The circle of confusion is the largest blur spot that still looks like a sharp point to the person viewing your photo. Every lens records a point of light that is slightly out of focus as a small disc, and the circle of confusion is the size at which that disc stops passing for a point. It matters because it is the hidden number behind every depth of field figure, every hyperfocal distance chart and every calculator app, and it is not a fact about your lens at all. It is a choice about how closely your pictures will be looked at.
In a picture of a person standing a few meters in front of a distant background, the person reads as sharp and the background does not. Nothing sudden happens between the two. The blur discs grow steadily with distance from the plane of focus, and somewhere along the way they get big enough for your eye to notice. Where that “somewhere” sits is the circle of confusion. This page explains where the usual value of 0.03 mm comes from, how to work out your own for a big print or a close look on screen, and why two calculators can give you different answers for the same lens.
How a point of light becomes a blur disc
Light leaves each point of your subject in every direction. The lens gathers a cone of that light, as wide as the aperture, and bends it into a second cone that narrows to a point behind the lens. If the sensor sits exactly where that point forms, the subject point is recorded as a point. That is what “in focus” means.
Light from a subject farther away comes to its point a little in front of the sensor, then spreads out again, so it lands as a small disc. Light from a nearer subject would come to its point behind the sensor, and it is cut off while still a disc. Only one distance, the plane of focus, gives true points. Everything else is a disc of some size.
A disc small enough is indistinguishable from a point, so the sharp zone in a photo has depth even though focus is exact at only one distance. Depth of field is the range of distances whose discs are no bigger than the circle of confusion, usually written c. Three things follow from this picture:
- Sharpness fades, it does not switch off. The disc grows smoothly with distance from the plane of focus. The near and far limits of depth of field are lines drawn across a gradual slope, and c decides where you draw them.
- A smaller aperture makes every disc smaller. The cone is narrower, so it spreads less on either side of its point. That is why stopping down adds depth of field. The page on aperture covers the light side of the same setting.
- The disc takes the shape of the aperture. A “circle” is a convenient name. With a hexagonal or nine-bladed diaphragm, a defocused point of light is a hexagon or a nonagon, which is what you see in the highlights of bokeh.
Where the 0.03 mm figure comes from
The disc on the sensor is tiny. Nobody looks at it there. What a person sees is that disc after the whole picture has been enlarged into a print or onto a screen, from wherever they stand. So the circle of confusion has to start with the viewer and work back to the sensor.
The traditional standard is an 8 by 10 inch print (about 20 by 25 cm) held at a comfortable reading distance of about 25 cm. At that distance, a common estimate of what normal eyesight can resolve is a dot about 0.2 mm across. Anything smaller reads as a point. Now work back to a full-frame sensor:
- The full-frame sensor is 36 by 24 mm, with a diagonal of 43.3 mm.
- The 8 by 10 inch print has a diagonal of about 325 mm. Comparing diagonals, the print is about 7.5 times bigger than the sensor.
- Every blur disc is enlarged 7.5 times too. A disc 0.2 mm across on the print was 0.2 divided by 7.5, or about 0.027 mm, on the sensor.
Round that up a little and you have the familiar 0.03 mm. Because the reasoning is the same for any sensor, it is usually written as a rule of thumb: c is the sensor diagonal divided by 1,500. For full frame that gives 0.029 mm. The rule bakes in three assumptions at once: a print of about that size, a viewer at about that distance, and normal eyesight. Change any one of them and the right value of c changes. It is a convention, not a law of optics.
Circle of confusion by sensor size
A smaller sensor has to be enlarged more to make the same print, so each of its blur discs is magnified more, and it needs a smaller c. Here are the values the diagonal rule gives, with two other divisors you will meet. Some older references use the diagonal divided by 1,000. Some calculators use the diagonal divided by 1,730, a stricter figure whose origin is unclear.
| Format (typical size) | Diagonal | Diagonal / 1,000 | Diagonal / 1,500 (usual) | Diagonal / 1,730 |
|---|---|---|---|---|
| Medium format digital, 44 x 33 mm | 55.0 mm | 0.055 mm | 0.037 mm | 0.032 mm |
| Full frame, 36 x 24 mm | 43.3 mm | 0.043 mm | 0.029 mm | 0.025 mm |
| APS-C, 1.5x crop, 23.6 x 15.6 mm | 28.3 mm | 0.028 mm | 0.019 mm | 0.016 mm |
| APS-C, 1.6x crop, 22.3 x 14.9 mm | 26.8 mm | 0.027 mm | 0.018 mm | 0.016 mm |
| Micro Four Thirds, 17.3 x 13 mm | 21.6 mm | 0.022 mm | 0.014 mm | 0.013 mm |
| 1 inch type, 13.2 x 8.8 mm | 15.9 mm | 0.016 mm | 0.011 mm | 0.009 mm |
Exact sensor sizes vary a little between cameras, which moves these figures by a few thousandths of a millimeter and changes nothing you would see. The rounded values you meet everywhere, 0.03 mm for full frame, 0.02 mm for APS-C and 0.015 mm for Micro Four Thirds, are these numbers to one significant figure.
