Exposure, ISO Settings and Digital Noise

The ISO setting on a camera is a measure of the sensitivity of the image sensor. The more sensitive it is, the more information it can capture at a given light level. However, if it is too sensitive, you can get a very noisy image.

You should have a basic understanding of how aperture and shutter speed work together to control the exposure of a photograph to make sense of the information below. If you are not sure about these terms, you can read up on them at the Cambridge in Colour site.

A Practical Example

Consider the two images shown below. The camera settings for each image is shown below. At the resolution and size used here, typical of a web image, it is very hard to tell the two images apart. If you look at the image data, you can see that the one on the left had an ISO setting of 100, which with the aperture fixed at f/3.0 allowed the camera to get the correct exposure with a shutter speed of 1/25 of a second. On the right, under exactly the same conditions, the ISO setting was 1600. This is 4 times as sensitive. Therefore the camera was able to get a correct exposure at the same aperture with a shutter speed of only 1/500 of a second.

Low ISO setting High ISO setting

ISO: 100
Aperture: f/3.0
Exposure: 1/25


zoom
ISO: 1600
Aperture: f/3.0
Exposure: 1/500

So, increasing the sensitivity of the camera's sensor allows quicker shutter speeds at the same aperture or, conversely, would allow smaller apertures at the same shutter speed.

So, why not just set your ISO as sensitive as possible all the time? Click on the zoom button to see a zoomed section of each image.

This image was chosen because it highlights two important aspects of the digital noise that can arise at high ISO settings. Firstly it has a region of uniform colour and secondly it has a region with detailed edges.

With the low ISO setting you can see that the areas of uniform colour are very smooth and that the edge details are very crisp. In contrast, the high ISO setting gives an image where the areas of uniform colour are actually very speckly and this, in turn, results in a noticable reduction in the definition of the edges.

This is the trade-off: increased sensitivity comes at the expense of increased noise in the image. This is not just a problem for digital photography, but for is a problem for all sensors including microphones and pressure sensors as well as the light sensors in cameras.

This means that, in practice, you will want to keep your ISO setting as low as possible in any set of circumstances. This ensures that you get the lowest noise level in your images.

Links

You can find additional information about exposure, ISO settings and image noise here:

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