Film Reciprocity Failure Calculator
Fill in the values and the answer updates as you type.
How the Film Reciprocity Failure Calculator works
Film loses sensitivity in long exposures. The common Schwarzschild correction is Tc = Tm^p, applied for metered times over one second. The exponent depends on the film: Ilford quotes roughly 1.26 for FP4 Plus and 1.31 for HP5 Plus, but always check the datasheet for your stock. Colour film may also need filtration to cancel colour shifts. Example: A 10 second metered exposure with p = 1.31 becomes 20.4 s, which is 1 stop more.
Inputs and results
The Film Reciprocity Failure Calculator takes 2 inputs and gives 3 results.
What you enter
- Metered exposure (s) — starts at 10.
- Reciprocity exponent — starts at 1.31.
What you get
- Corrected exposure time
- Corrected exposure (seconds)
- Extra exposure (stops)
Example: Film Reciprocity Failure Calculator with the starting values
With metered exposure 10 s and reciprocity exponent 1.31, the calculator returns:
| Corrected exposure time | 20.4 s |
|---|---|
| Corrected exposure (seconds) | 20.4 |
| Extra exposure (stops) | 1 |
Change any value above and the results update straight away.
Film Reciprocity Failure Calculator FAQ
How do I use the Film Reciprocity Failure Calculator?
Enter metered exposure and reciprocity exponent in the fields, then read corrected exposure time, corrected exposure (seconds) and extra exposure (stops) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Film Reciprocity Failure Calculator calculate?
Corrected exposure time for long film exposures. It returns corrected exposure time, corrected exposure (seconds) and extra exposure (stops) from the 2 values you provide.
What values does the Film Reciprocity Failure Calculator start with?
It opens with metered exposure 10 s and reciprocity exponent 1.31 so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Film Reciprocity Failure Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Film Reciprocity Failure Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.