darker areas seem to be others! Is there any physical explanation for this?

Another weird thing was that when analyzing the test strips under transmitted light (lit from behind, in a dimly lit room), I seem to see the blacker blacks happening under exposure values of more than 16 minutes… which seem to me a bit exaggerated for a 4’000 W metal halide unit (lamp placed at around 1.20 m).

Should we only evaluate the reflective densities in order to analyze the blacks when calculating the exposure time? Is it normal the reflective and transmissive densities are different? (I would say almost contradictory!)

I will also take advantage and ask you another thing. I have seen most people out there calculate the exposure times by using Stouffer step tablets (31 steps for instance, in 0.10 steps), by placing a piece of Pictorico on top of it and seeing at which step the next one becomes indistinguishable. What I do not understand however is that this calculated exposure level would be the one providing Dmax when exposing a negative whose density is the addition of the density of the Pictorico film + the Stouffer negative. And so, if we use that exposure level when exposing only Pictorico, we will be giving too much light… Should not we take into account the subtraction of the Base+fog density of the Stouffer step wedge to reduce the exposure level accordingly?

Thanks and warm regards,

Rafael

 


#2

Hi Walker,

Thanks for your quick response.

I am using a Metal Halide lamp whose spectral behaviour is attached… Normally, it is peaking up at around 400-420 nm… so it should be more or less adapted to pt-pd I guess. By the way, my unit has not an integrator, but based on my tests once the lamp gets ready (heated for 2 min more or less) the exposures seem quite consistent…

Otherwise, I reached that high value of 16 minutes when assessing the blacks in my test strips by helping myself of the light coming through the prints (I am putting them on a big light table, blocking the light around the print). It was only when looking “through” the print that I saw higher densities in the last patches (16 minutes), when I could not see that with the densitometer, scanner or visually when looking at the print in normal conditions (reflective light). I was thinking that both densities were going hand in hand, and therefore if a patch of printed paper shows Dmax with transmissive light, then that would also be the patch with maximum density with reflected light. It is quite interesting to know this is not the case!

Yes, I am putting the Pictorico film on top of the Stouffer neg… My doubt comes from the fact we calculate the time with this set up (two layers of material), when in reality we will only expose the pictorico later on (one single layer, blocking less light). As as a result, we will always over-expose a bit in the real printing situation…

Using PiezoDN is a really fantastic thing by the way… I just love this.

Warm regards,

rafael

uv-lamp.jpg


#3

while you were typing your response I probably was editing my first comment to be more exact.

thanks for your graph and the compliments.

best

Walker


#4

I’m attaching a chart of spectral output of the range of Olec lamps. (Not sure if PDFs work as attachments.) This may be useful for anyone who has one of these or is thinking of getting one. I have an Olec AL15 with the L-1250 bulb. It’s in storage and I haven’t used it in several years due to space limitations, but I hope to have a bit more room someday and to put it to use again. This is a 1500 watt unit, and if I remember right, my base exposure time for Pt/Pd was 300 units, which roughly translates to about 6 minutes. This was at a light to print distance of about 30 inches.

The output of this bulb is similar to yours according to the graph, so I’m surprised the your 4000 watt unit would require such a long exposure time. A couple questions that come to mind are (1) what is your light-to-print distance? (never-mind, I see you wrote 1.2 meters, why so far?), and (2) how old is the bulb? These bulbs do slow down with age.

As for determining optimum exposure time (aka minimum time to reach maximum black), don’t worry about the base density of the Stouffer. It is only about 0.05 or 1/6th of a stop. The reason you want to expose until steps 1 and 2 merge is that this is the best way to be sure you are actually reaching maximum black. However, the time that gets you to this point is not going to be your actual exposure time for printing negatives. You now need to back off by 1/3 of a stop if you are using a 31-step Stouffer wedge, or by 1/2 of a stop if you are using a 21-step. For the 31-step, multiply your time by 0.8; for the 21-step, multiply by 0.71. (This is something I learned a long time ago, I don’t remember the source.)

For example, if the time required to merge steps 1 and 2 on a 31-step is 16 minutes, your actual exposure time for printing would be 12.8 minutes, which I think it is safe to round up to 13.

Unless you plan to display the prints illuminated from behind, I don’t think trying to take into account what you see on a light table is going to do anything useful. At least not with opaque papers such as the usual ones used for Pt/Pd and other alt-processes. If the prints will be viewed by reflective light, that is how they should be analyzed. But if you are printing on something translucent such as thin Japanese papers, and if they may be viewed both against a white background and without a background such as in a book or suspended in a way that it is not against a background, that is a whole other story.

Olec-lamps.pdf (38.3 KB)


#5

I will answer the last question first. Stouffer strip calculation must be done with the pictorico film (over it) but often only half way. The exposure amount difference in film base + picto is less than stouffer without. So film-base + picto gives you a little wiggle room. You can mathematically figure this out by only covering half the stouffer with picto and leaving half bare. Find the darkest value of both and then find the middle. That will be your exposure time. (correct me if I’m wrong those of you who are more knowledgable here).

I’m working on a more elegant way to dial in exposure time but have yet to finalize it conceptually.

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Optical density through metal/paper is totally different that reflected density. There are tons of variables at play such as paper structure, metal depth, metal catalyst purity, developer used (and its age or strength), humidity (the big one), surfactant, etc.

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If your bulb is under 250nm or over 500nm in UV spectrum this can cause fairly long exposure times. I suggest you look up its specs. PtPd likes 360nm from what I’ve read, but I’m still learning myself.

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best,

Walker