Film Photography Exposure Calculator
Adjust film exposure for bellows extension and reciprocity failure to estimate a more practical real-world shutter time.
Last updated: 2026-03-26
Film photography exposure calculator
Enter your values
Adjust a metered exposure for bellows extension and reciprocity so the real shutter time is easier to set.
Recommended exposure
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Enter the metered exposure, focal length, bellows extension, and reciprocity profile to estimate a corrected shutter time.
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Example calculations
Tap an example to prefill the calculator with sample values.
Close-focus B&W portrait
Bellows extension plus mild reciprocity
A longer extension nearly doubles the exposure before the film’s reciprocity behavior adds a little more time on top.
Result: Bellows compensation gets the exposure close to two seconds, and reciprocity nudges it a bit higher.
Macro color setup
Long exposure with a stronger reciprocity penalty
Bellows extension and reciprocity stack quickly on longer metered exposures, which is why slide or color work can jump into much longer real times.
Result: A four-second meter reading can turn into a much longer real exposure once both corrections are applied.
Studio copy work
No reciprocity penalty and no extension lift
When extension stays near the focal length and the film stock behaves well, the meter reading remains a solid guide.
Result: With no extra bellows factor and negligible reciprocity penalty, the metered exposure stands.
How the film exposure correction works
First, the calculator applies bellows compensation using the square of bellows extension divided by focal length. That lifts the metered exposure to account for light loss when the lens is extended for close focus or macro work.
Then it applies a reciprocity profile if the adjusted exposure crosses the profile threshold. The result is a practical shutter estimate that combines both bellows and long-exposure film behavior in one step.
Film photography exposure FAQs
These profiles are intentionally practical rather than film-stock exact, so treat the result as a strong starting point and bracket when precision matters.
What does bellows extension compensation mean?
As bellows extension increases, the lens sits farther from the film plane and less light reaches the film. Exposure has to rise by the square of extension divided by focal length, which is what the bellows factor models.
Why is reciprocity handled as a profile instead of an exact film stock?
Exact reciprocity curves vary by emulsion and can change with age or processing. This calculator keeps things practical by using broad profile families rather than pretending to offer film-manufacturer precision.
Should I still bracket exposures?
Yes. Bellows and reciprocity corrections are best treated as informed starting points. Bracketing is still smart when the shot matters, especially on unusual film stocks or very long exposures.
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