Presets
Playback
Aperture and lens
Light passed is proportional to the clear area, so the diameter falls by sqrt(2) per stop. Closing the iris deepens the sharp zone but eventually diffraction blurs the image.
Model checks
The science
What is modelled
Nine leaves swing together so the opening is a nine-sided polygon. A 51-point calibration table gives the equivalent clear diameter for each control position; optics follows from the diameter and the lens focal length.
Equations
- f-number
N = f / D; light~ D^2; stops2 log2(Dmax / D). - Airy disk diameter
2.44 lambda N. - Thin-lens blur circle
b(x) = f^2 |x - s| / (N x (s - f)); near and far limits are the distances where b equals the allowed circle. - Nonagon area
n a^2 tan(pi/n).
Checks run in the page
- Calibration table is monotonic and interpolates exactly.
- One stop equals an area ratio of one half.
- Polygon area equals the equivalent circle.
- Depth-of-field limits give exactly the allowed blur circle.
Honest simplifications
Blades are drawn as straight leaves around a regular polygon of the calibrated area; real leaves have curved edges and slight overlap errors. Diffraction uses the on-axis Airy disk only; no aberrations or vignetting.
What to try
- Press Stopped down: depth of field widens but the Airy disk exceeds the blur limit.
- Press Wide open: shallow focus, maximum light.
- Press Auto cycle to watch all nine leaves swing together.