Tutorial to create add lens blur effect in photoshop CS4. Technique used in Fake Tilt Shift Photography.
Original tutorial at http://help.adobe.com/en_US/Photoshop/11.0/WSA75837E3-FE05-4f86-A9DF-3C0DD602CA63a.html
Add lens blur
Adds blur to an image to give the effect of a narrower depth of field so that some objects in the image stay in focus and others areas are blurred.
You can use a simple selection to determine which areas are blurred, or you can provide a separate alpha channel depth map to describe exactly how you want the blur added.
The Lens Blur filter uses the depth map to determine the position of pixels in an image. With a depth map selected, you can also use the crosshair cursor to set the starting point of a given blur.
You can use alpha channels and layer masks to create depth maps; black areas in an alpha channel are treated as though they’re at the front of the photo, and white areas are treated as if they’re far in the distance.
To create a gradual blurring (none at the bottom to maximum at the top), create a new alpha channel and apply a gradient so that the channel is white at the top of the image and black at the bottom.
Then select the Lens Blur filter and choose the alpha channel from the Source pop‑up menu.
To change the direction of the gradient, select the Invert check box.
The way the blur appears depends on the iris shape you choose. Iris shapes are determined by the number of blades they contain.
You can change blades of an iris by curving them (making them more circular) or rotating them.
You can also reduce or magnify the preview by clicking the minus button or the plus button.
Other website that you want to refer on Fake Tilt Photography are as follow: