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The Exposure Triangle: Part 1 ISO

There are three crucial components to creating every exposure.

ISO, Aperture and Shutter Speed

When taking a photo either in automatic or in full manual mode, there are three crucial components to creating every exposure. ISO,  Aperture and Shutter Speed. Each component influences the other and reducing one component means increasing another. When using automatic, the camera decides the setting for you; however if you want to take control and have more creativity with your photography, it is a good idea to understand how the exposure triangle works and switch to Manual or Semi Manual. This is Part 1 of a 3 part series of blog posts on the exposure triangle.

Part 1: ISO

ISO (International Standards Organisation) refers to the amount of light that reaches the sensor of the camera. ISO on a digital camera is the same idea as ISO on film cameras. It is the film speed. ISO100 for bright situations, ISO400 for darker situations. The lower the ISO number the less sensitive the sensor is to light. Therefore the higher the number, the more sensitive the sensor.

Putting this a little more simply, when there is lots of light, like being outside during the day, the sensor of our camera doesn’t need to be very sensitive in order to make an exposure. Typically we would use ISO100 or ISO200, as there is plenty of light. When there is less light, like in the evening or when we are inside a room, we need a more sensitive sensor, so we could use a higher ISO number like ISO800 or ISO1600 to avoid camera shake in our images. By changing the ISO it will however affect the image quality of the final photo, however sometimes it is necessary to get the shot.

The ISO numbers double as they go up, or halve as they come down.

ISO100, ISO200, ISO400, ISO800, ISO1600, ISO3200, ISO6400 etc.

Moving up or down one ISO setting is known as moving up or down one stop.


Changing the ISO will affect one of the other two components of the exposure triangle, because we are effectively changing the amount of light entering the camera. If we change our ISO from 100 to 200, we are moving one stop or opening up by one stop and allowing one stop more light into the camera. This means that to keep a good exposure, we will have to either reduce the shutter speed by one stop or the aperture by one stop to compensate for the increase in light from the ISO.

For example, we are taking a hand held photo of a landscape and to make the correct exposure we start with an ISO of 100, a shutter speed of 1/30th second and an aperture of f/11.0. Because we are shooting hand held and using a shutter speed of 1/30th second we could incur camera shake in our images due to our pulse and the slow shutter speed. By increasing the ISO from 100 to 200 we are letting more light in to the camera by one stop, so to avoid any camera shake we could now increase our shutter speed by one stop to compensate and avoid camera shake. Our shutter speed was 1/30th second, but now we can change it to 1/60th of a second. This will reduce the amount of light and bring the photo back to correct exposure. Remember as mentioned above, the higher the ISO number the more sensitive the camera’s sensor and the more interference or noise there is in the final image. However moving from ISO 100 to ISO 200 won’t make too much difference in the quality of the photo.

An alternative to changing the shutter speed would be to change the aperture. If the ISO changed from ISO 100 to 200, leave the shutter speed at 1/30th of a second and open up the aperture by one stop. This means the aperture would go from f/11.0 to f/8.0. By changing the aperture it won’t affect the shutter speed and potential camera shake. Changing the aperture will do something else which is called the changing the depth of field. Depth of field is a very creative tool and is explained in my blog The Exposure Triangle: Part 3 Aperture.

The exposure triangle is a very important concept to understand if we want to take control of our photography. By simply relying on our camera’s automatic mode we are very restricted and often cant’ take the creative and interesting photos that we visualise. Check out my other posts to learn more about the exposure triangle. The Exposure Triangle: Part 2 Aperture and The Exposure Triangle: Part 3 Shutter Speed.


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