If you were like me, you grew up in the era in which cameras used this strange and unusual stuff called "film". Naturally, I am exaggerating a bit because you can still purchase rolls of film in many places, but even strong advocates of film insist that it is a dying method of photography. It is digital photography that is taking over, and though it does not use the same tools as classic camera photography, there is one thing that has hung around...the ISO.
It is an acronym for the "International Standards Organization" but it means the same thing as the "film speed". For instance, you can pick up a roll of film and see that it might say 400 or 600, and that is a figure that explains its level of sensitivity. The higher the number, the more sensitive the film. This is why people who did a lot of sports photography would tend to use film that was more sensitive because it allowed the camera settings to be suitable to the action of the shot, even if the lighting was not the best.
Today, we still have ISO, but it is a setting on the camera and is the level of sensitivity of the "sensor" inside of the camera. The sensor is the thing that actually receives the view through the lens and makes the image that is sent over into the camera's memory card. This is often a confusing issue when people start talking about the sensor and the ISO because they seem like they might be the same things, but they are not.
Here is what we mean...you go out to buy a digital camera and see that it has a 14MP sensor. What does it mean? It means that the number of horizontal megapixels multiplied by the number of vertical megapixels results in a figure somewhere in the area of 14 million pixels. Pixels are the tiny dots that come together to make an image. You can lean in really close to an image on a screen and you'll be able to see those pixels/dots.
So, if a camera has a bagillion pixels it is going to make better photos...right? Not really. That seems like a lot, but it doesn't mean that the camera can make better images because of the sensor size. Instead, the real quality of any picture made with a DSLR or Digital Single Lens Reflector, or even a standard point and shoot camera, is the technique used for the shot and the camera settings.
This is where the ISO comes into the equation because it is something that people often use in the wrong way to get the results that they want, and then they get a shot that is not so satisfying.
To begin with, as you dial up the ISO to increase the sensitivity of your sensor it is opening up the risks for "noise". This is a graininess that you may not automatically detect with a smaller image, but which is blatant when you enlarge the shot. It is most often a problem with a sensor that is too small for the number of pixels PLUS the use of a higher than necessary ISO.
Why would someone use the higher ISO? Often if you need to shoot a photograph with a point and shoot camera or a DSLR, you can make adjustments to camera settings. Let's say that you are taking pictures in a low light setting and don't want to use the flash. You may feel more comfortable hand holding the camera, and so you use a faster shutter, a larger f/stop, and a high ISO. This lets you get that image, but if you were to do enlargements such as photo prints or canvas prints you would notice that the image was very grainy and unsatisfactory.
How do you make the shot without using the higher ISO? You would need to understand how to dial the proper shutter speed and aperture, and even consider using a tripod to prevent camera shake with a slow shutter speed. Dialing up the ISO is an "easy fix" but it often ruins the actual results.
In modern digital photography, the general rule is to think about the exposure settings suitable to the lighting - and this means that you need to learn how to choose the best ISO first; and to then address the situation for the best shot by changing the shutter and the aperture.