Determining base exposure with Stouffer Step Wedge

[alexreltonb]

Hi all, I am getting ready to begin my journey with Van Dyke Brown Printing. I just need a little clarification on how to use a Stouffer 21 step wedge to determine my base exposure as I am finding some contradictory instructions online.

As I understand it I need to lay the step wedge over a coated strip of paper, greatly overexpose it, process the strip and then find the first step where subsequent steps get no darker. This would be the first step where the dmax of the process is reached. I then plug this step number and the full exposure duration into a formula to find the exposure duration required to reach that dmax.

How is that all sounding so far?

My questions are:
- Is there a definitive formula that I should be using? I have tried using Clay Harmon's Exposure Calculator and also the instructions in Don Nelson's book and they produce different results.
- Do I need to include a blank piece of the transparency material I will be printing my negatives onto? How do I arrange this with the coated strip and the step wedge? In which order from UV light to coated strip? I have also read that I should potentially lay the transparency material so that it only covers half of the coated strip and then lay the step wedge so that it straddles both the part that is covered by the transparency material and the part that is uncovered - why would this be? If it is the case that I need to expose through the transparency material, why would I also need to see the results without the transparency material?

Thanks in advance for any help!

 

[koraks]

How is that all sounding so far?

That's a possibility. If you greatly overexpose, you could then trace back the approximate exposure where the black point is just above the film density. The advantage of this approach is that it takes (potentially) a single exposure to determine the base exposure and perhaps a second one to verify.

Alternatively, you could do a series of exposures to iteratively determine an exposure that creates the desired maximum density at step 0 of the Stouffer.

Btw, with many alt. processes and Van Dyke as well, there's a bit of a fuzzy area around where dmax (maximum density) starts. This makes it a bit of a judgement call. I would personally advise to not expose too deeply for a 'really good' dmax as it will just create problems with differentiation in the shadows afterwards.

- Do I need to include a blank piece of the transparency material I will be printing my negatives onto?

If you later on intend to make digital negatives with e.g. an inkjet printer then yes, determining the base exposure must be done through a blank piece of the film you will be using.

How do I arrange this with the coated strip and the step wedge?

Put the film on top of the coated paper and the step wedge on top of that. Or put the step wedge on the paper and the film on top. Doesn't really matter which, really. The second approach will make the step wedge image a little more sharp and it'll avoid some dot gain issues, but ignore that bit. It just doesn't matter a whole lot.

I have also read that I should potentially lay the transparency material so that it only covers half of the coated strip and then lay the step wedge so that it straddles both the part that is covered by the transparency material and the part that is uncovered - why would this be?

IDK, possibly for entertainment purposes. It gives you a bit of a feeling of how much light the film blocks, but it's barely relevant to know this. Btw I can already tell you that most inkjet film fill present a UV density of roughly 0.10logD, so a little less than one step on your 21-step Stouffer.

Go do some testing; it's better to try a couple of things out and make sense of what you're seeing than to try and do it 'right' on a theoretical approach. The latter isn't bad per se, but you learn a lot by just experimenting and reflecting on the results. Besides, Van Dyke is quick, easy and cheap to test with so there's no reason to avoid experimentation.

 

[ic-racer]

Each step of the 21 step wedge is 1/2 stop. So count the number of steps until just lighter than black and multiply by 1/2 to get the number of stops to decrease the base exposure.