Light meter conversion
Turn an incident light reading in lux or foot-candles into a stop for your ISO and shutter speed. The calculator also works backwards: how much light you need on the subject to shoot at a target stop.
Use it with a lux meter or a phone app, to read a lighting plot, or to check whether a fixture will reach the stop you want.
How to use it
- Incident reading: in lux or foot-candles.
- ISO / EI and shutter speed: your camera settings.
- Target stop: the stop you want to shoot at.
- Calibration constant (C): incident meters use about 240 to 340; set it to match your meter.
Example
1000 lux at ISO 800 and 1/48 s, with C = 250, gives about T8 (f/8.2). To shoot at T2.8 with the same settings you need about 118 lux.
Good to know
f-number = √(illuminance × ISO × exposure time ÷ C). One foot-candle is 10.764 lux.