How can I create a gradient blur? - Adobe Support ...
https://community.adobe.com/t5/photoshop/how-can-i-create-a-gradient-blur/m-p/1554916
The fastest and truest solution is to create a new channel, apply the gradient, then use lens blur, and select the alpha channel as a depth map. While Ho's procedure might look ok, it is a also fake gradual blur, as it can be described in this discussion: http://photoshoptechniques.com/forum/showthread.php?s=&threadid=169
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