Focus Stacking Step Calculator
Slices, step size and depth of field for a focus stack — where stopping down to shoot fewer slices runs into diffraction, and at 1:1 it runs into it early.
Depth of field is a judgement rather than a measurement — it rests on a chosen circle of confusion, and stacking software inspects far more closely than a print does. Real stacks also have to allow for focus breathing, which changes magnification slightly between slices. Treat the count as a floor to add margin to. Nothing is uploaded.
Stopping down cuts the slice count — until diffraction stops paying
Thicker slices mean fewer of them, and it works: covering 20 mm at life size takes 22 slices at f/8 and 8 at f/22. But at 1:1 the effective aperture is double the set one, so f/11 is already f/22 to the light and the Airy disc has passed the circle of confusion. Every slice past that point is thicker and softer, and the softness applies to the whole stack rather than its edges. The fewest slices worth shooting is 22, not 8 — fourteen extra frames is what sharpness costs here.
| Set | Effective at 1:1 | Depth of field | Slices | Airy disc |
|---|---|---|---|---|
| f/2.8 | f/5.6 | 0.325 mm | 62 | 0.0075 mm |
| f/4 | f/8 | 0.464 mm | 44 | 0.0107 mm |
| f/5.6 | f/11.2 | 0.650 mm | 31 | 0.0150 mm |
| f/8 | f/16 | 0.928 mm | 22 | 0.0215 mm |
| f/11 | f/22 | 1.276 mm | 16 | 0.0295 mm — past the limit |
| f/16 | f/32 | 1.856 mm | 11 | 0.0429 mm — past the limit |
| f/22 | f/44 | 2.552 mm | 8 | 0.0590 mm — past the limit |
Magnification is the bigger lever, and overlap is the hidden cost
At a tenth life size the depth of field already exceeds the subject, so one frame does it. At life size it takes 22; at 2× it takes 58. Aperture moves the count by a factor of eight across its whole range; magnification moves it by 58.
| Magnification | Depth of field | Slices for 20 mm |
|---|---|---|
| 0.1× | 51.067 mm | 1 |
| 0.25× | 9.281 mm | 3 |
| 0.5× | 2.784 mm | 8 |
| 1× | 0.928 mm | 22 |
| 2× | 0.348 mm | 58 |
Overlap costs slices in exactly the proportion it removes from the step — and here is where a fencepost hides. With no overlap the step is the depth of field, so the count is simply the depth over the step and nothing is lurking. Add overlap and the first slice keeps its full depth of field while every step after it shrinks, so the two part company: 43 slices at 50% overlap where depth-over-step would say 44. One frame, and it is the kind that quietly becomes a habit.
| Overlap | Step | Slices | Against no overlap |
|---|---|---|---|
| 0% | 0.928 mm | 22 | 1.00× |
| 20% | 0.742 mm | 27 | 1.23× |
| 30% | 0.650 mm | 31 | 1.41× |
| 50% | 0.464 mm | 43 | 1.95× |
How to use
- Enter your focal length, aperture and magnification.
- Enter how deep the subject is.
- Add an overlap if your stacker wants one.
- Read the slice count and whether diffraction is biting.
Frequently asked questions
How many slices does a focus stack need?
The subject depth divided by the depth of field, near enough. Covering 20 mm at life size with a 100 mm lens at f/8 takes 22 slices; at f/22 it takes 8.
So should I just stop down?
Only so far. At 1:1 the effective aperture is double the set one, so f/11 is already f/22 to the light and the Airy disc has passed the circle of confusion. Past that every slice is thicker AND softer.
What is the fewest slices worth shooting?
Whatever the diffraction limit allows, which here is 22 rather than the 8 that f/22 offers. Fourteen extra frames is what sharpness costs, and softness from diffraction affects the whole stack rather than its edges.
What matters more, aperture or magnification?
Magnification, by a long way. Across its whole range aperture moves the count by a factor of eight; going from a tenth life size to twice life size moves it from one slice to fifty-eight.
How much overlap should I use?
Twenty to thirty percent is usual, and it costs slices in exactly the proportion it removes from the step. A 30% overlap is about half again as many frames; 50% is roughly double.
Is the slice count just depth over step?
With no overlap, yes — the step IS the depth of field, so the two are the same expression. Add overlap and they part company, because the first slice keeps its full depth while every step after it shrinks.
What else should I allow for?
Focus breathing, which changes magnification slightly between slices and makes the stack harder to align. Stacking software also inspects far more closely than a print does, so treat the count as a floor and add margin.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.