The lens blur filter can be set to reference a layer mask to determine how the blur should be applied to the layer. As such, if we can control the mask, we can control the fall-off. We need to assess the image to figure out how depthof-field would naturally play across the scene. it’s all about planes of sharpness. Everything in the same plane as our focus point should be sharp, and we create fall-off by gradually transitioning the mask from black (sharp) to white (soft) – the shades of grey in between will come out as different strengths of blur.