ISO, along with the other two things we will be covering soon, is one of the fundamental things that needs to be understood in order to really understand how your camera works. Since these three things are so tightly interrelated, some of the things we will talk about won't make as much sense now as they will when we are done covering all three and we get to the recap at the end. So bearing that in mind let's dive in.
When we are talking about a digital camera's ISO, most basically, we are talking about how sensitive your camera's sensor is to light. Since most of you reading this are using digital camera's we are going to concentrate on those right now and we'll leave discussions of film for some other time. The basic question we are answering though is, how much light is it going to take until the sensor records it.
When you look at a camera's specifications it will often tell you the range of ISO's it is capable of. For example, if you happen to own a Cannon SD880 IS, your camera has an available ISO range of: Auto, 80 ,100, 200, 400, 800, and 1600. That Auto setting at the beginning there is why most people don't bother setting this themselves. But even if you can't set this directly with your camera there are still indirect ways to select your ISO, as we will learn in another post that will give better results than Auto.
When you are reading ISO numbers, the lower numbers are less sensitive and the higher numbers are more sensitive. With a low ISO number it takes a longer amount of time for the camera to record the same amount of light. With a higher ISO number, it takes progressively shorter amounts of time to record any amount of light.
So far, this is a little dry so lets take a look at an example to illustrate what I mean.
Take a look at this image I made of a chessboard in our apartment. You can see in the lower right of the picture the ISO setting I used and the shutter speed that was needed to take the picture. The shutter speed tells you how long it took to record that amount of light. Now position your mouse over the picture and it will switch to the second picture I took. The second picture was taken at a higher ISO setting. Look at the information in the lower right of the image and you can see that it took a much shorter amount of time to record the same amount of light.
This is an incredibly useful tool that you can use to reduce blurring in your pictures because it allows you to use much faster shutter speeds when taking pictures. This is why ISO is commonly referred to as ISO Speed, because it directly affects the speed with which you can take pictures.
Watch out though, there is a drawback, and its a big one. Not only is the speed with which you can take pictures directly affected by the ISO setting you use, but so is the final quality of your pictures. The bad part is that in this case the benefit goes the other way. The higher the ISO speed you use, the worse and worse your pictures will look. And if you are using a compact digital camera, instead of a big expensive SLR, (a difference we will get into later if I can remember to come back to it) image quality goes down A LOT and VERY FAST
The images you saw above of the chessboards are the entire thing, and at internet sizes they look ok, but lets look at those same images at the kind of size you might see if you decided to print them. The image you see now is a full size section of the photograph taken at ISO 200,but put your mouse over the picture again to see the same section at ISO 1600. You can see how much worse that photograph looks. If you were to try the same thing with a compact digital camera, or at night, the difference would be ten times worse.
As you can see, different ISO settings have differenct trade offs. Higher ISO setting allow you to take faster pictures with less blur, but lower settings increase your image quality. Which setting to use is an important decision if your camera allows you to make it. My wife's camera, for example, doesn't have a manual ISO setting at all. Many of you out there might have cameras like that. But don't worry, knowing how ISO works and the other topics we will cover soon will still help you have a much happier relationship with your digital camera.
So keep checking back for the next couple posts and this will all start to come together.