How do you deterime a starting exposure for contact printing a step wedge to film?

Now that I finally have a densitometer that works, I want to contact print my Stouffer step wedge to a sheet of film to check on my developing. My plan is to have my Gralab 450 timer flip an LED light bulb on and off rapidly to provide the exposure. At this point, I'm more interested in just getting all the steps recorded so I can look at the characteristic curve of the film than I am about knowing precisely to the photon how much light I gave it (in order to determine speed point).

How do I determine the correct exposure before making the first test?

Thanks as usual.

Cool! I'm working on this same task. I've ordered a solid state relay to make a repeatable timed exposure using an led bulb as well. My Stouffer tablet is for ~5 inches of 35mm film IIRC, so I was just planning to do a series of exposures on separate strips of film to get in the ballpark. Afterwards, hopefully I can figure out how to measure roughly how much light is emitted so I can calculate film speed, but that is a secondary goal for me. As you say, the HD curve is what's most informative. I'll let you know how it goes, and would like to hear others' solution to this challenge. All the best.

So if I read this correctly, you are trying to build your own sensitometer, correct?
I see a bunch of them for sale on eBay made by X-rite, some Kodak and EG&G remnants also out there.

I've not been able to find a suitable sensitometer on a limited budget. If I understand correctly, if you are interested in calculating film speed, you'd need to send the sensitometer away to be calibrated to find out the absolute exposure delivered (which I imagine would be prohibitively expensive for a hack like me). Additionally, most of the units I've seen on eBay seem to be for testing x-ray film, so they expose using blue or green light. While probably not a problem, I think it's preferable to use the same colour light that will be exposing my film in-camera.

Ok, that helps. I have a photo lab background, and through the years, we only used sensitometric strips for the monitoring of proper total processing measurements, but never to calculate film speed. Kodak made E-6 and C-41 process control strips you could buy for this purpose. Under some rare and very special conditions, with our sensitometer, we made custom control strips which used xenon(flash) with lots of neutral density filtering for film types like ortho or graphic arts materials where strips were not available. For plotting pure H&D curves, this makes sense, but unless you are going to a lot of trouble to have everything else as controlled, for me it would be time better spent on nailing down better development times by looking at the high values of a b&w print.

That's really interesting, thanks for posting. Good advice about looking at a print, however I'm interested in b&w slides and want something a bit more quantified.

Educated guess

Make a (un)educated guess for the first exposure time. Uneducated, as it depends on many variables like:

- intensity of the LED (depending on it´s power, opening angle, distance, spectrum, ...)
- accuracy of the LED flash times
- properties of the sheet film
- your development process

Turn on the LED, take a camera with integrated exposure meter, make a reading at fully open aperture, adjust for ISO difference (if your camera has a different setting than the film you are using), and go with that time. Make the reading at aprox. the same distance to the LED you will have for final exposure.

The exact time of the first exposure is not that important, as the stouffer wedges have a range of at least 5 stops. Unless you are totally off, you will get a useful result. For the second attempt, adjust the time according to the readings you get on copy of the step wedge, leave everything else the same.

williaty said:

At this point, I'm more interested in just getting all the steps recorded

Click to expand...

Which DMax does your step wedge have? What is the reproduction range of your sheet film? Stouffer wedges have a DMax of either 1.4 (equal to 5 stops), 2 (7 steps) or 3 (equal to 10 stops). Reproducing 5 stops should work, 7 stops might be difficult already, 10 stops might not work.

There are a few things you need to consider when you do this:

1. Depending on Dmax you will need strong exposure and sensitive film. On my Durst Laborator 138 (2x300 W halogen bulbs, light spread out over about 20x30 cm) I need about 1/2 second @ F/5.6 to get a good step wedge contact print on Tri-X.
2. You'll say "1/2 second is very short!": it isn't, because it already gets you deep into reciprocity failure territory. Tri-X enters it at exposure time of about one second for normal exposure, and the toe region of your film is proportionally deeper in reciprocity failure area! Unless you test with Fuji Neopan Acros, be prepared to test with strong illumination and very short exposures, or face distorted toe regions which look a lot like underdevelopment.
3. Watch out for the angle your light source shines at your film, lest you get strong vignetting artifacts.
4. Since your Stouffer wedge holds a high density range, and since overexposure is an unlikely event, I'd say create the strongest evenly spread out light source you can, use multiple light sources to maintain low incident angle, then expose somewhere between 1/10 and 1/2 seconds and see what you get.