A meter measures the light; you choose the picture.
Incident metering measures light arriving at the subject. A useful starting exposure method for a three-dimensional portrait is to place the raised dome at the subject position and face it toward the camera. Keep your own body from blocking the source. For a specific contrast or flat-surface measurement, use the receptor configuration and orientation appropriate to that task; follow the meter manual and document the method.
Set the meter ISO to match the camera, choose the correct flash measuring mode and use the relevant shutter setting when ambient is included. Flash-only, ambient-only and combined readings answer different questions. Mixing them produces convincing-looking numbers with the wrong meaning.
Read aperture tenths correctly.
A display of f/8 plus 0.3 stop means a reading three tenths of a stop above f/8. It does not mean f/8.3. In a mathematical f-number representation, multiply the base f-number by 2 raised to half the fractional stop: 8 × 2^(0.3/2), approximately f/8.88.
You usually do not need that decimal f-number. Enter the base aperture and the tenths separately into the target-aperture calculator. If your target is one stop above f/8, the difference from the measured f/8 + 0.3 is 0.7 stop. Re-meter after the proposed change.
Measure separate lights, then their combination.
- Choose one measurement point and record ISO, receptor configuration and orientation.
- Fire key only, then fill only. Keep the measurement method comparable if you intend to calculate their contributions at that point.
- Adjust the independent sources to the intended relationship. Two stops lower means one quarter of the illumination, under the same measurement conditions.
- Fire both and take a combined reading. Independent f-numbers cannot simply be averaged.
- Inspect the photograph: final cheek-to-cheek contrast depends on how each source reaches each surface, plus reflections and ambient.
Understand what a sophisticated meter cannot guarantee.
The Sekonic L-858D-U supports incident and spot readings, flash/ambient analysis, HSS measurement and flash-duration analysis. Optional radio modules support compatible systems, including Godox. Confirm the installed module and mode before assuming radio triggering is available.
A meter integrating the flash pulse can disagree with a camera capturing only a short section of that pulse. A duration reading does not reveal the camera’s correct timing offset. If your α9 III exposure changes at a fast shutter, verify with repeat camera frames and the timing calibration assistant.
YOUR NEXT 10 MINUTES
Predict. Measure. Refine.
You measure f/5.6 + 0.4 stop and want f/8. How much more light is needed?
Predict approximately +0.6 stop on the conventional full-stop scale. Apply that measured power change and re-meter.
Allow full recycle. Check whether the display reaches the target, then compare a camera frame.
Record a repeatable correction only for the tested camera, meter, mode and workflow—not as a universal flash adjustment.