Almost nothing will ruin an image more than blur that results from camera shake. Some systems have image stabilization built into the camera, but many systems choose to put it in the lens. To understand this, you first need to understand the different types of blur effects that can ruin your images.
Different Types of Blur
Camera shake is a common problem that can cause a blurry image. How do you tell if your images are suffering from camera shake? Take a look at the image and remember what the camera was focusing on. For argument's sake, we're going to say that you were using someone's face as your subject. Assuming that the camera focused sharply and accurately on your subject, proceed to determine if there is blur that looks like something was moving around in the frame (also called trails). If you can clearly see all the details, then you've got a sharp and blur-free image. If you can see trails of some sort on the subject you were focusing on, then your images are suffering from camera shake
Out of Focus Image
Your photos are out of focus when the details in your subject are soft. You can tell the image above is not in focus because the intention was to focus on the camera but it is blurry. However, the blur here is a nice, smooth, and creamy out of focus look that is characteristic of faster aperture lenses. This was shot with a Canon 85mm F/1.8.
When an image is in focus, you can clearly tell.
As you can see in this image, all the details of this Lomography Action Snapper are very clear. To make absolutely sure that your camera isn't misfocusing, calibrate your viewfinder's diopter by turning the dial to either + or -. Have your camera autofocus on an image and then look into the diopter until you see a clear image in the viewfinder.The diopter adjusts the viewfinder according to your eyesight.
For more technical users, their lens can also be calibrated in order to ensure their camera is focusing. In that case, the Spyder LensCal is a great item to help.
The following images were shot after six cups of coffee to simulate the unsteadiness that one may encounter.
This image was shot on a Canon 5D Mk II with 24-105mm F/4 L IS at 105mm, 1/50th of a second, F/4 and at ISO 800 using spot metering and focusing on the "FM10" characters. Image stabilization was on, resulting in a much sharper image. As you can see, there is no camera shake in the 100% crop of the image below.
The details are very clear and blur free despite shooting at a slow shutter speed. The rule of thumb is that your shutter speed should be the reciprocal of your focal length. Since this photo was shot at 1/60th of a second and at 105mm, the image stabilization was able to compensate shaky hands.
Like the previously shown image, this image was shot on a Canon 5D Mk II with 24-105mm F/4 L IS at 105mm, 1/50th of a second, F/4 and at ISO 800. This time though, image stabilization was off and camera shake became very apparent.
This image has a lot of blur and upon even closer glancing, you can see some trails. This is camera shake. Had the image stabilization been turned on or shot at above 1/100th of a second at a higher ISO,this image may have been sharper.
Another way to ensure that your images will be sharp is to shoot at the reciprocal of your focal length. Meaning that if you are shooting at 100mm on your 35mm full frame sensor camera, that you should shoot at 1/100th of a second. To be fair, there are professionals that can shoot down to 1/15th of a second with image stabilization enabled and still shoot photos without camera shake. Many of these image stabilized lenses and systems allow for up to three to four stops of stabilization.
Do You Need It?
So how do you tell if you need image stabilization or not? Well, if you use Canon, Nikon, or Panasonic and are seeing blurry photos with trails, then you'll need to perhaps shoot at a higher shutter speed combined with a higher ISO setting and a wider aperture. If that combination doesn't do the trick, then it's time to consider purchasing an image stabilized lens.
Other systems like Olympus, Sony, and Pentax put the image stabilization into their camera bodies. This is done by moving the sensor around inside the camera body to compensate for camera shake. This makes any lens attached to the camera image stabilized.
When you're not using a tripod, what do you do to not get camera shake in your images? Let us know in the comments below.