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How can I guesstimate the exposure time for a collodion wet plate? Any helpful hints?

I have a lightmeter, a Sekonic 308, so I can take measurements. I know that the age of the collodion plays into the exposure time needed as well, but its a minor factor. I've done my research on the whole process of developing a collodion wet platen, but I haven't seen any recommendations or hints on the exposure time.

Is there anyway to get close to a good exposure time by some indicators? Or is it just a matter of experience and wasting a few plates until you get it right by guessing? I haven't done my first plate yet, I'm still waiting for the chemicals. I have zero personal experience with wet plate photography but I'm familiar with regular modern day DSRL photography and not a beginner in photography.

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This material has super low sensitivity to light. Additionally, it has no sensitivity to red so you can use a safelight. Assume an ISO of 2. The sunny 16 rule of thumb is workable. This will be: Shutter speed is 1 over ISO – this shutter speed is ½ second at f/16 in bright sun. Applying the reciprocity law – your trial exposures can be any of the following:

  • 1 second @ f/22
  • 1/2 seconds @ f/16
  • 1/4 sec. @ f/11
  • 1/8 second @ f/8
  • 1/15 second @ f/5.6
  • 1/ 30 second @ f/4
  • 1/60 second @ f/2.8
  • 1/125 second @ f/2

This material can be worked under safelight; you can easily create multiple exposures on a single plate. This will allow you to test for ISO consuming a minimum time and material.

Cover all but a thin vertical strip by carful placement of an opaque sheet or card over the entire plate except for a thin strip running top to bottom on one edge. Load this plate into the camera and expose an outdoor scene.

Now return to the darkroom and cover this initial slit area and the remaining area of the plate except for an adjacent strip area. Repeat, exposing as many strips as you can, each will have a unique exposure. Best if they each very by a 2x increment (same as f-stops). You can use exposure time or f-stop increment. The idea is to test for a suitable ISO on a single plate.

I would use aluminum foil to do the covering and uncovering. I would prefer a material without the mirror finish but foil is opaque and crimpable, it molds to shape and holds. If you don’t like the foil idea, come up with a substitute.

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