Tutorial - ISO Sensitivity

In Part 3 of the Photography Basics Tutorial Series we will explore what ISO is and how does it affect your photos.

ISO stands for "International Standards Organization", which is the organization that sets the standard for light sensitivity in digital camera sensors.  A digital camera's sensor is what actually records the scene in front of the camera. The shutter speed and aperture affect how much light or how the light hits the sensor, but the sensor actually captures the image. The amount of light needed for the sensor to record something is affected by how sensitive it is. Regardless of the camera, the user can amplify the signals produced by the sensor. This helps in darker areas where there is less light or when we need a faster shutter speed than the ambient light will allow.

Most cameras start their ISO at around 100 or 200. Every time you double the ISO (from 400 to 800) you make the sensor twice as sensitive. This means you need half the amount of light to get the same shot.

So why don't you just have a highly sensitive sensor then? Well, nothing comes without a price unfortunately. When the ISO is increased, the image quality gets degraded due to an increase in "Noise".

There is always noise in an image, as it is due to electrical fluctuations in the circuitry of the sensor. When the ISO or sensitivity of the sensor is low, most of the unwanted fluctuations in electrical signals are ignored. However, when the ISO is high, the electrical fluctuations cannot be ignored in case they are genuine signals. Due to the fact that electrical fluctuations are relatively low signals, they mainly affect the darker areas of the image.

Full Image

Mushroom in a forest

You can notice the grainy look to the image, specifically in the darker areas. This is due to high-ISO noise. Below you can see a larger section and notice the grain even more.

Larger section

Crop of mushroom in the forest


The amount of noise in a picture is controlled by the sensor and the cameras processor. The processor receives the signals from the sensor and then assembles the picture. As it is a factor of the camera itself, some cameras are better than others when it comes to handling noise. However, no camera is immune to noise. As it is such a common factor in photography, most editing programs offer a way to lower noise in a picture. Obviously some programs will be better at this than others but they will always soften the focus. The extent to which they soften the image is a factor of the program and the amount of noise reduction used. The more noise reduction used, the less detail in the image.

Due to the fact that noise can really degrade an image and there is no foolproof way of getting rid of it, you want to keep your ISO as low as possible while still getting the shot. One trick, if you are forced to use a high ISO, try turning the image Black and White when editing. The grain from high ISO looks more natural in black and white and gives it an old-time feel.

In part 4 of the Basic Photography series, we will learn how aperture, ISO and Shutter Speed all work together to give you a properly exposed images.