Enter the luminance, ISO value, and aperture value into the calculator to determine the exposure time.

Exposure Time Formula

The following formula is used to calculate the exposure time in photography.

ET = (LN / (ISO * AP^2)) * 100


  • ET is the exposure time (seconds) LN is the luminance (cd/m²) ISO is the sensitivity of the camera sensor (ISO value) AP is the aperture value (f-stop)

To calculate the exposure time, divide the luminance by the product of the ISO value and the square of the aperture value. Multiply the result by 100 to get the exposure time in seconds.

What is an Exposure Time?

Exposure time, also known as shutter speed, is a photography term that refers to the length of time a camera’s shutter is open when taking a photograph. The amount of light that reaches the camera sensor is directly impacted by the exposure time. Short exposure times (like 1/2000th of a second) freeze action, while long exposure times (like 1 second or longer) capture the blur of motion.

How to Calculate Exposure Time?

The following steps outline how to calculate the Exposure Time using the given formula:

  1. First, determine the luminance (LN) in cd/m².
  2. Next, determine the sensitivity of the camera sensor (ISO).
  3. Next, determine the aperture value (AP) in f-stop.
  4. Then, substitute the values of LN, ISO, and AP into the formula: ET = (LN / (ISO * AP^2)) * 100.
  5. Finally, calculate the Exposure Time (ET) in seconds.
  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 (LN) = 50 cd/m²

Sensitivity of the camera sensor (ISO) = 800

Aperture value (AP) = 2.8 f-stop