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What the circle of confusion is
A camera lens never focuses a point of light into a perfect point. It focuses it into a tiny disc, and that disc is called the circle of confusion. If the disc is small enough, your eye still sees it as a sharp point, and the photo looks sharp. If the disc grows too large, your eye sees a blurry blob instead, and the photo looks soft. The largest disc that still looks like a point is the circle of confusion limit, and it sits behind every depth-of-field calculation, every hyperfocal distance, and every claim about what is sharp in a photo.
Why does sensor size change the limit? Because a smaller sensor’s image must be enlarged more to make the same size print, and enlargement enlarges the blur discs too. A full-frame sensor, with a diagonal of about 43.3 mm, has a standard circle of confusion near 0.030 mm. An APS-C sensor, with a diagonal near 28.2 mm, has a standard value near 0.019 mm, and a Micro Four Thirds sensor near 0.014 mm. Smaller sensors demand smaller blur limits, because their images get stretched further for the same print.
Print size changes the limit in the same way. A photo printed at 20 by 30 inches enlarges the sensor image far more than the same photo printed at 4 by 6 inches, so the acceptable blur disc on the sensor must be smaller. This calculator starts from the standard value for your sensor, based on an 8 by 10 inch reference print, and adjusts it for your intended print size. A bigger print means a smaller circle of confusion, and therefore a stricter demand on focus and depth of field.
A worked example
Take a full-frame sensor with a diagonal of 43.3 mm. Dividing by 1500, the standard rule, gives a base circle of confusion of about 0.0289 mm, close to the commonly quoted 0.030 mm. At the reference print size of 8 by 10 inches, with a diagonal of 12.8 inches, the adjusted value equals the base value. That is the number most depth-of-field tables and calculators use for full-frame cameras, and it matches the standard viewing assumption of a print viewed from about 25 cm.
Viewing distance and eyesight
The standard assumes a print viewed from about 25 cm, roughly normal reading distance, by a viewer with normal eyesight. Real viewing breaks that assumption in both directions. A large wall print is usually viewed from metres away, not 25 cm, so its effective sharpness demand is lower than the calculator suggests. A photo examined closely on a high-resolution screen, pixel by pixel, faces a far stricter demand than any print standard. Use the calculator value as a reference, then adjust for how your photo will actually be seen.
Eyesight matters too. The standard circle of confusion comes from an old rule about what the human eye can resolve, roughly one minute of arc, and real eyes vary. A viewer with sharp eyesight looking closely will see blur that another viewer misses. For critical work, such as a print for exhibition or a photo that will be cropped heavily, use a circle of confusion half the standard value as a safety margin, and check focus at 100 percent on screen before printing.
How it connects to depth of field
Every depth-of-field number you have ever used depends on this value. A smaller circle of confusion gives a smaller depth of field, because less blur is tolerated, and a larger circle gives a larger depth of field. That is why depth-of-field tables differ between sources: they assume different circles of confusion. When you compare two calculators and get different answers for the same lens and aperture, check the circle of confusion first, because that is usually the hidden difference.
Remember that circle-of-confusion values are conventional reference values, not physical constants. They vary by sensor and print size, and they rest on assumptions about viewing distance and eyesight that your real audience may not match. Treat the calculator result as a planning guide, confirm sharpness at the size you will actually use, and build in a margin for critical work.
Frequently asked questions
Why divide the sensor diagonal by 1500? That rule comes from the standard print assumption: a print viewed at 25 cm, where the eye resolves about 5 lines per millimetre, and the sensor diagonal enlarged to the print diagonal. It is a convention that gives sensible values, not a law of physics.
Does the circle of confusion affect exposure? No. It affects only sharpness calculations. Aperture, shutter speed, and ISO decide exposure, while the circle of confusion decides what counts as sharp at a given aperture.
Should I use a smaller value for screen viewing? If viewers will zoom to 100 percent, yes. Screen pixel-peeping is stricter than print viewing, so halve the standard value or check focus directly at full resolution instead of trusting any table.
Use the calculator above with your sensor and intended print size, carry the adjusted value into your depth-of-field and hyperfocal planning, and check the final result at the size it will be seen. Sharpness is not a property of the file alone: it is a property of the file, the print, and the viewer together.
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