Reciprocity Failure Calculator
The corrected exposure time for film at long exposures, by the Schwarzschild law, including any filter stops.
Results
Corrected time
6.063
s
Time before the correction
4.000
s
Extra stops this costs
0.600
Times longer
1.5157
The same in minutes
0.101
min
Seconds added
2.063
s
What this tool does
Below about a second, film stops trading light for time fairly: doubling the exposure no longer doubles the density. The Schwarzschild law describes this with a single exponent p, and the corrected time is the metered time raised to that power. **The exponent belongs to the film, not to this page** — it comes from the manufacturer's data sheet and typically sits between 1.1 and 1.5, so it is a field here and not a guess made on your behalf.
Formula
Schwarzschild law: corrected t = metered t^p · extra stops = log₂(corrected t ÷ metered t)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tm | Time the meter says | s |
p | Schwarzschild exponent p, from the film's data sheet | — |
nd | Stops of filter on the lens | — |
TC | Corrected time | s |
TB | Time before the correction | s |
ES | Extra stops this costs | — |
FA | Times longer | — |
MN | The same in minutes | min |
DF | Seconds added | s |
Worked example
- Time the meter says4 s
- Schwarzschild exponent p, from the film's data sheet1.3
- Stops of filter on the lens0
- Corrected time6.063 s
- Time before the correction4.000 s
- Extra stops this costs0.600
- Times longer1.5157
- The same in minutes0.101 min
- Seconds added2.063 s
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.