ISO forms part of the exposure triangle. We'll be looking at the exposure triangle in a later tutorial. Where we put together ISO, Shutter Speed, and Aperture. In this tutorial, we will look at ISO as an individual element.
ISO actually stands for International Standards Organisation. A body that measures all sorts of things. It just so happens that photography has embraced those letters to acknowledge a camera's sensitivity to light.
So what exactly is sensitivity to light in photography? Back in the day when we were shooting film, we would load a film that was bound to a certain sensitivity, and we were stuck with that sensitivity until the roll of film was finished. Luckily, nowadays we can adjust the ISO for each photograph. We can move from bright daylight taking a picture of a still life project to shooting an indoor sport, and ISO doesn't give us the headache that it once did.
Photographic film was manufactured with grains of silver halide within the emulsion. The larger the grains, the more sensitive to light the film became. But the pay off was noise.
We will learn about how this affects a photograph in the exposure triangle tutorial, so for the moment just accept the science.
An easier way to picture this may be - ISO 100 is represented by grains of sand, ISO 400 by grains of gravel, ISO 800 by small stones and ISO 1600 by pebbles. Get the idea? The bigger the grain, the more light catches it.
So why bother with this, why not have one ISO setting and just alter the shutter speed or change the camera's aperture?
The aim in all photographs (unless for artistic reasons) is to use the finest grain available so that the photograph is as noise free (it's never entirely noise free) as we can make it. In given light, if we were stuck to one low ISO and the available light wasn't that great, we would need to slow the shutter right down. On a tripod, this may be achievable, but doubtful if handheld. Have I mentioned that we would be putting all of this together in the tutorial on exposure triangle?
Now you shouldn't worry about this. In my youth, the thought of shooting a photograph in ISO 3200 would have me breaking out in a cold sweat. The photograph would have more noise than a punk rock concert. Nowadays, cameras are so sophisticated, and technology so advanced, that it's not something that we should worry about too much. You will get an acceptable image. In fact, while I recommend that you learn about this, and practice it via the exposure triangle and the manual shooting tutorials, this is the one element that I would suggest it's ok to leave to the camera and leave in auto ISO mode. Only taking control where you feel it necessary.
An example of taking control - You are taking a landscape, and the camera is suggesting ISO 200. You could use ISO 100 by simply introducing a tripod and using a slower shutter speed.
The other thing to watch out for, and this is just for your information, and only relevant when buying a camera or arguing the point in the pub. Not all readings are equal. Your camera could say ISO 400 for a given image, and it shows the same amount of noise that your friend took with their camera at say ISO 200. Could this be because one camera outperforms another or could it simply be one manufacturer mislabeling the ISO settings to make themselves look better? Just some food for thought. Just get to know your camera and remember that the best camera is the one that you have in your hand.