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The Bokeh Calculator quantifies the size of out-of-focus background blur (bokeh) produced by a lens-camera system, given the aperture, focal length, focus distance, and the distance to the background. Bokeh (from the Japanese word 'boke', meaning blur or haze) refers to the aesthetic quality of out-of-focus areas in a photograph, particularly the rendering of background light sources as soft, circular discs (called 'bokeh balls'). The size of these discs — and therefore the intensity of background separation — depends on four factors: wider aperture produces larger bokeh; longer focal length produces larger bokeh; shorter focus distance produces larger bokeh; and more distant background produces larger bokeh. The calculator computes the diameter of the circle of confusion for a background element at a given distance behind the focus plane. This allows photographers to predict, before shooting, whether a given setup will produce the desired background separation and bokeh intensity. Bokeh is a defining characteristic of portrait photography — the soft, creamy background (especially for small-aperture lenses like f/1.2–f/1.8) separates the subject from the background and draws viewer attention to the face. Cinema lenses (especially vintage glass like Zeiss Super Speeds, Cooke S4s, and Leica Summilux primes) are valued partly for their distinctive bokeh rendering. Understanding bokeh physics helps photographers and cinematographers choose the right lens and camera format for a desired look, understand why full-frame cameras produce more bokeh than crop sensors for the same composition, and design studio setups that maximize or minimize background blur.
Bokeh Blur Circle Diameter (mm) = (Aperture × Focal Length × (Background Distance - Focus Distance)) / (Background Distance × (Focus Distance - Focal Length)) Simplified for close focus: d_bokeh ≈ (f × D) / (N × d_focus) Where: f = focal length (mm), D = background distance beyond focus (mm), N = f-number, d_focus = focus distance (mm) Equivalent Full-Frame Aperture = Actual Aperture × Crop Factor (for DOF comparison only, not exposure)
- 1Step 1: Set your lens parameters: focal length (mm) and aperture (f-number).
- 2Step 2: Set your focus distance to the subject in meters.
- 3Step 3: Estimate the distance to the background elements you want blurred.
- 4Step 4: Calculate background blur circle: d_bokeh = (f × D_bg) / (N × d_focus) where D_bg = background distance beyond focus.
- 5Step 5: Compare the blur circle to the pixel pitch: if d_bokeh > 2× pixel pitch, bokeh is clearly visible.
- 6Step 6: To increase bokeh: reduce aperture number (wider aperture), increase focal length, move closer to subject, or move subject further from background.
d_bokeh ≈ (85 × 6000) / (1.4 × 2000) = 510,000 / 2800 ≈ 182mm — this is simplified for distant bg. At f/1.4 and 85mm with 6m background separation, the blur is extreme. Background elements render as large, soft circles.
Same f-stop, same framing (same subject size in frame). 85mm requires more distance but provides more background separation due to telephoto compression. Longer focal lengths provide more bokeh for equivalent framing.
On APS-C (1.5× crop), 50mm gives 75mm equivalent FOV. To get the same framing on full-frame, use 75mm. The 75mm f/1.8 on FF produces more background blur than 50mm f/1.8 on APS-C for identical composition.
At 100mm, 0.5m focus, 1m background separation: f/5.6 produces approximately 0.3mm blur circles (50 pixels at 6μm pitch) — clearly visible bokeh. Wider apertures increase this proportionally.
Portrait photographers choosing lens combinations for maximum subject-background separation.. This application is commonly used by professionals who need precise quantitative analysis to support decision-making, budgeting, and strategic planning in their respective fields
Cinematographers selecting lenses for narrative films to achieve a specific visual style.. Industry practitioners rely on this calculation to benchmark performance, compare alternatives, and ensure compliance with established standards and regulatory requirements
Rental house staff advising clients on lens choices for portrait vs. group work.. Academic researchers and students use this computation to validate theoretical models, complete coursework assignments, and develop deeper understanding of the underlying mathematical principles
Photography instructors explaining depth of field and aperture relationships with visual examples.. Financial analysts and planners incorporate this calculation into their workflow to produce accurate forecasts, evaluate risk scenarios, and present data-driven recommendations to stakeholders
Anamorphic bokeh
{'title': 'Anamorphic bokeh', 'body': 'Anamorphic cinema lenses (1.33×, 1.5×, 2× squeeze ratio) produce characteristic oval/elliptical bokeh discs and elongated horizontal lens flares. The oval bokeh is a deliberate aesthetic hallmark of anamorphic photography, distinguishing it from spherical lens rendering. Many modern content creators use anamorphic adapters or native anamorphic lenses specifically for this bokeh character.'}
Macro bokeh and focus stacking
{'title': 'Macro bokeh and focus stacking', 'body': 'At macro distances (1:1 or closer), depth of field is a fraction of a millimeter. Bokeh circles are enormous relative to the subject scale. Focus stacking is often the only practical way to capture macro subjects with front-to-back sharpness while maintaining the bokeh character of the background.'}
Negative input values may or may not be valid for bokeh calc depending on the domain context.
