Hi Vaughan...you missed out the second 2s exposure in your calculation....i.e. the second strip only got four seconds (the intial 2s for the whole sheet plus another 2s), the 3rd strip got those 4 sec0nds plus another 4seconds...i.e. 8 etc., the fourth strip got those 8seconds plus another 8seconds...i.e. 16secs.etc.

Rgds, K