Please keep in mind, that this is not gospel. It’s based on an increase of 10 times the exposure at ISO 3. That seems to be a fair estimate for the dry collodion process. You may find that it’s more like 8 times or 6 times (as the old literature says), but it only takes a plate or two and you’ll know.
When developing the plate, you should see the highlights within about 3-5 minutes (the sky, bright objects, etc.). The entire plate should be fully out within 10 minutes. If the highlights appear in 1 minute, you are overexposed. If they don’t appear for 5 minutes, you are underexposed. Simple.
To begin with, you need to determine what preservative you’re using. I’m testing T. Sutton’s “Rapid Dry Plate” process right now. That takes a “special” collodion and gum arabic as the preservative. Sutton claims these are as fast (exposure) as wet collodion negatives. We’re going to find out in the next couple of days.
For this question, I’ll address how I figure out exposure for Sutton’s Collodion Dry Plate using tannic acid. It would be nice to know what the ISO/ASA is, but it’s so slow (10x slower than wet collodion) not really feasible to approach exposure from that angle. Here’s what I do.
First, download the “Pinhole Assist” app. I’ve been using this app since 2010. A workshop student in Paris developed it and turned us all on to it during the workshop. He incorporated more wet collodion features in it after the workshop to include super low ISO/ASA and a really good/accurate calculation for exposure. The pinhole features were already there and help with what we do. Now, the collodion dry plate needs some help. Here’s what I do: Meter the scene at ISO/ASA 3.2 and set your f/stop. I have screenshots here showing the scene and the numbers. If the app says 1/2 second (.5) times that by 1024 = 512/60= 8.53 minutes for exposure. It’s a 10-stop increase from ISO 3.2. Easy, quick, and simple.
ISO/ASA 3.2 with f/16 and .5 seconds. Translated for Collodion Dry Plate: .5 seconds times 1024 = 512/60= 8.53 minutes for a Tannin Dry Plate. Notice where I am metering the scene - very important to expose for the shadows/midtones.