Apertia · Library
How can aperture change the number of frames needed for a stack?
Compare stack frame counts at f/4, f/8 and f/11 for a 50 mm 1000–2000 mm target, and see why Apertia stacks at f/11 instead of the f/32 one frame needs.
Two limits pull in opposite directions
Stopping down widens each frame's depth of field, so fewer focus positions cover a fixed range. It also enlarges the Airy disk, 2.44 × 0.00055 mm × N. Apertia settles the trade-off with a fixed rule: a single frame is accepted only while the Airy disk stays at or below half the circle of confusion. On full frame that limit is f/11.18, and the planner stacks at the largest third-stop below it, f/11.
The shot planner therefore never asks you to choose an aperture for a stack. When the required single-frame f-number, rounded up to a listed stop, is beyond that limit, the plan shows how many shots to take at the stack aperture. Comparing other apertures by hand is still useful for understanding why the app chose that number.
Worked example
The 50 mm full-frame model covers the stated 1000–2000 mm range in six frames at f/4.
- Frames at f/4
- 6
- Target covered in the model
- Yes
Worked example
Keeping the other inputs unchanged gives three frames at f/8. The smaller list does not account for the longer exposure or the image’s complete sharpness budget.
- Frames at f/8
- 3
- Target covered in the model
- Yes
Compare apertures on one target
- Fix the sensor preset, focal length, circle of confusion and the near and far target.
- Build the stack at each aperture, starting with the near limit of frame 1 at the near target.
- Count frames and confirm that the last far limit reaches the far target.
- Compute the Airy disk for each aperture and compare it with half the CoC.
- Keep only apertures your lens actually offers.
Worked example: 50 mm, 1000 to 2000 mm
A single frame covering 1000–2000 mm would need f/23.485, rounded up to f/32 — far past the f/11.18 limit — so the planner builds a stack at f/11 and needs three shots. By hand, f/4 needs six frames and f/8 three. At f/4 the Airy disk is 0.005368 mm, at f/8 0.010736 mm and at f/11 0.014762 mm, just under half of the 0.030 mm CoC. Here f/8 and f/11 give the same frame count, so the smaller disk at f/8 costs nothing in frames.
- f/4 · frames · Airy disk
- 6 · 0.005368 mm
- f/8 · frames · Airy disk
- 3 · 0.010736 mm
- f/11 · frames · Airy disk
- 3 · 0.014762 mm
- Single-frame requirement
- f/23.485 → f/32
Common mistakes
- Picking the aperture with the shortest frame list without checking the diffraction estimate.
- Comparing frame counts for two apertures with different near or far targets.
- Assuming the full-frame f/11 rule applies to APS-C, where the planner stacks at f/6.3.
What the comparison omits
Frame count and Airy disk are model numbers. Exposure time, subject movement and lens aberrations also change the result and are not part of the plan. The DOFMaster equations and the University of Arizona camera-systems notes listed below cover the depth and diffraction relations used here.