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Film Reciprocity Failure Calculator

Calculate film reciprocity failure correction from metered exposure time, film profile, p-factor exponent, threshold, and bracketing range.

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Film reciprocity failure calculator

Estimate corrected long-exposure time for film reciprocity failure using generic p-factor/exponent rules.

Corrected exposure Estimated shutter time after reciprocity failure correction.
1 min 32 sec
92.17 seconds
Correction factor Corrected time divided by metered time. 2× equals one stop more exposure.
3.07×
+1.62 stops
Bracket exposures Suggested lower and upper long-exposure brackets around the corrected time.
1 min 9 sec
to 1 min 55 sec
Profile used The p-factor/exponent and no-correction threshold used in the estimate.
Traditional B&W rule of thumb
p=1.33, no correction ≤ 1s
Reciprocity failure is film-specific. Use the manufacturer datasheet or your own tested curve when accuracy matters; this calculator is a rule-of-thumb estimator.

Score from the build-order list

This calculator is item #5 — Film Reciprocity Failure Calculator from the build-order list, with score:

Vol / Comp / Ratio = 3 / 1 / 3.0

What this estimate does and does not do

This calculator estimates long-exposure reciprocity correction for film. Enter the metered exposure time and choose a generic film profile. It returns a corrected exposure time, correction factor, stop correction, and bracket suggestions.

It does not replace the film manufacturer’s reciprocity table, a tested curve, or notes from your own darkroom workflow. Films of the same speed can have very different reciprocity behaviour. Processing, temperature, colour shift, development compensation, and metering error can all matter.

Formula / method used

For p-factor profiles, the calculator uses:

corrected time = metered time ^ p

Where p is the film/profile exponent. For example, the traditional black-and-white rule of thumb uses:

corrected time = metered time ^ 1.33

The correction factor and stops are:

correction factor = corrected time ÷ metered time
correction stops = log2(correction factor)

Some profiles apply no correction below a threshold. For example, a profile may say no correction below 50 seconds because its p-factor was intended only for very long metered times.

Worked example

For a 30-second metered exposure using the traditional B&W 1.33 rule:

corrected time = 30 ^ 1.33 ≈ 92.2 seconds
correction factor = 92.2 ÷ 30 ≈ 3.07×
stops = log2(3.07) ≈ 1.62 stops

With a ±25% bracket, the calculator suggests roughly:

low bracket ≈ 69 seconds
high bracket ≈ 115 seconds

Assumptions and limitations

  • Exponents are generic planning values.
  • Manufacturer data should override this calculator.
  • Some p-factor formulas only apply above a specified metered time.
  • Long exposures can drain batteries, drift with changing light, and cause colour shifts.
  • Development changes may be needed for some films and contrast conditions.
  • Bracket when the exposure matters.

For another photography-related tool, see the Astrophotography Bortle Exposure Equivalency Calculator.

Frequently asked questions

What is reciprocity failure in film photography?

Reciprocity failure happens when very long or very short exposures no longer follow the simple rule that doubling time equals doubling exposure. Many films need extra time for long exposures.

What formula does this calculator use?

It uses a p-factor rule of thumb: corrected time equals metered time raised to a film-specific exponent, after any no-correction threshold.

Is this as accurate as a film datasheet?

No. Film datasheets, tested curves, and your own exposure tests are better. This calculator is for field estimates and planning.

Should I bracket long film exposures?

Yes, especially when the correction is more than a stop or the corrected time is many minutes. Bracketing helps because film, processing, temperature, and metering all vary.

Does reciprocity correction affect colour?

It can. Colour films may shift colour during long exposures, and correction can require both extra time and colour filtration according to the film datasheet.