Exposure and power
A stop is a factor of two in light. Aperture changes use 2 × log₂(new f-number / old f-number). Under a consistent complete-pulse setup, doubling power adds one stop. Displayed full-stop aperture labels such as f/5.6 and f/11 are treated as the conventional rounded labels of an exact stop sequence.
The calibrated power model is P₂ = P₁ × (N₂/N₁)² × (d₂/d₁)² × (ISO₁/ISO₂) × 2^(additional modifier loss). It needs a real reference reading. Watt-seconds, model name, and distance alone do not establish the light reaching your subject.
A meter reading has a notation
The Sekonic’s f/8 plus 3 tenths means 0.3 stop above f/8. The effective aperture is 8 × 2^(0.3/2), approximately f/8.88. It does not mean f/8.3. Enter the base aperture and tenths separately.
Several lights add in linear light
At the same measuring point, ISO, receptor configuration, and orientation, contributions combine as Ntotal = √(N₁² + N₂² + …). Two equal independent readings add one stop. The ratio designer allocates contributions toward a total at one point; it does not infer a portrait’s facial contrast or shadow appearance.
The spill mapper uses isolated meter readings at three regions. It squares those readings to form a contribution matrix, then solves for bounded, nonnegative power multipliers. If the targets cannot be met, it reports the residual mismatch. Ambient must be negligible; geometry, modifiers, and light response must remain consistent. A mathematical result still needs a test.
Subject and background
For an ambient-lit background, shutter can control its brightness while manual flash controls a flash-dominated subject, provided the camera captures the same flash energy. The tools account for ambient already on the subject. If ambient alone exceeds the requested result, reducing flash cannot solve it.
Independent power controls only give independent regions when cross-spill is negligible. Shared-light targets may require moving or feathering lights, flags, grids, or changing subject–background separation.
Distance, grids, and source size
The inverse-square model predicts illumination proportional to 1/d². Large nearby modifiers, reflected light, feathering, and nonuniform beams can depart from it. Distances are measured from the effective source to the subject. Always use consistent units.
The visual placement planner uses d_far = d_near × 2^(drop/2). With fixed separation S and a positive target drop, the near distance is S / (2^(drop/2) − 1). A group uses this same relationship with its depth and maximum permitted stop difference. Exactly zero variation across a positive depth has no finite solution.
The measured-falloff tool fits E ∝ 1/d^n from two isolated readings, with n = measured stop drop / log₂(far distance / near distance). It normally restricts predictions to the tested span. Its independent third-point check uses a 0.2-stop comparison tolerance; neither the fit nor that check proves the curve at other positions.
Nominal beam coverage uses width = 2d × tan(angle/2); apparent source size uses 2 × atan(size/(2d)). These are geometric estimates, not guarantees of uniform exposure or softness. A grid has no universal stop loss. Compare actual before/after readings without changing other variables.
Sony α9 III timing: estimate, then verify
The camera can advance flash firing from 0 to 1,000 µs. The tool uses Sony’s 20 µs adjustment steps. Flash duration—whether t0.1 or t0.5—does not establish the trigger delay, time to peak, pulse shape, or captured energy.
Godox’s V100 manual gives approximate α9 III starting values of 160 µs with wireless off and 560 µs with wireless on for V100S. Those references are clearly labeled. They are not measured profiles for every X3S or X3ProS configuration. The calculated mode integrates an assumed triangular pulse over the shutter interval and compares all 51 supported offsets. It uses an editable delay (initially 500 µs), peak position (initially 50%, retaining the original’s centering idea), and t0.1 duration. The original user-supplied AD100, AD200 and AD600 duration tables are unverified family approximations, with logarithmic interpolation for fine power steps. AD400 and V100 calculated mode require a custom duration. The original formula is shown for comparison and its delay error is explained. Exposure alternatives use your chosen brightness change, and the ND planner accounts for both flash and ambient attenuation. These assumptions are not factory profiles; verify the result in camera.
At short exposures the camera can capture only part of the pulse. A whole-pulse meter reading may then disagree with the photograph, and changing power can change the waveform. Recheck timing, brightness, and color after changing power, shutter, trigger path, mode, or firmware. The calibration assistant selects the best stable, unclipped sample you entered; it cannot prove an untested offset is worse.
Read the equipment references
- Sony α9 III Help Guide: Flash Timing Setting ↗
- Sony: adjusting flash timing in practice ↗
- Godox V100 manual: global shutter synchronization ↗
- Sekonic L-858D specifications and support ↗
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