How do I make the camera's depth of field affect an imported image?

Question

I'm using Eevee in Blender 2.9 and I have imported an image (with alpha information) as a plane. My camera's focus distance is extremely low but the image I imported isn't being affected or blurred by the depth of field. I'm not sure if this is able to be solved, but it seems like a fairly big limitation if there is no way to make an imported image affected by the camera. If it can't be fixed, is there any alternate way to blur the background image? Perhaps with nodes?

I'm also curious if I can make the camera affect an HDRI in the same way. I've included a picture for reference, thanks in advance for any advice. (Note that the mountains in the distance are too sharp).

related: blender.stackexchange.com/questions/9394/… and blender.stackexchange.com/questions/48240/… — lemon, Commented Nov 18, 2020 at 18:04

susu · Accepted Answer · 2020-11-18 17:52:34Z

The image contains no depth information, to blender it is just a plane, so you won't be able to have any selective defocusing based on the distance to the camera. If you want to blur parts of it selectively, you need to create some kind of depth map: a black and white image in which the close areas are dark and the far ones are bright.

Then use that image in the compositor. Feed the original image to a defocus node and the fake "depth map" as the Z information. You can use a color ramp to change the values to determine what areas get blurred.