Apertia · Library
What can an ideal circular-aperture model leave out of a real photograph?
Apertia warns past the f-number where an ideal Airy disc equals the CoC, f/22.35 on full frame and f/5.96 on iPhone, but the disc ignores the real lens.
Where the app uses the ideal disc
Apertia takes one number from diffraction theory: the diameter of the Airy disc formed by a perfect circular aperture in green light, 2.44 × 0.00055 mm × N. It uses that number in three places. The calculator shows a note when the selected aperture is beyond the f-number where the disc equals the sensor's circle of confusion. The hyperfocal table fades every column past that same f-number. The shot planner keeps single frames at or below half of it and stacks beyond.
Each of those thresholds is a comparison between two model diameters. None of them is a measurement of your lens, which may already be limited by aberrations before that point or hold up a little beyond it.
Read the warnings for your sensor
- Open the hyperfocal table and look for faded columns; the hint line names the f-number where fading starts.
- In the calculator, note whether the diffraction note appears for the aperture you chose.
- Remember that the threshold moves with the sensor's CoC and halves under the strict setting.
- If the result matters, photograph a detailed flat subject at the candidate stops and compare the files.
Worked example: thresholds by preset
On full frame the disc equals c = 0.030 mm at f/22.35. At f/22 the disc is 0.029524 mm, a ratio of 0.984, so no note appears; at f/32 it is 0.042944 mm, a ratio of 1.431, and the note appears. The table's columns run from f/1.4 to f/16, so none are faded on full frame or medium format (f/26.83). APS-C (f/14.16) and Micro Four Thirds (f/11.18) fade f/16, 1-inch (f/8.2) fades f/11 and f/16, and the iPhone main-camera preset (f/5.96) fades f/8, f/11 and f/16.
- Full frame, f/22 · f/32
- ratio 0.984 · 1.431
- APS-C · Micro Four Thirds
- f/16 faded
- 1-inch
- f/11 and f/16 faded
- iPhone main camera
- f/8, f/11 and f/16 faded
Common mistakes
- Reading a faded table cell as an unusable setting rather than a model warning.
- Assuming no warning means the lens is diffraction-free at that stop.
- Comparing two cameras by their Airy-disc thresholds alone; pixel count and processing are not included.
- Using the f/1.78 iPhone preset's fading at f/8 as if the phone could set f/8; the main camera's aperture is fixed.
What the model leaves out
The disc assumes a perfect circular opening and a single wavelength. Aperture blade shape, aberrations, sensor filters, demosaicing and sharpening all change the recorded result; the University of Arizona camera-systems notes listed below derive the ideal case only.