Assessing film speed and development time without a darkroom.

[Drew Wiley]

Cute cartoon.

[JohnF]

I now have a Stouffer 21 step wedge, and have exposed several frames on roll film of a white card out of focus at zones 1-8. I am wanting to assess visually the tones on my negatives against the step wedge - I do not have a darkroom.
I have read that a negative has a density of 0.1 at Zone 1 and 1.3 at Zone 8. So I need to get the Zone 1 tone on my negative to match Step 2 on the Stouffer wedge for film speed.
The specification on the Stouffer wedge is that a density of 1.3 approximately matches between Step 9 and Step 10. Since each step is half a stop, this is only a difference of about 4 and a half stops and not the 7 stop difference on Ansel Adams Zone system.
Can I in fact use the step wedge in this way to assess film speed and development time, and how can I reconcile the above disparity?

[Michael R]

Think of the stouffer steps as neutral density filters you might put on a taking lens. Each density increment of 0.15 reduces the amount of light passed by 1/2 stop of exposure. Those increments would only be equal to the increments in the developed negative if the contrast gradient (ie slope) of the negative was the same as that of the wedge. The wedge has a slope of 1. However in typical photography we don’t develop negatives to a gradient of 1. “Normal” is typically a gradient between 0.5 and 0.6. This explains the apparent disparity.

Given the methodology you are using, don’t think of the stouffer wedge increments in terms of zones. Just use the densities of the wedge for visual comparisons with your test negative.

Keep in mind your roll film test negatives include the densities of the film base and fog. You are interested in image density above the constant density of base+fog. One relatively simple way to do this visually is to sandwich an unexposed but fully processed piece of your chosen film with the stouffer wedge. Just to keep things simple for the sake of this example, suppose you target a Zone I density of 0.2, and a Zone VIII density of 1.4. You would then want the density of the Zone I exposure on the test roll to match the combined density of step 2 on the wedge sandwiched with the unexposed, processed piece of film. Likewise for Zone VIII, you would want the density of the Zone VIII exposure on the test roll to match the combined density of step 10 on the wedge sandwiched with the unexposed, processed piece of film.

Alternatively, you could first try to find a close match between the unexposed, fully processed piece of test film and the stouffer wedge. This would give you a density number for base+fog. You could then compare the test film densities with the wedge directly and do the math to subtract base+fog density. Same result in the end.

[JohnF]

Hi, Michael. That was really helpful.
I tried the first option, overlaying the Stouffer strip on an unexposed developed frame and aligning it with the Zone 1 and then Zone 8 exposures to compare.
To my delight, Zone 1 matched step 2 and Zone 8, step 10. This was FP4 exposed at ASA 80 and developed in Ilford DD-X for 9 mins.
I enjoy trying different films so now feel emboldened to testing as I go along.
Thanks again.