Originally Posted by Existing Light
Oh, I forgot to add how I got my times. I found the exposure time for the couch picture by finding the exposure at F/16. The exposure was 4seconds @ F/16. I used my aperture chart and saw that f/512 was 10 stops under f/16, so I added 10 stops to 4 seconds. That's what gave me about 70 minutes. Actually, I came up with a little more than 68 minutes, but I rounded up

I agree with your f/stop calculation, but I think f/360, which is 9 stops from f/16 is close enough. Now to the exposure time:

You said you measured 4s at f/16 (are you sure, it wasn't 1/4 s at f/16). Assuming it was 4 s and the 9 stops above, this calculates to:

1st stop > 8s at f/22
2nd stop > 16s at f/32
3rd stop > 32s at f/45
4th stop > 1m at f/64
5th stop > 2m at f/90
6th stop > 4m at f/128
7th stop > 8m at f/180
8th stop > 16m at f/256
9th stop > 32m at f/360

and this doesn't even include reciprocity, which would take it to:

> 2h at f/360 (I don't think so)

It must have been 1/4 s, wasn't it? In which case:

1st stop > 1/2s at f/22
2nd stop > 1s at f/32
3rd stop > 2s at f/45
4th stop > 4s at f/64
5th stop > 8s at f/90
6th stop > 16s at f/128
7th stop > 32s at f/180
8th stop > 1m at f/256
9th stop > 2m at f/360

and with reciprocity failure:

6-12 minutes, depending on the film.