pinhole photo overexposure

Worked with the grad student group today, introducing pinhole photography. I had them make pinholes and we calculated the exposure time. However, something in our math was way off. It seems certain that we were going to need to treat the cameras (made from cans) as between f-400 and f-600 which means we'd need to use very long exposures on a cloudy snowstorm day like today. However,everything was very overexposed. We didn't have time to do a second round.