PhotoCalcs
Background Blur Calculator — Portrait Background Blur
Calculate the amount of background blur for portraits based on lens settings and distances.
How We Calculate This
Background Blur Disc
d = (f × D × |b − s|) / (b × (s − f))
Where D = f/N (entrance pupil diameter), f = focal length, s = subject distance, b = background distance. The result is the diameter of a background point's blur disc projected onto the sensor.
Key Insight
Blur increases with entrance pupil size and subject-to-background distance. As the background moves farther away the blur disc increases, rising toward the limit f × D / (s − f) as background distance approaches infinity.
Frequently Asked Questions
Use the widest aperture, longest focal length, shortest subject distance, and maximum subject-to-background distance. An 85mm f/1.2 with subject at 2m and background at 20m produces extreme blur. Each factor contributes independently.
Yes, significantly. Background objects further from the focal plane are more blurred. A background at 5m (2.5m behind a 2.5m subject) is far less blurred than a background at 20m. Choosing locations with distant backgrounds dramatically improves portrait blur.
The entrance pupil (focal length / f-number) is the physical diameter of the lens opening. A larger entrance pupil creates more blur. An 85mm f/1.4 has a 60.7mm entrance pupil. A 200mm f/2.8 has a 71.4mm entrance pupil — more blur despite being 2 stops slower.
On full frame: 85mm f/1.2-1.4, 105mm f/1.4, 135mm f/1.8, or 200mm f/2.0 produce outstanding background blur. Budget options: 85mm f/1.8 or 50mm f/1.4. The 135mm f/1.8 offers an excellent balance of blur, working distance and compression.
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Last updated: February 2026
All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.