You can safely ignore every option except for two of them - the Depth Map option that we just talked about a moment ago and the Radius option. This brings up the Lens Blur dialog box, which consists of a large preview area on the left and a bunch of potentially confusing options along the right. Go up to the Filter menu at the top of the screen, choose Blur, and then choose Lens Blur: We're going to create our depth of field effect at this point, and we're going to do it using Photoshop's Lens Blur filter. Now that we're back in the Layers palette, make sure you have 'Layer 1' selected (the currently selected layer is highlighted in blue). Switch back over to your Layers palette to continue. We'll need to tell Photoshop to invert our depth map in the Lens Blur filter's dialog box, and we'll see how to do that in a moment. The black area is where we want to apply the blurring!' You're absolutely correct. If you were paying attention there, you may be thinking 'Wait a minute, isn't that exactly the opposite of what we want? You said black areas remain in focus while white areas are blurred out, but in the alpha channel we just created, it's the white area that we want to keep in focus, not the black area. Areas that fall between pure black and pure white, such as the transition area around the selection edges, will be blurred to a lesser degree depending on how light or dark they are.

Any areas in the depth map that are pure black remain 100% in focus while areas that are pure white are completely blurred out. A depth map is simply a grayscale (black and white) image that Photoshop uses with the Lens Blur filter to decide which parts of the image to blur out and by how much. This is important because in a moment, we're going to be using this black and white alpha channel we've created as a depth map for the Lens Blur filter. The saved selection becomes a new 'Alpha channel' in Photoshop's Channels palette.

Adobe photoshop 7.0 effects tutorials