Not sure which of these sensors is in your camera, or what its multiplier is? Crop factor explains the number and how it relates to the full-frame diagonal.
Working out your own circle of confusion
The standard is built for one print size at one distance. Your pictures may end up much larger, much smaller, or on a screen. The same reasoning gives a formula you can use for any of them:
c = 0.0008 × viewing distance × sensor diagonal ÷ print diagonal
Work in millimeters throughout. The 0.0008 is the 0.2 mm the eye resolves at 250 mm (0.2 ÷ 250), so it scales with how far away the viewer stands: twice as far, twice the blur tolerated. The other two terms undo the enlargement. Here it is for a full-frame camera (diagonal 43.3 mm):
| Print and viewing distance | Print diagonal | c on full frame | Compared with 0.03 mm |
|---|---|---|---|
| 8 x 10 inch, held at 25 cm | 325 mm | 0.027 mm | About the same |
| A3, 297 x 420 mm, at 40 cm | 514 mm | 0.027 mm | About the same |
| 20 x 30 cm, held at 25 cm | 361 mm | 0.024 mm | A little stricter |
| 60 x 90 cm, viewed from 60 cm | 1,082 mm | 0.019 mm | About one third stricter |
| 60 x 90 cm, viewed from 1.5 m | 1,082 mm | 0.048 mm | More forgiving |
Two lessons hide in that table. First, a big print is only demanding if people walk up to it. Second, there is a neat special case: if viewers stand about as far back as the print’s diagonal, which is how most people look at a picture on a wall, the print size cancels out of the formula and c becomes 0.0008 times the sensor diagonal, or the diagonal divided by 1,250, whatever the size of the print. People naturally step back from big pictures, which is why the standard works as well as it does.
Screens follow the same logic, measured in pixels. A picture shown 1,080 pixels wide, a common size for phone apps, spreads the 36 mm width of a full-frame sensor over 1,080 pixels, so each screen pixel covers about 0.033 mm of the sensor. A blur disc smaller than that cannot even fill one pixel, which is why so many photos look sharp on a phone and soft as a print. A 3,840 pixel wide display showing the whole frame puts about 0.009 mm of sensor in each pixel, and a 100 percent view on the camera’s own pixels is stricter still.
Planning a big print and wondering whether your file has the pixels for it? The print size calculator checks resolution for any print size.
Pixels, diffraction and the 0.03 mm disc
The 0.03 mm standard was set long before digital sensors. Compare it with today’s pixels and it looks generous. A 24 megapixel full-frame sensor is 6,000 pixels across 36 mm, so each pixel is 6.0 thousandths of a millimeter (6.0 µm) wide and a 0.03 mm disc covers 5 pixels. On a 45 megapixel sensor (8,192 pixels across) a pixel is about 4.4 µm and the same disc covers about 7 pixels. Zoom to 100 percent and a 5 to 7 pixel smear is plain to see, even though the calculator called it sharp.
That is the point people miss about megapixels: they do not change depth of field. The blur on the sensor is set by the optics. A higher resolution sensor only lets you enlarge further before you run out of detail, and when you use that extra enlargement you are choosing a smaller c. If you want depth of field that holds up at 100 percent, a common pixel-level choice is about two pixel widths: about 0.012 mm on 24 megapixels and about 0.009 mm on 45.
The dashed circle is the other limit. Even a perfectly focused point is spread by diffraction into a small disc whose diameter is about 2.44 times the wavelength of light times the f-number. For green light (0.00055 mm) that comes to about 0.011 mm at f/8, 0.015 mm at f/11, 0.021 mm at f/16 and 0.030 mm at f/22. At f/22 the diffraction spot alone is as large as the whole full-frame budget, so the frame softens everywhere, including at the plane of focus. Stopping down to gain depth of field only works until the diffraction spot starts to eat the same allowance.
Seeing the whole frame go soft at small apertures? Diffraction in photography shows where it starts for each sensor size and how to balance it against depth of field.