Some formulas accept negative numbers (e.g., temperatures, rates of change), while others require strictly positive inputs. Users should check whether their specific scenario permits negative values before relying on the output. Professionals working with bokeh calc should be especially attentive to this scenario because it can lead to misleading results if not handled properly. Always verify boundary conditions and cross-check with independent methods when this case arises in practice.
| Focal Length | f/1.4 | f/1.8 | f/2.8 | f/4.0 |
|---|---|---|---|---|
| 35mm | Strong | Moderate | Mild | Subtle |
| 50mm | Strong | Strong | Moderate | Mild |
| 85mm | Extreme | Very Strong | Strong | Moderate |
| 135mm | Extreme | Extreme | Very Strong | Strong |
| 200mm | Maximum | Maximum | Extreme | Very Strong |
What makes 'good' bokeh vs. 'bad' bokeh?
Subjective but widely agreed criteria: Good bokeh features smooth, round blur circles (especially from circular aperture blades), no harsh edges or bright rings ('cat-eye' effects at edges), smooth tonal transitions in backgrounds, and pleasing rendering of highlights. Bad bokeh shows nervous or 'busy' rendering with double-edge outlines (longitudinal chromatic aberration), hard circular edges, non-circular shapes (from fewer aperture blades), and distracting busy backgrounds. Vintage lenses are often valued specifically for distinctive bokeh rendering — the 'character' of the blur.
Why do vintage film lenses often produce better bokeh than modern lenses?
Vintage cinema and still lenses (Zeiss Super Speed, Cooke Panchro, Leica Noctilux) were designed before computer-optimized aberration correction. They retain spherical aberration that modern lenses correct away — spherical aberration contributes to the smooth 'creamy' bokeh rendering that cinematographers value. Modern lenses correct for maximum technical sharpness at wide apertures, sometimes at the expense of bokeh smoothness. Cinematographers deliberately use vintage glass for its 'character.'
Does stopping down always reduce bokeh?
Stopping down always reduces the size of bokeh circles and the degree of background blur. However, the shape of bokeh changes with aperture: wide open, bokeh discs are circular (iris is near-circular); stopped down, aperture blades create polygonal shapes (hexagonal for 6-blade iris, octagonal for 8-blade). Lenses with more aperture blades (9+, or rounded blades) maintain rounder bokeh at intermediate apertures — a key feature in portrait lenses.
How does background distance affect bokeh?
Background blur increases as the background moves further behind the focus plane. This is why portrait photographers ask subjects to step away from walls and backgrounds — even a few meters of separation dramatically increases bokeh. The blur circle diameter is approximately proportional to the background distance beyond the focus plane: doubling the background distance roughly doubles the bokeh circle size for typical portrait distances.
What is 'swirly' bokeh?
Swirly bokeh is a distinctive optical rendering where out-of-focus backgrounds show a spiral or vortex pattern radiating from the center of the frame. It's caused by specific optical designs — particularly older Petzval-design lenses (pioneered in 1840 by Joseph Petzval). The Lomography Petzval lens and some vintage portrait lenses (Helios 44-2 from the USSR) produce this effect deliberately. It's highly sought after for portrait and artistic photography.
Can bokeh be simulated in post-processing?
Portrait mode on smartphones uses AI depth mapping to simulate lens bokeh by computationally blurring backgrounds based on estimated depth. The results are improving rapidly (Apple, Google, Samsung all use advanced neural network models) but still show artifacts at complex edges (hair, transparent objects, glass). True optical bokeh from large-aperture lenses on large sensors remains distinguishable from computational bokeh for critical applications. Lens blur in Photoshop/Lightroom can add lens-realistic blur to non-portrait images with manually created depth maps.
Does the number of aperture blades affect bokeh quality?
Yes. Fewer blades produce more angular bokeh shapes: a 5-blade aperture creates pentagonal bokeh shapes at intermediate apertures. 7-blade apertures produce 14-pointed starburst highlights (when stopped down). 9+ blades (especially rounded blades) maintain near-circular bokeh at most apertures. Professional portrait lenses prioritize rounded aperture blades (9–11 blades) for smooth bokeh throughout the aperture range. Cinema lenses typically use 9–11 blade irises for this reason.
Pro Tip
When shooting portraits for maximum bokeh: use the longest focal length available, open to the widest aperture, focus close to the subject, and move the subject as far as possible from the background. Each of these actions independently increases bokeh — combined, they produce maximum background separation.
Did you know?
The word 'bokeh' entered English photography vocabulary only in the late 1990s, popularized in articles in Photo Techniques magazine. Before that, English-speaking photographers called it simply 'out-of-focus blur.' The Japanese photographic tradition had always used 'boke' (ボケ) to distinguish intentional, aesthetic blur from undesirable blur — a distinction Western photography had not formally made.