Background Blur (Bokeh) Calculator
Background blur is the size of the out-of-focus disk that distant points of light become in a photograph — the quality most people mean when they say "bokeh". This calculator computes the exact blur disk for your camera and lens, in millimetres on the sensor and as a percentage of frame height.
Your setup
Result
How to use this bokeh calculator
Pick your camera (48 bodies in the database — the sensor dimensions are exact, not "full frame approximations"), set the lens, aperture and distances, and read the blur disk. Three things drive background blur: aperture (wider = more blur), focal length (longer = more blur), and how far behind the subject the background sits. Doubling the background distance roughly doubles the blur until it approaches the infinity value — which is why a compressed background (long lens, subject far from both you and the backdrop) blurs hardest.
Worked example
85 mm f/1.8, focused at 3 m, background 30 m away on full frame: blur disk = 85²·(30−3)/(1.8·3·(30−0.085)) ≈ 1.21 mm — about 5.0% of frame height. That's the classic "creamy separation" portrait look. The same lens on Micro 4/3 (17.3×13 mm sensor) gives the same 1.21 mm disk but 9.3% of frame height — the smaller sensor makes the blur look stronger relative to the frame, which is why M4/3 portraits with fast lenses still blur nicely.
Background blur FAQ
How is background blur calculated?
b = f²·(x−s) / (N·s·(x−f)) with all lengths in millimetres; at infinity it simplifies to f²/(N·s). This is the standard geometric formula for the out-of-focus disk diameter on the sensor.
What does the blur disk size mean for bokeh?
As a percentage of frame height: under ~2% looks nearly sharp, 3–5% is gentle separation, 6%+ is the strong blur portrait shooters chase. The disk's edges (smooth vs. busy) are the lens's optical character — that's the part of bokeh no number can capture.
Does sensor size affect background blur?
Yes — same lens and distances, the blur disk is the same physical size, but it occupies a bigger slice of a smaller sensor's frame. That's why Micro 4/3 can look more blurred at the same settings, and why full frame "isolates" subjects with normal lenses.
What's the difference between blur and depth of field?
DoF is about what's acceptably sharp; blur is about how soft the out-of-focus areas are. You can have a thin DoF with modest background blur (wide-angle close-up) or a deep DoF with strong blur (telephoto at f/5.6). They follow different math.