Camera Lens Blur Calculator
Calculate blur circle diameter for any camera lens and sensor. Enter focal length, aperture, and distances for blur disk size and sensor percentage results.
About This Calculator
The Camera Lens Blur Calculator helps photographers, videographers, and optics enthusiasts quantify the amount of background blur their camera setup produces. Unlike depth of field calculators that tell you what is sharp, this tool measures the actual blur circle diameter — the size of the disk that a distant out-of-focus point creates on your camera sensor. Understanding blur circle size helps you choose the right lens, aperture, and shooting distance for portraits, wildlife, sports, and any situation where background separation matters.
The calculator uses the fundamental thin-lens formula for blur circle diameter: blur = f² × (db − ds) / (N × (ds − f) × db), where f is the focal length in mm, N is the aperture f-number, ds is the subject distance in mm, and db is the background distance in mm. This exact formula accounts for the subject distance being comparable to the focal length, making it accurate for both standard photography and close-up work. For background at infinity (db → ∞), the formula simplifies to blur = f² / (N × (ds − f)). The calculator also expresses the blur as a percentage of both sensor width and height for easy comparison across different sensor formats.
Key factors affecting background blur: focal length has a squared relationship (doubling focal length quadruples blur), aperture has a linear inverse relationship (halving the f-number doubles blur), subject distance has an inverse relationship (moving closer increases blur), and background distance matters most when it is within 10 times the subject distance — beyond that, additional background distance adds little blur. Sensor size affects the percentage representation: a 1 mm blur circle is 2.8% of a full-frame sensor width but 4.2% of an APS-C sensor width, making the blur appear more pronounced on smaller sensors when using the same absolute focal length.
Tips for Maximizing Background Blur
To maximize background blur: use the longest focal length your composition allows, open your aperture to its widest setting (lowest f-number), move your subject as far from the background as possible, and get as close to your subject as your framing allows. For portraits, an 85mm f/1.4 on full frame focused at 2 meters with the background 10+ meters away produces a blur circle of about 2.6 mm (7.2% of sensor width) — ideal for creamy, professional-looking bokeh. For landscape where you want everything sharp, use a wide-angle lens at f/8 or f/11 and focus at the hyperfocal distance.
Frequently Asked Questions
What is camera lens blur and how is it calculated?
Camera lens blur, also known as bokeh, is the aesthetic quality and size of the out-of-focus areas in a photograph. The blur circle diameter on the sensor is calculated using the formula: blur = f² × (db − ds) / (N × (ds − f) × db), where f is focal length, N is aperture f-number, ds is subject distance, and db is background distance. A larger blur circle means smoother, more pronounced background blur.
How does aperture affect background blur?
Aperture has the most significant impact on background blur. A wider aperture (smaller f-number like f/1.4) creates a larger physical aperture opening, resulting in a larger blur circle diameter and smoother bokeh. A narrower aperture (larger f-number like f/11) reduces blur, keeping more of the background in focus. Doubling the f-number roughly halves the blur circle diameter.
What is the blur circle diameter and why does it matter?
The blur circle diameter is the size of the disk that a distant point of light forms on the camera sensor when it is out of focus. It is measured in millimeters on the sensor. A blur circle of 0.03 mm or less appears sharp to the human eye at normal viewing distances. Values above 0.1 mm produce visible blur, and values above 1 mm create strong, creamy bokeh. Expressed as a percentage of sensor width, values above 1% produce noticeable background blur while values above 5% create extreme bokeh.
Does sensor size affect background blur?
Yes, sensor size affects background blur in two ways. First, a larger sensor (full-frame, medium format) produces a larger blur circle for the same lens and framing because the blur circle is a fixed size on the sensor but represents a smaller percentage of a larger sensor. Second, to achieve the same field of view, larger sensors require longer focal lengths, which further increases blur. This is why full-frame and medium format cameras are preferred for portraits with strong background separation.
How do focal length and subject distance affect blur?
Focal length has a squared relationship with blur — doubling the focal length quadruples the blur circle diameter at the same aperture and distances. Subject distance has an inverse relationship — moving closer to the subject increases blur significantly, while moving farther reduces it. Background distance relative to the subject also matters: once the background is more than about 10 times farther than the subject, further increasing background distance adds negligible additional blur.
What lens gives the most background blur?
Lenses with long focal lengths and wide apertures produce the most background blur. Popular choices for maximum bokeh include 85mm f/1.4, 135mm f/1.8, 200mm f/2, and 50mm f/1.2 lenses on full-frame cameras. For example, an 85mm f/1.4 lens focused at 3 meters with the background at infinity produces a blur circle of approximately 1.72 mm on a full-frame sensor, which is about 4.8% of the sensor width — enough for very smooth, pleasing bokeh.
What is the difference between blur circle diameter and depth of field?
Blur circle diameter measures the size of the out-of-focus disk for background points and directly quantifies how much background blur you get. Depth of field measures the distance range over which objects appear acceptably sharp. They are related but different concepts: a shallow depth of field means the background becomes blurry sooner, but the blur circle diameter tells you how blurry it actually is. This calculator focuses on quantifying the blur itself rather than the zone of sharpness.
Can I use this calculator for macro photography?
This calculator is designed for standard photography distances where the subject distance is significantly larger than the focal length. For macro photography where the subject distance is close to the focal length, the exact thin-lens formula accounts for the (ds − f) term in the denominator. While the calculator uses the correct formula, macro photography introduces additional considerations like effective aperture change due to lens extension and pupil magnification that are beyond the scope of this tool.