Apertia · Library
Why are diffraction and defocus separate parts of a sharpness budget?
Stopping a 24 mm lens from f/8 to f/16 moves the near limit at hyperfocal from 1236 mm to 636 mm while the ideal Airy disc doubles to 0.0215 mm.
Two blurs that move in opposite directions
Depth of field in Apertia is geometric: a point is called sharp while its defocus disc stays within the circle of confusion, and a larger f-number widens that zone. Diffraction is separate: the ideal Airy disc, 2.44 × 0.00055 mm × N, grows in proportion to the f-number for every point in the frame, in focus or not. Stopping down therefore buys depth while softening the whole image.
The app keeps the two apart with two rules. The calculator shows a note once the aperture passes the f-number where the Airy disc equals c (f/22.35 on full frame). The shot planner is stricter and stacks frames rather than going past the f-number where the disc reaches half of c (f/11.18).
Worked example
With the same 0.00055 mm wavelength, the ideal diameter is 0.005368 mm at f/4.
- Ideal diameter at f/4 (mm)
- 0.005368
Worked example
The diameter doubles at f/8. A wider geometric focus interval at that aperture does not cancel this separate diffraction change.
- Ideal diameter at f/8 (mm)
- 0.010736
Balance the two
- Work out the depth you need, for example with Shot plan from the nearest to the farthest point.
- Read the f-number it needs and compare it with the half-CoC limit for your sensor.
- If it stays below the limit, use one frame at that stop.
- If it goes beyond, prefer the stack the planner proposes over stopping down further.
- When you do stop down past the limit, watch for the calculator's diffraction note.
Worked example: hyperfocal at f/8 and f/16
Full frame, 24 mm. At f/8 the hyperfocal distance is about 2448 mm, and focusing there keeps everything from 1236 mm to infinity within 0.030 mm. At f/16 the hyperfocal distance drops to 1248 mm and the zone starts at 636 mm, so the foreground gains about 600 mm. Over the same change the ideal Airy disc grows from 0.010736 mm to 0.021472 mm, from about a third of c to nearly three quarters of it. The calculator shows no note at f/16, but f/16 is beyond the planner's single-frame limit.
- f/8 · hyperfocal 2448 mm
- sharp from 1236 mm · Airy 0.010736 mm
- f/16 · hyperfocal 1248 mm
- sharp from 636 mm · Airy 0.021472 mm
Common mistakes
- Adding the defocus disc and the Airy disc and treating the sum as a measured sharpness figure.
- Assuming diffraction only affects the background; it applies to the focused subject too.
- Stopping down to f/22 for depth when a two-frame stack at f/11 would cover the same range.
- Comparing apertures without keeping focal length, focus and c fixed.
Where the comparison stops
Both quantities come from idealised models, so neither predicts final image quality on its own; aberrations, sensor sampling and sharpening also matter. The University of Arizona camera-systems notes and DOFMaster equations below give the two formulas.