When an image gets out of focus, a point source of light (a small shiny object) becomes a circle of light, as shown in the following examples. If you are lucky enough to have a point source in your photo, you can measure its diameter and enter that figure for the Blur Width.
This medicine bottle in the background of a photo has light reflecting off it. This point source became a circle when the image went out of focus. The image on the right zooms into the point source and shows that its diameter is about 18 pixels (the distance between the black dots).
We have placed the black dots approximately in the middle of the blurred edges — between the blue dots.
This image of some trees in the background of a photo has a point source. If you look carefully, it is in the bottom-left of the image.
Sometimes you can have a point-source bonanza. These two photos, one taken out of focus and one taken shortly afterwards in focus, clearly show the circle effect for point sources.
Here is another difficult one. There are three point sources in this blurred picture of a car's licence plate. Their diameter determines the Blur Width setting to use in Focus Magic. See the Tutorials page for more information.