One Stop of Exposure


Increasing or decreasing the exposure effectively doubles or halves the amount of light reaching the sensor. Photograph/Raj Lalwani

Imagine a room with no light except for a torch that is switched on. If you were to switch on a second torch of equal intensity, the light in the room will effectively double. This represents a one-stop increase in the exposure of the room.

The exposure of an image is calculated based on the ISO, shutterspeed and aperture values. Each of these factors is divided into ‘stops’ to help you control the overall exposure of the image.

The Shutterspeed-Exposure Connection

Doubling the shutterspeed increases the exposure by one stop. Halving it decreases it by one stop. If you decrease the shutterspeed from 1/50sec to 1/25sec (effectively doubling the exposure time), the exposure increases by one stop.

The ISO-Exposure Correlation

When shutterspeed and aperture remain constant, but ISO is doubled, the exposure increases by one stop. Conversely, if the ISO is halved, the exposure decreases by one stop. For example, if the ISO is increased from 100 to 200, the exposure of the image increases by one stop.

The Aperture-Exposure Interaction

With aperture, increasing the exposure by a stop means doubling the area of the aperture. While the formula to calculate this is cumbersome, there is a simple way to remember.

A two-stop increase in the exposure results in the aperture’s f-number doubling. For example: f/1, f/2, f/4, f/8 and so on, show a two-stop difference.

Now, a one-stop increase is going to be slightly less than the average of two adjacent f/stops. For example: f/1.4, f/2.8, f/5.6, f/11 and so on.