The calculation
spare time = start-to-start interval − occupied time
Worked example
An 8-second exposure, another 8 seconds for a dark frame and 2 seconds of processing occupy 18 seconds. A 20-second interval leaves 2 seconds. An 18-second interval has no spare time and is flagged.
Make the timing assumption explicit
Some camera settings add processing or a dark-frame exposure after the visible capture. This model optionally budgets a second period equal to the shutter exposure. It does not assert that every camera operates that way. Confirm the behavior in your camera manual and test it with the intended settings.
Positive spare time is a model result
A positive number means the entered allowances fit arithmetically. It is not a guarantee that a camera will maintain the requested cadence. Autofocus, file writing, temperature, buffer state and automatic exposure changes can alter timing. Leave additional headroom and examine a short trial sequence.
For a day-to-night sequence, check the longest exposure you expect to use. A plan based only on bright daylight can become too tight later. If your timer defines interval as a delay after exposure rather than start-to-start, convert its setting before using this tool.
Common questions
Is zero spare time enough?
The checker flags it because there is no margin for variation. Increase the interval or revise the capture settings after testing.
Should I always turn noise reduction off?
This tool does not make that recommendation. Use your camera’s documentation and image-quality requirements to choose settings.
Further reading
Official or original references for the workflow; these organizations do not endorse this independent tool.