Pinhole negs part 2: flash plus exposure time

Okay that makes more sense. Exposures 16 and 17, see bottom right were the two best exposures.

As I noted in my last post:

Frame 16: 1 x flash 180 secs.
Frame 17: 1 x flash + 120 secs.

The exposure value was 7.6. On my chart  EV 7 calls for 4 min and EV 8 calls for 2 min. So around 3 min is what we're shooting for.

To me it looks like the flash contributes 60 secs to exposure time. If we look at exposures 18 and 21, the two next best exposures (middle row left and middle row right), we get:

Frame 18: 1 x flash + 60 secs.
Frame 21: 2 x flashes + 30 secs.

And indeed they are slightly underexposed. I think we can safely say 1 flash on Yellow on the Vivitar 283 equals around 60 sec. exposure time. I good round number and usable for my purposes.