Originally Posted by RalphLambrecht

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.
For the shot in my living room, the exposure was actually 4 seconds @f/16. It was hard for me to believe, too. You have me doubting myself now, so I'll check again tonight. I cant check right now because I took the picture of my couch at night, so there was no (or very little) light coming in through the windows.

I didnt adjust for reciprocity at all on the first one, and I kinda just guessed at reciprocity adjustment on the second shot I did outside.

I noticed that you adjusted for reciprocity by adding about two stops overexposure. Is that a rule of thumb kind of thing or is there an actual equation for adjusting for reciprocity? I've read the data sheet for hp5+ and found the recirpocity chart, but I dont think it covers exposures longer than 75 seconds. I dont have it printed out, but I'll look it up again and make sure about that