Measured Light Falloff & Sekonic Calibration Tool
Start with your own readings. Calculate the change, verify a frame, and save what works.
Measured falloff
Use Sekonic readings to check your modifier’s actual falloff.
Calibrate falloff with your own light and meter
The measured-falloff tool compares isolated incident readings at two distances with the ideal inverse-square model. It fits a local curve for the exact source, modifier, grid and measuring path you tested. A separate third reading checks whether that fit is useful at another point.
- Keep the light, modifier, aim, power, ISO, meter mode and receptor orientation fixed. Move the meter along the same unobstructed beam path and record two distances and aperture-plus-tenths readings.
- Compare the measured drop with inverse-square falloff. Predict only within the tested span unless you deliberately enable an unverified extrapolation.
- Add a third, distinct distance and reading. Investigate a discrepancy greater than the displayed 0.2-stop comparison tolerance, then verify a photograph and save your notes.
If the meter reads f/8 at four feet and f/4 at eight feet, the measured drop is two stops, matching an exponent of 2. The fitted one-stop threshold is about 5.66 feet from that same distance origin.
Is this a universal correction for my softbox?
No. Two readings define one local fit. Changing aim, source-distance origin, grid, power, room reflections or the measuring path can change it. Take new readings when the setup changes.
Why is a predicted distance withheld?
The requested point may lie outside the tested span, or the readings may not show decreasing illumination. By default the tool asks for a new measurement instead of treating an extrapolated distance as established.
Does a third-point check prove the whole curve?
No. It tests agreement at one additional point. The 0.2-stop threshold is a practical comparison setting, not a guarantee of accuracy everywhere or a specification of your meter.
Can I use these readings at very fast Sony a9 III shutter speeds?
The tool predicts whole-pulse meter readings. At very short exposures, the camera may capture a different fraction of the pulse. Verify the captured photograph and timing separately.
Learn the decision behind the setting.
By Dwayne of DNA Premium Portraits, Sacramento-area portrait photographer. See formulas, assumptions and equipment sources.
Want help turning this plan into a photograph?
Bring this lighting goal to a private Sacramento workshop. Three hours, your equipment, one focused learning plan.