How the Reciprocity Failure Calculator works
Adds the extra exposure film needs at long shutter speeds.
Where to find it
Applied automatically inside the meter when the loaded film stock has a non-trivial reciprocity exponent and the shutter is past the threshold.
Summary
Reciprocity correction for long exposures. Past about 1 second, film no longer responds linearly to light, so the meter's geometric exposure is too short. This adds the per-stock correction so the actual exposure is long enough.
How it works
At very long exposures, film stops doubling its density when you double the time. Two seconds of light gives less than twice the density of one second; eight seconds gives less than four times. This is reciprocity failure, and it has bitten film photographers since 1899.
How the calculator handles it
The film stock you pick in Equipment carries a reciprocity exponent. When the meter computes a shutter past the threshold (usually 1 second), it transparently extends the time according to the Schwarzschild formula for that stock. The number you see in the readout is the corrected time, not the geometric one.
How much extra time
Modern emulsions like T-Max 400 need very little correction. Older traditional grains like Tri-X need a lot: a 4-second geometric exposure can become 8 or 10 seconds after correction. The exponent is in each film's data sheet and the app bundles values for the major stocks.
Bulb timer pairing
When the corrected time is tens of seconds or minutes, use the Bulb Timer (1.5) to count it accurately. The timer shows the corrected time, not the geometric one, so you can rely on it.
Stocks without a reciprocity profile
If your stock is not in the database, pick the closest match (same speed, same film type) and the correction will be approximately right. For exposures past 30 seconds, bracket; one stop over and one stop under, plus the metered time, gives you a survival window.