Light Meter Calculator
Turn a meter reading in cd/m² or lux into a shutter speed, for incident or reflected metering, using the ISO 2720 constants.
Results
Shutter this calls for
0.003200
s
The same, as one over
312.5
Exposure value at ISO 100
14.288
Exposure value at that ISO
14.288
f-number for one second
141.421
The same reading in footcandles
0.000
fc
Calibration constant used
12.50
Is this an incident reading?
No
What this tool does
A light meter does not measure exposure, it measures light — and turning one into the other needs a calibration constant that the standard leaves partly to the maker. Reflected meters use K, usually 12.5 or 14; incident meters use C, usually 250 for a flat disc or 340 for a dome. The constant is a field here rather than a hidden assumption, because two meters reading the same scene can honestly disagree by a third of a stop.
Formula
reflected: N² ÷ t = L S ÷ K · incident: N² ÷ t = E S ÷ C (ISO 2720)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
mo | How the meter reads | — |
lv | What the meter reads, in cd/m² or lux | — |
iso | ISO | — |
ap | f-number | — |
SH | Shutter this calls for | s |
FR | The same, as one over | — |
EV | Exposure value at ISO 100 | — |
EI | Exposure value at that ISO | — |
AP | f-number for one second | — |
FC | The same reading in footcandles | fc |
KC | Calibration constant used | — |
IS | Is this an incident reading? | — |
Worked example
- How the meter reads12.5
- What the meter reads, in cd/m² or lux2500
- ISO100
- f-number8
- Shutter this calls for0.003200 s
- The same, as one over312.5
- Exposure value at ISO 10014.288
- Exposure value at that ISO14.288
- f-number for one second141.421
- The same reading in footcandles0.000 fc
- Calibration constant used12.50
- Is this an incident reading?No
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.