Lenses and Focus
Circle of Confusion Calculator
Derive acceptable sensor blur from print size, viewing distance, and enlargement.
Enter the optical values
For Circle of Confusion, keep sensor format, units, focus state, and reference planes attached to the entries.
The lens or focus result will appear here with supporting checks.
Define the optical setup for Circle of Confusion
Derive acceptable sensor blur from print size, viewing distance, and enlargement.
Circle of Confusion Calculator reports circle of confusion from Sensor width, Print width, Viewing distance, Visual acuity. Derive a sensor-plane blur criterion from an intended print.
Do not mix nominal specifications with measurements taken after cropping or digital correction.
Inputs that define circle of confusion
Record Sensor width and Visual acuity from one camera, lens, focus setting, and frame. Preserve the unit printed beside Sensor width and Visual acuity; converting one without the other changes the geometry behind circle of confusion.
Identify whether Sensor width came from EXIF, the lens scale, a test frame, a sensor specification, or direct measurement before comparing another Circle of Confusion setup.
The calculation behind Circle of Confusion
Derive acceptable sensor blur from print size, viewing distance, and enlargement.
For Circle of Confusion, the page keeps the intermediate geometry visible and rounds only the displayed circle of confusion. Use a familiar full-frame or identity case as an independent check.
Worked check with the published values
For Circle of Confusion, calculate the defaults and save the displayed circle of confusion with its supporting quantities. Change only Sensor width in the Circle of Confusion example, predict the direction of circle of confusion, and calculate again.
Restore Sensor width and confirm the first result returns. That reset test distinguishes a real change in circle of confusion from a stale Sensor width value or an unnoticed unit conversion.
Where the circle of confusion model stops
Visual acuity, contrast, sharpening, printer behavior, and viewing distance all affect acceptable blur.
Circle of confusion cannot judge the lens rendering, autofocus repeatability, decentering, field curvature, contrast, stabilization, or the success of the intended composition.
Measurement boundary for Circle of Confusion
Define the reference plane for Sensor width and what is included in Visual acuity. For Circle of Confusion, a distance referenced to the sensor cannot silently replace one measured from the lens front, mount, principal plane, or subject surface.
For Circle of Confusion, list any crop, correction profile, teleconverter, extension, close-up element, breathing, or shift already represented by Visual acuity. Apply each Circle of Confusion adjustment once—either inside the entered measurement or explicitly afterward, never in both places.
Reading the result in practice
Compare circle of confusion with a test frame, focus chart, measured field width, sensor specification, or known limiting case. If that check disagrees with circle of confusion, inspect the units and the reference plane used for Sensor width before changing the formula.
The Depth of Field Comparison Calculator addresses a nearby lens question, but its output should be transferred only when the format and geometry remain the same.
Precision and available settings
Keep full precision while circle of confusion feeds another calculation, then round to the precision supported by the lens scale, rail, sensor specification, or field measurement.
Extra decimals in circle of confusion cannot repair uncertainty in Sensor width, Visual acuity, pupil location, focus breathing, or the selected sharpness criterion.
Saving a reproducible lens record
Save Sensor width, Print width, Viewing distance, Visual acuity, circle of confusion, camera format, lens state, focus distance, and date. Mark nominal specifications separately from measured values.
For Circle of Confusion, when the lens, format, crop, focus, or camera position changes, preserve the earlier Circle of Confusion case and calculate a labeled replacement.
Designing a useful test for Circle of Confusion
For Circle of Confusion, choose a subject with measurable edges or planes and keep the camera position fixed unless distance is the quantity under test. Record Sensor width before the first frame and confirm Visual acuity from the same lens state. For Circle of Confusion, note the marked focal length and focus state beside Sensor width; internal focusing or zoom position can change the effective geometry.
After the published Circle of Confusion baseline, create a second case that changes only Visual acuity by a measurable amount. A doubled distance, one-stop aperture change, known crop, measured rail movement, or fixed pixel dimension gives circle of confusion an independent directional expectation.
For Circle of Confusion, inspect both center and edge when using circle of confusion; projection, coverage, vignetting, distortion, diffraction, or field curvature may vary across the image. A center-frame Circle of Confusion measurement may agree while the edge departs because of behavior outside this simplified equation.
Using circle of confusion in a shooting decision
For Circle of Confusion, translate the number into one concrete choice: move the camera, change focal length, select an aperture, set a rail position, confirm image-circle coverage, or decide whether the subject will occupy enough pixels. Keep that action beside the saved circle of confusion so another person knows why the calculation was made.
If circle of confusion falls between marked Circle of Confusion controls, retain the unrounded value and document the setting actually used. Test the neighboring available Visual acuity value instead of claiming more adjustment precision than the lens, rail, focus scale, tilt mechanism, or camera position provides.
For Circle of Confusion, after the photograph, compare the recorded frame with the expected circle of confusion. Save deviations that repeat across tests; they can reveal focus breathing, a nominal sensor dimension, an entrance-pupil offset, a lens-specific projection, or another stable correction worth applying in future Circle of Confusion work.
Questions about circle of confusion calculator
What does Circle of Confusion Calculator calculate?
It calculates circle of confusion from Sensor width, Print width, Viewing distance, Visual acuity. Derive acceptable sensor blur from print size, viewing distance, and enlargement.
Which measurement should I verify first?
Confirm Sensor width, its unit, and its reference plane before using the other entries.
What is the main limitation?
Visual acuity, contrast, sharpening, printer behavior, and viewing distance all affect acceptable blur.
How can I check circle of confusion?
Run the defaults, change only Visual acuity, predict the direction, and restore the starting value.