Variable Contrast UV Exposure Unit for Pyro Negatives

[koraks]

may allow for

may be slow to bulid

Doesn't sound very 'accepted' to me; more like a hypothesis.

Moreover, while Ethan's suggestion relies on a clear causality, the reasoning underlying the supposed print-out-contrast-control scheme is vague and does not seem to fit experimental evidence so far.

Again, a simple test would verify this pretty easily.

 

[FotoD]

Doesn't sound very 'accepted' to me; more like a hypothesis.

Try it, you may like it. It's a good tool to have when you need a little more detail in the shadows, or when trying to preserve the brightest highlighats.

Moreover, while Ethan's suggestion relies on a clear causality, the reasoning underlying the supposed print-out-contrast-control scheme is vague and does not seem to fit experimental evidence so far.

A theory of causality is great. But "the experimental evidence" doesn't even convincingly show that there *is* a difference in contrast yet.

The plot below shows that one of the two LEDs provides more sensitivity (brighter? more suitable wavelength? who knows?), but IMO, the two curves (with their little imperfections) show essentially the same contrast.

 

[koraks]

bernard_L's comment you quoted pertained to the previous experiment. I haven't run the numbers on the new one. Have you?

Try it, you may like it.

I might, but currently I'm not doing much with iron- or silver-based alt. processes, so it'll likely take some time.
Have you run side by side comparisons? You could easily settle this by showing the proof; I would be interested in learning from it and so would others, I assume. It would beat the crap out of us theorizing up against each other.

 

[FotoD]

Have you run side by side comparisons? You could easily settle this by showing the proof

Yes. I've used it in ferric processes, when a print was too contrasty. The effect is not huge, but it could well be as large as what we've seen from the supposed "wavelength-effect" in this thread.

I'm not here to settle or prove things. I was sharing my experience and trying to help the OP find an explanation for their results, or rule out other causes. But you already seem to have settled on an explanation for the difference in contrast.

 

[koraks]

Insofar as I've settled on anything, which is not much, is that the explanation you offered doesn't seem to apply in this case.

Thanks for the suggestion though; I'll be sure to try it sometime.

 

[nmp]

I don't know which side I am on in the this debate, but for salt prints, the conventional wisdom has been that slowing the exposure gives higher contrast. In the old days, they used to put the frame in the shade (perhaps there is another reason - shade has different mixture of wavelengths?) to increase contrast. Here is one instance I found more recently that uses the approach suggested above - putting a plain piece of tracing paper over the glass to get "better contrast and range of tones":



This from Steve Anchell desribes various ways for contrast control (one being slowing down exposure):

Printing-Out Processes – AlternativePhotography.com

www.alternativephotography.com

:Niranjan.

 

[FotoD]

Insofar as I've settled on anything, which is not much, is that the explanation you offered doesn't seem to apply in this case.

Fair enough, we can all believe anything we like. So far the OP has not ruled out this alternative explanation by testing though.

So in my mind the explanation could be one of a difference in wavelengths, or it could be another alternative explanation that hasn't been ruled out yet.


"Split Exposure to Increase Shadow Details
One additional technique that you may wish to apply to your knowledge of exposure is a split exposure technique. When you are halfway through your pre-determined UV exposure time, suspend the exposure. Then, take the printing frame and its contents into a low level light environment and wait for 15 – 30 minutes before going back for the rest of the exposure time. This interrupted printing results in noticeably better shadow details and separation without losing highlight or D-max integrity. If you are looking for more sophisticated levels of value in your cyanotype images this is a good technique to try. My friend, Mike Ware, suggested that this technique may allow for a greater degree of print-out, which may be slow to build, and therefore more self-masking."
Christopher James, same book as above, 3rd ed

In my exerience the benefits of this technique is not limited to cyanotypes.