Flash Power Calculator from a Meter Reading
Start with your own readings. Calculate the change, verify a frame, and save what works.
Meter → flash power
Translate a measured reference into a useful power setting.
Calculate manual flash power from a measured reference
A flash model’s watt-second rating does not tell you the exposure at the face through your softbox. This flash power calculator uses the output you actually measured, then accounts for target aperture, ISO, distance and a measured modifier change.
- Fire one light at a known manual power and record its incident meter reading.
- Enter the target reading and any changes in ISO, distance or modifier loss.
- Set the suggested supported power and verify it with the meter and camera.
A measured f/5.6 at 1/16 power becomes approximately f/8 at 1/8 power with unchanged ISO, distance and modifier under complete-pulse capture. Moving the light twice as far away would require two additional stops of output.
Can I use a Godox AD100, AD200, AD400 or AD600?
Yes. The calculation uses your measured reference rather than an assumed output for a model. Set the actual power limits and increments supported by your flash.
How do I enter Sekonic f/8 plus 3 tenths?
Choose f/8 as the aperture and 3 in the tenths field. It means 0.3 stop above f/8; it is not an f/8.3 reading.
By Dwayne of DNA Premium Portraits, Sacramento-area portrait photographer. See formulas, assumptions and equipment sources.
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