Hi Bryan
This is what I think may be happening:
The interval is from the start of Exposure 1 to the start of Exposure 2 to the start of Exposure 3
to the start of Exposure N. So, in your example, with ICNR mode (C.Fn II-1 long exposure nose reduction = Auto or 2 = On), then the following sequence takes place:
- Light exposure 01 15 secs is successful (interval 1 of 10 secs causes a failure, interval 2 starts)
- Dark exposure 01 15 secs (interval 2 ends causing a failure. Interval 3 starts, possibly causing a failure)
- Download time say 1 sec
- Light exposure 04 15 secs is successful (occurs on interval 4)
- Dark exposure 04 15 secs
- Download time say 1 sec
- Light exposure 07 15 secs is succesful
- Dark exposure 07 15 secs
- Download time say 1 sec
And so on
- Light exposure N 15 secs
- Dark exposure N 15 secs
- Download time say 1 sec
So, the interval between exposures is 30 seconds plus download/processing time.
If you set the interval as 10 seconds, then the camera will still be exposing the 1st light and 1st dark exposures after 3 intervals have elapsed (10+10+10 secs) so only the 4th interval will cause another Light exposure to be taken and the 3 earlier attempts at 10 sec intervals will be registered as failed.
Cheers
Dennis