Key Aperture
f/19.3
Fill Aperture
f/9.7
Key:Fill Ratio
4:1
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The Studio Lighting Power Calculator helps photographers and cinematographers determine the correct flash power, guide number, aperture, and flash-to-subject distance relationship for any studio or on-location strobe setup. The guide number (GN) is the fundamental measure of flash power, formally defined in ISO 1230:2007 as the product of the aperture (f-number) and the subject distance (in meters or feet) at ISO 100 with the flash at full power. Understanding the guide number relationship allows photographers to calculate the correct aperture for a given flash distance, the maximum distance a flash can effectively illuminate, and the flash power required for a specific aperture and distance combination. Studio monolights and pack-and-head systems (Profoto, Broncolor, Elinchrom, Godox) express power in watt-seconds (Ws) or joules (J), which relate to guide number through the flash unit's efficiency and reflector design. The inverse square law governs how flash intensity decreases with distance: doubling the flash-to-subject distance reduces light intensity to one quarter (2 stops less light). This non-linear relationship means that small changes in flash position produce large exposure changes close to the light source. Additionally, modifiers (softboxes, umbrellas, reflectors) affect both the quality (hardness/softness) and quantity of light — a 1m softbox may reduce light by 1–2 stops compared to bare flash output. The calculator also handles flash exposure compensation, high-speed sync power reduction, and lighting ratio calculations for multi-light setups.
Guide Number (GN) = f-number × Distance (meters or feet) [at ISO 100] f-number = GN / Distance Distance = GN / f-number Inverse Square Law: I₂ = I₁ × (d₁/d₂)² Stops Change = 2 × log2(d₁/d₂) [distance change] Power in Ws: GN ≈ √(Ws × Efficiency × 2.5) [approx, varies by flash design] Lighting Ratio = Key Light Power / Fill Light Power
- 1Step 1: Find your flash unit's guide number from the manufacturer's spec sheet (specified at ISO 100 in meters or feet).
- 2Step 2: Choose your desired aperture for depth of field or set your target aperture.
- 3Step 3: Calculate working distance: Distance = GN / f-number. Position the flash accordingly.
- 4Step 4: For multiple lights, calculate each light's contribution separately and set ratios.
- 5Step 5: Adjust flash power in stops: reducing power by half (1 stop) requires either doubling the distance or opening aperture 1 stop.
- 6Step 6: Account for modifiers — a large softbox reduces effective GN by 0.5–2 stops depending on size and design.
Distance = 60 / 8 = 7.5m. Position the bare speedlight 7.5m from subject for f/8 exposure at ISO 100. For a 1m softbox (reducing GN by ~1.5 stops to effective GN 42): 42/8 = 5.25m.
A 600 Ws monolight with 60cm octabox has an effective GN of approximately 38m (modifier reduces output ~1.5 stops from bare). 38/8 = 4.75m bare; with modifier factor 0.5, approximately 2m working distance for f/8.
A 2:1 ratio means the key light provides 2 units of light to the fill's 1 unit. This equals 1 stop of difference. At this ratio, shadow detail is retained — ideal for professional headshots and commercial work.
Doubling distance reduces intensity by 2² = 4× = 2 stops. f/8 loses 2 stops: f/8 → f/5.6 → f/4. To maintain f/8, increase flash power by 4× (2 stops) or return the flash to 1m distance.
Studio portrait photographers setting flash positions for consistent, repeatable lighting setups., representing an important application area for the Studio Lighting Calc in professional and analytical contexts where accurate studio lighting calculations directly support informed decision-making, strategic planning, and performance optimization
Commercial photographers calculating flash power requirements for large product or group shots., representing an important application area for the Studio Lighting Calc in professional and analytical contexts where accurate studio lighting calculations directly support informed decision-making, strategic planning, and performance optimization
Photography assistants setting up multi-light setups from a lighting diagram., representing an important application area for the Studio Lighting Calc in professional and analytical contexts where accurate studio lighting calculations directly support informed decision-making, strategic planning, and performance optimization
Photography instructors teaching flash exposure and lighting ratio concepts., representing an important application area for the Studio Lighting Calc in professional and analytical contexts where accurate studio lighting calculations directly support informed decision-making, strategic planning, and performance optimization
flash', 'body': 'LED continuous lights (Aputure, Nanlite, Arri Skypanel) are measured in lux or lm/W rather than guide numbers. They offer the advantage of WYSIWYG exposure monitoring via live view, video capability, and no recycling time — but are dimmer than equivalent Ws flash units. For video, continuous lighting is standard; for stills with motion-stopping requirements, strobe flash is preferred.'}
Flash color temperature
{'title': 'Flash color temperature', 'body': 'Standard studio strobes produce light at approximately 5500–5600K (daylight balanced). Some flash units (Profoto, Broncolor) have TTL automatic color temperature control for consistency. Gels (CTO warming, CTB cooling, color) are used to match mixed ambient light or create creative color effects. Always color balance or apply custom white balance when mixing flash with ambient light.'}
When using the Studio Lighting Calc for comparative studio lighting analysis
When using the Studio Lighting Calc for comparative studio lighting analysis across scenarios, consistent input measurement methodology is essential. Variations in how studio lighting inputs are measured, estimated, or rounded introduce systematic biases compounding through the calculation. For meaningful studio lighting comparisons, establish standardized measurement protocols, document assumptions, and consider whether result differences reflect genuine variations or measurement artifacts. Cross-validation against independent data sources strengthens confidence in comparative findings.
