Circle of confusion: what counts as sharp?
Depth of field needs a tolerance. Changing it changes the reported sharp range, even though the lens and the photograph stay the same.
A blur circle is not a sensor pixel
A point away from the focused distance forms a defocused footprint on the sensor. A depth-of-field calculator needs an acceptable diameter for it. This threshold is the circle of confusion, abbreviated CoC and expressed in millimetres at the sensor.
Sharpness changes gradually across a scene. The tolerance depends on how much you enlarge the image, how far away you view it and how much detail you expect. Sensor pixel pitch alone does not determine an appropriate viewing criterion.
Same shot, two definitions of acceptable focus
Keep a 50 mm lens at f/1.8, focus at 3 m, and place the background at 12 m on a 36 × 24 mm sensor. Compare a 0.030 mm criterion with a stricter 0.015 mm criterion.
| Acceptable CoC | Near limit | Far limit | Total DOF |
|---|---|---|---|
| 0.030 mm | 2.820 m | 3.204 m | 0.384 m |
| 0.015 mm | 2.907 m | 3.099 m | 0.191 m |
The stricter criterion roughly halves the acceptable interval. Background blur does not shrink: changing the threshold changes the classification of sharpness, not the physical light cone. Hyperfocal distance changes from about 46.346 m to 92.643 m.
Compare both criteria side by side →The simulator's default
We use sensor diagonal divided by 1500. For 36 × 24 mm that is about 0.02884 mm. This example explicitly chooses a rounded 0.030 mm to make its inputs reproducible. Other camera presets use their actual sensor dimensions.
The default assumes a consistent relative enlargement convention across formats. It is not a promise that every pixel will be equally crisp. Advanced controls let you choose a different CoC. When comparing cameras, state whether output size, viewing conditions and framing match.
Choose a criterion for the photograph you need
A small screen image can tolerate blur that becomes obvious in a large print or a heavy crop. Greater enlargement makes the same sensor-plane footprint larger in the output. Close viewing also changes how much detail you can distinguish.
Use the default to plan, try a stricter threshold to see how much tolerance you lose, and inspect a real frame at its intended size. Stopping down increases geometric DOF, but diffraction, motion and focusing errors can still reduce visible detail.
Changing CoC alone does not sharpen the preview. The numerical DOF and blur-strength label change because their threshold changes. Aperture, focus and object distances determine geometric defocus.
Continue with hyperfocal distance, matching sensor formats, or the documented equations.
Verify the numbers: reproducible reference examples · equations and limitations · publisher and corrections.