So, I shot several exposures of the same scene, with the same light, at the same time, and did some development tests.
The film was 35mm
AgfaPhoto "new emulsion" APX 100, fresh. My camera (Mamiya NC 1000s) was set at
ISO 80. It has a shutter speed priority mode, so I moved the speed until the TTL reflective metering chose an aperture of f/16. Lens : mamiya-sekor f1.7 50mm, no filter.
The following was kept constant :
- No prewash
- Temperature : 20°c
- Agitation method : continuous for the first 30s, then 10s at the start of every minutes
- Water stop bath
- PH measurement : paper test strips, 0.5 precision according to the box. It's not ideal, but I didn't want to invest in a PH meter. The biggest issue was reading the result with the caffenol solutions, because of the brown stain.
- Pictures of the negative : APS-C Canon EOS M100 mirroless camera, 50mm M42 fujinon f/1.8, no filter. Without and with extension tube for the close up shot. ISO 100, f/11, aperture priority mode. Light source was a phone. No edit on the first picture except rotation. The positive was done with the following method : 1) jpeg opened in GIMP 2) rotation/crop 3) colors->invert 4) colors->levels->auto 5) mode->grayscale 6) export.
Note : I couldn't upload the full resolution images, theses are resized to 1080 px. if anyone's interested, I can see how to share them in full size.
1. "Rodinal 1:50 control strip" : 500ml, 10ml rodinal (Adox Adonal). PH : n/a. Dev time: 10min
Just a control strip with a regular commercial developer, following
the manufacturer times, to compare to the others.
=> As APX 100 is my main film and i've done rodinal 1:50 10min @ 80 several times, both for scan and prints, I'm confident that the dense highlight is the result of the metering and not overdevelopment.
2. "Caffenol CM" : 500ml, 27g sodium carbonate, 8g ascorbic acid, 20g instant coffee. PH : ~9.5. Dev time : 12min
Regular
CCM Recipe.
=> Quite dense ... I was excepting less development : the recipe starting point is 15min so I thought 12min would be OK. Either this film need less time in CCM, or I screwed up somewhere (I think it's the second option .. most likely the temperature).
3. "Caffenol CM but with thyme" : 500ml, 27g sodium carbonate, 8g ascorbic acid, 3g dried thyme. PH : n/a. Dev time : 12min
Still the regular CCM recipe, but with 3g dried thyme instead of instant coffee. The thyme was boiled for 15min. I forgot to test the PH ..
=> I think it's OK. Less development happened.
4. "Vitamin C and washing soda only : round 1" : 500ml, 10g sodium carbonate, 8g ascorbic acid. PH : ~9.5/10. Dev time : 12min
So, do we really need coffee, or thyme, or whatever ? I diluted the 8g of ascorbic acid of the CCM recipe, and got a PH of 2. Then, 10g of sodium carbonate put it close to the CCM value. Same dev time, same agitation method :
=> thin, underdeveloped negative.
5. "Vitamin C and washing soda only : round 2" : 500ml, 27g sodium carbonate, 20g ascorbic acid. PH : ~10. Dev time : 12min
How much more vitamin C do we need to get a "good" negative ? for no particular reason, I chose 20g of ascorbic acid and 27g of sodium carbonate. Same dev time, same agitation method :
=> It's ... not bad at all, I think ? slightly more contrast than the rodinal strip.
6. "Raw caffenol" : 500ml, 12g sodium carbonate, 10g iodised salt, 18g coffee. PH : ~ 10/11. Dev time : 30min
Apologies to Donal Qualls, I didn't used the recipe he kindly provided because I found
this one. It was easier for my european brain as it was already in grams, and the fp4+ was closer to the APX 100.
=> development did happen, but not enough. I don't know if it needs more coffee and/or less restrainer (and I'm of course interested in advices). PH was higher than CCM, might be a factor ?
7. "Raw caffenol but with thyme" : 400ml, 10.8g sodium carbonate, 9g iodised salt, 4g thyme. PH : ~ 11. Dev time : 30min
Same thing but with 4g of dried thyme boiled for 15min instead of coffee. I used a second single reel tank, that's why the solution is 400ml.
=> nothing. Nothing at all.
Grain / sharpness comparaison : rodinal and Caffenol CM with thyme
My personnal interpretation, which of course you may or may not agree with : I havent't discovered anything, but merely confirmed what I read in several articles and forum posts. Caffenol is nice because the development time and each composants are kept low by using two developing agent : vitamin C and coffee. At least thyme can be that second developing agent instead of coffee, but peoples have been reporting success with a wide variety of other plants. Coffee alone can develop film, but thyme cannot. Vitamin C alone can develop film, but you need a lot more than what is used in the caffenol C* recipes. My preference would be toward a home-made developer that don't cost much and use everyday products that you can find in your garden or at the next store, but to each their own .. some won't mind the cost of ascorbic acid, and I'm sure great result can be achieved with vitamin C and washing soda only.
Sorry for the long post and approximate English. Thank you for your advices and i'd be happy to discuss the results.