This was a final shot and the only one that came out for me this day, after many trials. Light was very changeble - sun and clouds. Everyone was running here and there checking the exposure time. What worked by chance for someone in one moment didn't work for somebody else in next minute. But I was determinated and got the image. Collodion has very low light sensitivity, range of 1-3 ASA, so it is impossible to measure the exposure, it's just a good guess.This one was ca 3 seconds, f8. Black plexiglass ambrotype.