| Flash Model | Max Power (Ws) | Guide Number (ISO 100, m) | Recycle Time |
|---|---|---|---|
| Canon Speedlite 600EX-RT | ~70 Ws equiv | 60m | 0.1–5.5 s |
| Godox AD200Pro | 200 Ws | 52m (bare bulb) | 0.01–2.1 s |
| Profoto B10 Plus | 500 Ws | 58m | 0.8–1.8 s |
| Elinchrom ELC 500 | 500 Ws | 68m | 0.9–1.5 s |
| Broncolor Siros 800 | 800 Ws | 72m | 0.02–1.5 s |
| Profoto Pro-11 2400 Ws | 2400 Ws | 105m | 0.05–2 s |
| Godox QT600IIIM | 600 Ws | 76m | 0.05–1.5 s |
What does the guide number tell me in practical terms?
The guide number is a direct calculator for flash exposure. If your speedlight has GN 60 (meters, ISO 100) and you're shooting at f/8, you place the flash at 60/8 = 7.5 meters. If you're shooting at f/4, the distance becomes 60/4 = 15 meters. If you know your working distance (e.g., 3 meters) and want to know the resulting aperture: f-number = 60/3 = f/20. Guide numbers assume bare flash with full power at ISO 100 — adjust for modifiers, ISO, and power reduction.
How do I compare watt-seconds (Ws) to guide numbers?
Guide number and watt-seconds measure different things: GN is the practical output measured at the film/sensor plane, while Ws is the electrical energy stored. The conversion varies by flash efficiency, reflector design, and tube characteristics. A rough approximation: GN ≈ √(Ws) × K, where K is approximately 2.5–3.5 for typical studio strobes and 1.5–2 for speedlights. A 500 Ws monolight might have GN ≈ √500 × 3 ≈ GN 67 meters. Always rely on manufacturer GN specifications rather than Ws calculations.
What lighting ratio is best for portraiture?
Common portrait ratios: 1:1 (flat lighting, even shadows — used for beauty and glamour). 2:1 (soft, commercial — one stop difference, good for headshots). 3:1 (standard portrait — 1.5 stops, classic Rembrandt lighting). 4:1 (dramatic — 2 stops, strong shadows). 8:1 or higher (very dramatic, chiaroscuro — for artistic editorial). The ratio is measured between the key light + fill light combined vs. fill light alone on the shadow side.
How does high-speed sync (HSS) affect flash output?
High-speed sync (above the camera's native sync speed, typically 1/200–1/250 s) pulses the flash rapidly to illuminate the sensor as the shutter curtain sweeps across. This reduces effective flash output by approximately 2–4 stops compared to standard sync. At 1/1000 s HSS, you may lose 3 stops of flash output vs. the same power at 1/200 s. Account for this when using HSS for outdoor fill flash in bright sunlight — you'll need maximum flash power at close distances.
What modifier reduces light the most?
Light modifier output reduction (relative to bare flash): Reflective umbrella: -1 to -1.5 stops. Shoot-through umbrella: -1.5 to -2 stops. 60cm softbox: -1 to -2 stops. 120cm softbox: -2 to -3 stops. Beauty dish (direct): -0.5 to -1 stop. Parabolic reflector: -0.5 stop (concentrates rather than diffuses). Grid on softbox: additional -0.5 to -1 stop. Gel on flash: varies by color/density (-0.5 to -4 stops). Larger modifiers generally reduce output more due to greater light spread and surface area.
How do I calculate correct exposure for multiple lights?
Sum flash contributions in terms of their effective intensity at the subject. Each light's contribution = (GN/distance)² (proportional to intensity). Add all contributions: total intensity = Σ(GN_n/d_n)². Total f-number = √(total intensity / 1) — then compare to meter reading or histogram. Practically: use a handheld flash meter (Sekonic L-858D) at the subject position, reading each light individually then combined, for accurate multi-light exposure.
Why does doubling flash power only add one stop?
Because exposure follows a logarithmic scale. Doubling power (Ws) doubles the intensity, which is one stop (2× more light). To gain 2 stops, you need 4× the power. This is the inverse square law applied to flash power: halving the distance gains 2 stops; quadrupling power gains 2 stops — both achieve the same exposure increase. This non-linearity means that moving a flash closer is usually far more efficient than increasing power output.
プロのヒント
Invest in a handheld flash meter (Sekonic L-858D, Gossen Digiflash) rather than relying on trial-and-error. A meter reads incident light at the subject position, giving accurate aperture readings regardless of modifier complexity or multi-light setup. It pays for itself in saved studio time and consistent results.
ご存知でしたか？
The world's most powerful photographic flash system ever built was constructed for scientific applications — Harold Edgerton's ultra-high-speed strobe systems at MIT in the 1940s–60s produced flash durations of 1 microsecond (1/1,000,000 second) at power levels that could freeze the motion of bullets. His 1952 image of a bullet piercing an apple became one of the most reproduced scientific photographs in history.