Exposure time and capture volume share fields but answer different questions. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Exposure time and capture count are different ledgers
The first schedule has 4 fields at dwell time 5 s, so exposure time is 20 s. With 3 channels it records 12 captures.
t=4⋅5s=20s,Ncap=4⋅3=12
Equal exposure time can hide a different capture count
The second schedule changes to 5 fields at 4 s, so the time is still 20 s. The channel count is 2, giving 10 captures.
t=5⋅4s=20s,Ncap=5⋅2=10
More fields can shorten time but expand captures
The third row uses 6 fields and dwell time 3 s, giving 18 s. Its 4 channels produce 24 captures, so the scan separates time budgeting from readout volume.