Why Blur Circle Size Matters More Than “In or Out of Focus”

Depth of field tells you whether a point is technically sharp or not, but it doesn’t tell you how blurry an out-of-focus point actually looks. Two lenses can have the same depth of field on paper and produce very different-looking backgrounds, because the size of the blur circle (the defocus disc a point of light spreads into) depends on focal length, aperture, and the distance to that specific point — not just where the near and far focus limits fall.

How the Blur Circle Is Calculated

This tool uses the standard defocus formula relating focal length, f-stop, and the distances involved to the diameter of the blur circle projected onto the sensor. Because that circle is tiny compared to your frame, it’s shown both as a physical size in millimeters and as a percentage of your frame width, which is usually a more intuitive read on how “melty” your background will render.

Why This Is an Estimate

Real lenses don’t render a perfectly uniform disc — aperture blade shape, spherical and other aberrations, and coatings all affect the character of bokeh (edges can be soft or hard-outlined, discs can be swirly or neutral). This calculator estimates the geometric size of the blur, not its exact rendered shape or color fringing, which vary by lens design.

This tool is especially useful for:

• Portrait photographers comparing how much background separation a given lens and aperture combo will actually produce.
• Anyone choosing between lenses for a shoot where background blur is a creative priority.
• Videographers planning foreground frame elements (like out-of-focus foliage) at a known distance from camera.