Apertia · Library
How can you inspect a focus stack for uncovered gaps?
Check a three-frame 50 mm f/8 stack for gaps: setting the focus to the displayed 1.33 m and 1.73 m leaves a 2.292 mm uncovered strip near 1.5 m.
How frames meet in the plan
Apertia places each stack frame so that its near limit equals the previous frame's far limit exactly, so in the model there is neither overlap nor gap. That exactness lasts only as long as you set the planned focus distances exactly. The app displays distances rounded for reading — in metres with two decimals between 1 and 10 m — and a focus ring is set by eye, so real frames start and stop a little off the plan.
An overlap check therefore compares the intervals of the frames you actually set, not the plan's: the far limit of each frame must reach at least the near limit of the next, and the first and last frames must reach the two ends of the target.
Worked example
For the full-frame 50 mm model at f/8, a 1000–2000 mm target produces three frames. Its interval boundaries progress through approximately 1198.401 mm and 1505.928 mm before the last frame reaches beyond 2000 mm.
- Requested target covered in the model
- Yes
- Stopped with an uncovered target at the frame cap
- No
Check the intervals you will really use
- Copy the planned focus distances and their near and far limits.
- Replace each focus distance with the value you can actually set, for example the displayed rounding.
- Recompute the near and far limit for each of those settings.
- Compare each far limit with the next near limit; a smaller far limit is a gap.
- Check that frame 1 starts at or before the near target and the last frame ends at or beyond the far target.
Worked example: rounding to the display
For full frame, 50 mm, f/8 and 1000–2000 mm, the plan focuses at 1089.799, 1333.98 and 1733.834 mm, with joints at 1198.401 and 1505.928 mm and a last far limit of 2046.437 mm. The app displays those focus distances as 1.09 m, 1.33 m and 1.73 m. Set exactly to 1090, 1330 and 1730 mm, the frames cover 1000.167–1198.648, 1195.235–1500.785 and 1503.077–2041.022 mm. The first joint still overlaps, but between 1500.785 and 1503.077 mm a 2.292 mm strip is outside every frame, and frame 1 now starts 0.167 mm behind the near target.
- Planned joint 2
- 1505.928 mm
- Rounded frame 2 · far
- 1500.785 mm
- Rounded frame 3 · near
- 1503.077 mm
- Gap
- 2.292 mm
Common mistakes
- Checking only the planned list, where joints always meet by construction.
- Rounding every focus distance down, which shifts all frames nearer and opens gaps at the far end.
- Ignoring a gap of a few millimetres on a product whose label sits exactly there.
Scope of an interval check
Interval arithmetic shows where the model leaves gaps; it does not prove that the merged image is sharp, and it does not align or blend frames. The DOFMaster equations linked below define each frame's limits.