Enter the exposure time, intensity of light, emulsion speed, and sensitivity of the film into the calculator to determine the reciprocity failure.

Reciprocity Failure Formula

The following formula is used to calculate the reciprocity failure.

RF = (T * I^2) / (E * S)


  • RF is the reciprocity failure (seconds) T is the time of exposure (seconds) I is the intensity of light (lux) E is the emulsion speed (ISO) S is the sensitivity of the film (ISO)

To calculate the reciprocity failure, multiply the time of exposure by the square of the intensity of light. Then divide this result by the product of the emulsion speed and the sensitivity of the film.

What is a Reciprocity Failure?

Reciprocity Failure is a phenomenon in photography where the relationship between shutter speed and aperture does not hold constant under extremely low or high light levels. Normally, if you increase the shutter speed, you can compensate by opening the aperture to maintain the same exposure, and vice versa. However, in conditions of very low or high light, this relationship breaks down, leading to under or overexposed images. This is known as Reciprocity Failure.

How to Calculate Reciprocity Failure?

The following steps outline how to calculate the Reciprocity Failure.

  1. First, determine the time of exposure (T) in seconds.
  2. Next, determine the intensity of light (I) in lux.
  3. Next, determine the emulsion speed (E) in ISO.
  4. Next, determine the sensitivity of the film (S) in ISO.
  5. Next, gather the formula from above = RF = (T * I^2) / (E * S).
  6. Finally, calculate the Reciprocity Failure.
  7. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

time of exposure (T) = 10 seconds

intensity of light (I) = 100 lux

emulsion speed (E) = 200 ISO

sensitivity of the film (S) = 400 ISO