Why depth of field calculators disagree
Put the same lens, aperture and distance into two apps and you can get two different answers. Almost always, the difference is c. Some apps use the diagonal divided by 1,500, some 1,730, some pick a value from the camera model, and some let you choose. The depth of field changes almost in proportion to c, and the hyperfocal distance changes in inverse proportion. Here is a 35mm lens at f/8 focused at 3 m on full frame, computed with the standard thin-lens formulas:
| Circle of confusion | Near limit | Far limit | Total depth | Hyperfocal distance |
|---|---|---|---|---|
| 0.030 mm (diagonal / 1,440, the usual rounding) | 1.90 m | 7.16 m | 5.26 m | 5.14 m |
| 0.025 mm (diagonal / 1,730) | 2.02 m | 5.81 m | 3.79 m | 6.16 m |
| 0.020 mm (large print, close look) | 2.16 m | 4.90 m | 2.73 m | 7.69 m |
| 0.015 mm (half the standard) | 2.32 m | 4.23 m | 1.90 m | 10.24 m |
| 0.010 mm (close to pixel level) | 2.51 m | 3.72 m | 1.21 m | 15.35 m |
The arithmetic behind it: the hyperfocal distance H is the focal length squared divided by (f-number times c), plus the focal length. For 0.03 mm that is 35 × 35 = 1,225, divided by (8 × 0.03 = 0.24), which gives 5,104 mm, plus 35, so 5,139 mm. The near limit is then s × (H − f) ÷ (H + s − 2f) and the far limit s × (H − f) ÷ (H − s), with the focus distance s in millimeters. Halve c and H doubles, and the total depth drops from 5.3 m to 1.9 m. Neither app is wrong. They are answering the question for different viewers.
Try the same numbers yourself in the depth of field calculator, which uses the rounded standard value for each sensor format (0.03 mm for full frame), or the hyperfocal distance calculator for landscape focusing.
Want to see how the near and far limits move as you change the aperture, focal length and focus distance? What is depth of field works through all four controls with photos and a hands-on tool.
Same lens, different sensor: the trap in the numbers
Because a smaller sensor needs a smaller c, the circle of confusion produces one of the most argued-over results in photography. Put the same 50mm lens at f/2.8 on full frame and on a 1.5x crop camera, focus both at 2 m, and the full-frame camera has more depth of field: about 26 cm (with c = 0.03 mm) against about 17.5 cm (c = 0.02 mm). Its larger allowance tolerates more blur before calling anything soft.
That comparison is unfair, though, because the two pictures are framed differently. To frame the same shot on the crop camera you would use a lens of about 33mm, and at 33mm, f/2.8 and 2 m the depth of field grows to about 40 cm. So for the same picture at the same f-number, the smaller sensor gives more depth of field and the larger one gives less. The circle of confusion is half of that story; the shorter focal length is the other half. The full comparison, including equivalent apertures, is on full frame vs crop sensor, and sensor size covers every format.
Background blur, measured in circles of confusion
The same disc that sets the edge of depth of field also sets how soft a background goes. For a lens focused at distance s, a point far away in the background makes a disc of about f² ÷ (N × (s − f)) on the sensor, where f is the focal length and N the f-number. Divide by c and you get a feel for how blurred the background will look:
- 50mm at f/1.8, focused at 2 m: 2,500 ÷ (1.8 × 1,950) = 0.71 mm, about 24 times the 0.03 mm circle. The background melts.
- 50mm at f/8, same distance: 0.16 mm, about 5 times c. Clearly soft, but shapes are still recognizable.
- 85mm at f/1.8, focused at 2 m: 2.1 mm, about 70 times c. Very little of the background survives as anything but color.
- 24mm at f/8, focused at 3 m: 0.024 mm, just under c. The far distance counts as sharp, because 3 m is beyond this lens’s hyperfocal distance of about 2.4 m.
Depth of field and background blur usually move together, but they are not the same question. Depth of field asks where discs cross c; background blur asks how far past c the background goes. The quality of that blur, smooth or busy, is a separate matter covered under bokeh.
Why Group f/64 could shoot at f/64
The circle of confusion also explains a piece of photographic history. Group f/64, the circle around Ansel Adams and Edward Weston, named itself after one of the smallest apertures on their big view cameras. On a full-frame digital camera, f/64 would be hopeless: its diffraction spot is about 0.086 mm, nearly three times the whole 0.03 mm budget.
