Enter the luminance of the scene, exposure time, sensitivity of the camera sensor, and aperture of the lens into the calculator to determine the Exposure Index.

Exposure Index Formula

The following formula is used to calculate the Exposure Index.

EI = (L * T * ISO) / (A^2)


  • EI is the Exposure Index
  • L is the luminance of the scene (cd/m²)
  • T is the exposure time (s)
  • ISO is the sensitivity of the camera sensor (ISO value)
  • A is the aperture of the lens (f-number)

To calculate the Exposure Index, multiply the luminance of the scene by the exposure time and the sensitivity of the camera sensor. Then divide this result by the square of the aperture of the lens.

What is an Exposure Index?

Exposure Index (EI) is a numerical representation used in photography to indicate a combination of aperture size and shutter speed in a camera. It is used to measure the amount of light that reaches the film or image sensor. The higher the EI, the more sensitive the film or sensor is to light, allowing for better images in low-light conditions. EI is part of the ISO system, which is a standard for film speed or sensor sensitivity.

How to Calculate Exposure Index?

The following steps outline how to calculate the Exposure Index (EI).

  1. First, determine the luminance of the scene (L) in cd/m².
  2. Next, determine the exposure time (T) in seconds.
  3. Next, determine the sensitivity of the camera sensor (ISO).
  4. Next, determine the aperture of the lens (A) as the f-number.
  5. Finally, calculate the Exposure Index (EI) using the formula EI = (L * T * ISO) / (A^2).
  6. 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.

Luminance of the scene (L) = 100 cd/m²

Exposure time (T) = 0.5 seconds

Sensitivity of the camera sensor (ISO) = 800

Aperture of the lens (A) = 2.8