If you're new to photography or you've just been using an fully automatic camera and upgraded, there's lots of settings on the camera that you may not understand. One of those is ISO.
The letters simply stand for International Standards Organisation, so they don't really concern us.
Lets go back in time a little, to when everyone used film and digital was just a pipe dream. There was a huge variety of film, all recording the image in slightly different ways, that often made a huge difference to how you used them and/or the end result. One of the big differences was how quickly the film reacted to light.
If a film reacted slowly(it's speed) to light you would need longer shutter speeds and if it reacted quickly to light, you could use faster shutter speeds, if you wished. There was a large range of film speeds and it was standardised, several times. Many of the systems were complicated, so in the end it was agreed to use only the one system everywhere.
They chose the American system because it was beautifully simple. What was regarded as the standard film at the time was rated at 100. When a film reacted half as quickly to light, it was rated at half that, 50. If it reacted twice as quickly, it was rated at double 100, in other words 200. It made life easy for everyone.
In the days of film you were stuck with the same ISO speed for the full roll of film. Today, with digital, you can change the speed for each and every image, if you wish. There's a great advantage in that.
So, now that you know what it is, how do you use it to improve your images? The obvious thing to think is that you should set your ISO at a high rating and then you need not worry about shutter speeds and concentrate on composing your image. It's not as simple as that.
Firstly, ISO is part of the exposure triangle, which also includes Aperture and Shutter Speed, which I'll deal with in another blog. For now the important thing to remember is that any change in your ISO rating has to be compensated for by making a change to one or both of the other settings, to achieve the same exposure.
Higher ISO ratings allow you to use faster shutter speeds or higher F numbers (aperture) but also produces digital noise. This is where stray colours appear in your image, usually bright pixels in dark areas and dark pixels in bright areas. There will also be a grainy appearance to your image.
When this problem occurs will depend on how new your camera is and the quality range of it. If you look at the photo of the band above, it was taken with a camera I bought in 2004, aimed at the general amateur market. I used ISO 800 for the shot, it's highest and as you can see, it's very grainy, has lots of digital noise and is a poor quality image, as a result.
The second image, of the singer, was taken only a few months later but with a professional camera. I used ISO 6400 for that shot and the quality of the image is almost perfect, without any editing required. The level of light was roughly the same.
Newer cameras give better quality at high ISO ratings than older ones. More expensive cameras give better quality than cheaper ones. You'll have to test your own camera to see what it's limits are.
Another problem with high ISO ratings is that when there is a lot of light, your camera may not have fast enough a shutter speed to get the correct exposure. You will end up with over exposed images, having faded colours and little detail. If it's greatly over exposed, you may not be able to recover the lost detail, regardless of what programme you use.
When taking a portrait, too high an ISO rating may require to use a high F number. This will prevent you from isolating your subject from the background. Good portraits usually require a narrow depth of field and high F numbers give a wide depth of field. I will go into this in more detail in a future blog.
Even though this shot was taken in bright sunlight, I used an ISO rating of 640, 2.66 times faster that the default setting. The reason was that I was using a long lens and pointing it upwards, at a steep angle. The risk of camera shake was high. The statue itself was in shadow and I only got a shutter speed of 1/100 second. With an ISO rating of 200, the default in the camera used, I would have a shutter speed of 1/30. It would have been impossible to get a steady image, in those circumstances.
Finally, lower ISO ratings give more saturated, vibrant colours. In the days of film, plant photographers used stock rated at ISO 50, sometimes lower, to capture rich colour. Using higher ratings gives less saturated and less vibrant images. Again, like digital noise, more modern cameras, especially higher end ones, have this problem more or less solved. It is, however, worth experimenting with you camera to see where the difference begins to show.
The main point to remember is to use the lowest ISO rating at which you can achieve a correct exposure. It is also good to develop the habit of return your camera to the default setting when you have finished taking your image or images. This will prevent accidentally over exposing an image later on.