Weston made contact prints from 8 by 10 inch negatives, laid directly on the paper with no enlargement at all. The negative was the print. Its diagonal is about 325 mm, so the diagonal rule gives a circle of confusion of about 0.22 mm, seven times the full-frame figure, and the 0.086 mm diffraction spot fits comfortably inside it. A bigger format does not escape the physics; it simply needs less enlargement, so it can afford smaller apertures and more depth. Large format photography covers the cameras themselves.
Common mistakes with the circle of confusion
- Treating it as a property of the lens. No lens has a circle of confusion. It is a tolerance you choose for how the picture will be seen. Fix: pick c from your output, using the formula above, not from the lens.
- Confusing it with the image circle. The image circle is the full circle of light a lens projects, tens of millimeters across, which must cover the sensor. The circle of confusion is a blur tolerance a few hundredths of a millimeter across. Fix: keep the two words apart; they have nothing to do with each other.
- Trusting a calculator’s numbers at 100 percent. Most calculators assume a print of about 8 by 10 inches. On a high-resolution sensor the standard disc covers 5 to 8 pixels. Fix: if you will inspect at 100 percent or print large, set c to half the standard or less, or focus a little beyond the hyperfocal distance and check on the screen.
- Thinking more megapixels means less depth of field. The optical blur is identical. Only your viewing changes. Fix: compare cameras at the same output size; then sensor resolution makes no difference to depth of field.
- Stopping down to f/22 to “be safe”. At f/22 the diffraction spot alone is about 0.03 mm on any sensor, as big as the whole full-frame allowance, so the entire frame loses crispness, and smaller sensors, with their smaller c, lose more. Fix: on full frame, stay near f/8 to f/11 and place your focus distance carefully, or use focus stacking when one frame cannot hold the depth.
- Comparing sensors with the same lens. It answers a question nobody photographs. Fix: compare at the same framing, as in the section above.
Try this: find your own circle of confusion
Ten minutes, any camera or phone, and something with fine detail that recedes into the distance: a fence, a row of parked bikes, a bookshelf shot at an angle.
- Stand at one end and focus on an object about 2 m away. On a camera use a wide aperture, f/2.8 to f/4. On a phone use the main lens with portrait mode off, and tap to focus.
- Take the picture. Then note where things are: the focused object at 2 m, and a few marks in front and behind (a post, a bike, a book) whose distances you pace out.
- Look at the picture three ways: the whole frame on your phone, the whole frame on a computer screen, and at 100 percent.
- For each view, mark the nearest and farthest objects that still look sharp to you.
- Put your lens, aperture and the 2 m focus distance into the depth of field calculator and compare its near and far limits with yours.
How to judge it: the phone view should give a sharp zone deeper than the calculator’s, the computer screen something close to it, and the 100 percent view a much thinner zone. If that is what you found, you have just measured three different circles of confusion with your own eyes, and you know which one matches the way you show your work.
Ready to check how well the ideas behind sharp zones have stuck? The hyperfocal distance and depth of field quiz asks about apertures, distances and acceptable sharpness.
Circle of confusion FAQ
What circle of confusion should I use for full frame?
0.03 mm (strictly 0.029 mm) is right for prints up to about A3 viewed normally and for whole-frame viewing on most screens. For large prints you will stand close to, or work you will check at 100 percent, use 0.02 mm or smaller. The formula on this page gives the value for any print and distance.
Is the circle of confusion the same as bokeh?
No. Both start from the same blur discs, but the circle of confusion is a threshold: the largest disc still seen as sharp. Bokeh is about how the much larger discs in the out-of-focus areas look, smooth or busy, round or angular.
Why is it called a “circle of confusion”?
The term comes from older optics, where “confusion” meant the light from a point being mixed, or confused, over an area instead of meeting at one spot. It describes the blur disc itself, and photographers later used it for the largest disc they would accept.
Does a smaller sensor have a smaller circle of confusion?
Yes, in proportion to its diagonal, because its pictures must be enlarged more to reach the same size. About 0.02 mm for APS-C and 0.015 mm for Micro Four Thirds, against 0.03 mm for full frame.
Can I change the circle of confusion on my camera?
Not as a setting, since the camera does not use it. You change it in the calculator or app you plan with, and in effect you change it every time you decide how big to print or how closely to look. The depth of field scale on a lens barrel assumes one fixed value chosen by the lens maker, so treat its marks as a rough guide for prints of ordinary size.
Want to use all this to get a landscape sharp from front to back? What is hyperfocal distance turns the circle of confusion into a focus distance you can set in the field.