Photoshops lens blur is different from the usual blur filters. Unlike applying a mask to a gausian blurred layer, it doesnt fade the blurred image into the sharp image.
Instead it actually locally adapts the blur radius if a depth map is applied.
There are several options to set the focus point, and some options to tweak specular highlights, add noise (to simulate ISO noise and such) and adjust the 'iris' settings.
The iris settings can affect the way the blur looks. Generally, the more sides the shape has (= the more blades the iris has) the more smooth the blur intensity will be, less sides will generate some different more edgy blur patterns. It's hard to explain the visual result exactly, but its sometimes a very clear difference. This setting is for simulation of a camera's iris. The maximum blurryness is controlled by the iris radius, and will therefore probably be one of your most important controls.

• When in photoshop, open the depth mask, and the render you wish to apply DOF to.

• Select the entire depth mask, and press ctrl+c (copy).

• Now go to the render, and press the little grey square with a dot in it, on the bottom of the layer panel. This assigns a mask channel to the image.

• Go to the channels tab in the layer panel.

• Click the mask channel. make sure the eye icon is on, this means you will be actively working in the mask channel.

• Press ctrl+v (paste) to paste your depth mask into the mask channel. If render sizes of mask and render are equal it should automatically be aligned correctly.

• Go back to your layers tab, and right click on the mask. Disable the mask so it wont make the render transparent.

• Go to: Filter / blur / lens blur

• In the lens blur menu that will now load, select "Layer Mask" in the "source" dropdown box from the Depth Map settings.

• Your depthmap will now control the blurring. Set the vocal distance slider to control the point of focus. Anything closer to, or furder from this point, will blur. The blur radius increases as you get further from the focus point, and maxes out at the amount set by the iris radius.

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