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What a blur disc is
When a background point is out of focus, it does not vanish. It becomes a disc of light on the sensor, called a blur disc or circle of confusion for that point. Big discs make a creamy, melted background. Small discs leave the background recognisable and busy. The size of that disc is the real measure behind the word bokeh, which photographers often use loosely to mean a nice background. Bokeh quality, the shape and smoothness of the discs, is a separate matter of lens design. Blur disc size is pure geometry, and the calculator above computes it from your lens, aperture, and distances.
Enter the focal length, the aperture, the subject distance, and the background distance, using zero for an infinitely distant background. The tool returns the disc diameter in millimetres on the sensor and as a percentage of the frame height. A 50 mm lens at f/1.8, focused at 2 metres with a background at 5 metres, gives a disc near 0.43 mm, which is about 1.8 percent of a 24 mm frame height. Move the background to infinity and the disc grows to about 0.71 mm. Those numbers look small, but on a print they scale up with the image, and a disc near 2 percent of frame height reads as clearly blurred.
The four levers that grow background blur
Four things enlarge the blur disc, and the calculator lets you test each one. A wider aperture grows it directly: f/1.8 gives more than four times the disc of f/8, because the disc scales with the aperture diameter. A longer focal length grows it fast, since the formula includes the focal length squared, which is why portrait lenses near 85 to 135 mm blur backgrounds so well. A closer subject grows it, because the lens works at higher magnification. A farther background grows it too, up to the infinite-distance maximum, which is why stepping your subject away from a wall transforms a portrait more than any lens upgrade.
Notice what does not appear in the list: sensor size. Sensor size changes the field of view and therefore which lens you choose for the same framing, but for the same lens, aperture, and distances, the disc on the sensor is the same. This trips up many comparisons. A cropped-sensor camera with a 50 mm lens gives the same disc as a full-frame camera with the same lens, but a tighter framing, so the background looks different in the final photo even though the disc size matches. Compare discs only when the framing matches too, or the comparison misleads.
Using the number in real portraits
For a portrait, aim for a disc that clearly separates the subject from the background without turning the background into featureless mush, unless mush is the look you want. A disc near 1 to 3 percent of frame height usually reads as pleasantly blurred, with shapes still faintly recognisable. Beyond 5 percent, lights become large soft orbs and most background detail disappears. That can be beautiful for evening portraits with string lights, but it can also erase the sense of place that makes an environmental portrait work. The percentage output helps you judge this before you shoot, instead of discovering it on the back of the camera.
Finally, remember that disc size is not disc beauty. A lens with a rough aperture shape makes discs with hard edges or onion rings, while a well-designed lens makes soft, even discs that melt away. The calculator cannot tell you which lens renders beautifully, but it can tell you which combination of settings gives a large enough disc for beauty to matter. Set the geometry first with this tool, then judge rendering from real sample photos taken with the lens you are considering. Geometry gets you the blur. Lens character decides whether you love it.
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