Apertia · Library
How do you compare an Airy-disk estimate with a chosen blur tolerance?
Compare the ideal Airy disc 2.44 × λ × N with your circle of confusion, and see why Apertia stacks frames once the disc passes half of it.
What the comparison measures
Apertia estimates diffraction with the ideal circular-aperture formula: Airy-disc diameter = 2.44 × wavelength × f-number. The app fixes the wavelength at 0.00055 mm, green light near the middle of the visible range, and compares the result with the circle of confusion (CoC) of the selected sensor preset. Both quantities are diameters on the sensor, so they can be divided directly once they share the millimetre unit.
Two f-numbers follow from that ratio. Where the Airy disc equals the CoC, N = CoC ÷ (2.44 × 0.00055); on a full-frame preset with a 0.030 mm CoC this is about f/22.35, and past it the calculator shows a diffraction note. Apertia's planner is stricter: it treats a single frame as safe only while the disc is at most half of the CoC, N = CoC ÷ (4.88 × 0.00055), about f/11.18 on full frame. When a request needs more than that, the plan switches to a focus stack at the largest listed stop below the limit.
Worked example
At f/8 and a 0.00055 mm wavelength, the ideal diameter is 0.010736 mm. Relative to a 0.030 mm tolerance, the ratio is about 0.358. The ratio has no unit.
- Ideal Airy-disk diameter (mm)
- 0.010736
- Diameter divided by the chosen tolerance
- 0.357867
Work through one setting
- Note the sensor preset and its CoC; full frame uses 0.030 mm, APS-C 0.019 mm, Micro Four Thirds 0.015 mm.
- Multiply 2.44 × 0.00055 mm × f-number. At f/16 that gives 0.021472 mm.
- Divide by the CoC: 0.021472 ÷ 0.030 = 0.7157, so the disc is about 72 percent of the tolerance.
- Compare the f-number with the half-CoC limit for that preset before deciding between one frame and a stack.
- Full frame · stack aperture used by the planner
- f/11
- APS-C · stack aperture
- f/6.3
- Micro Four Thirds · stack aperture
- f/5
- 1-inch · stack aperture
- f/4
Common mistakes
- Entering a wavelength in micrometres (0.55) next to a CoC in millimetres; the ratio then comes out a thousand times too large.
- Reading f/22.35 as the point where photographs suddenly become unsharp. It is only the aperture where two model diameters are equal.
- Using the full-frame f/11 limit on a smaller sensor. The limit scales with the CoC, so APS-C reaches it at about f/7.08 and the planner steps down to f/6.3.
- Switching on the strict CoC setting without noticing that every diffraction limit halves with it.
What the estimate leaves out
The calculation is monochromatic and ideal. It does not include lens aberrations, the sensor's pixel pitch, sharpening or the print size you will view, so a ratio below one is not a sharpness guarantee. The University of Arizona camera-systems notes listed below describe the same Airy-disc relation.