is Alpha Blend enabled on the planes' materials, perhaps?

Alpha Blend will prevent the material from rendering in the depth buffer, and the DoF depends on it.
Use Alpha Hashed or Opaque, depending on your direct use-case for image-as-planes.

Right, the image/video must have alpha. I think can be closed, I figured it out myself actually. So if I put it as Alpha Hashed then it works just fine. No idea what is the difference between Alpha Blend and Alpha Hashed though.

Glad that fixed it!

In T80972#1021905, @Bo Maryniuk (isbm) wrote:

No idea what is the difference between Alpha Blend and Alpha Hashed though.

Let me explain the different "Alpha modes" Blender provides:

Alpha Blend uses native GPU "blending" functionality. Triangles that use blending are rendered separately, and then mixed on top of the rest of the render, inside of the GPU. Blending is pretty fast to render on modern hardware and is a good approximation of proper transparency, but has a few problems. For example, it is impossible to know the depth of a "blended" triangle and a regular triangle at the same time with a single depth image. Regular triangles have special priority in this regard.

Alpha Clip uses native GPU "pixel discarding" functionality. Any pixel on a triangle that uses Alpha Clip can be either fully opaque or fully transparent in this mode. This is the least visually-pleasing Alpha mode Blender provides, but unlike Alpha Blend, depth is fully preserved in this mode.

Alpha Hashed uses a modified variant of "Alpha Clip". Long story short, a noise texture is placed onto the mesh and used for Alpha Clip. On every render sample, the noise is randomized.

This emulates the transparency method from a ray-traced rendering engine like Cycles. A good few samples are needed to average out the noise, but Alpha Hashed is the most accurate transparency method you'll find here.

i should try contributing to blender someday

@Firepal64 very nice explanation, thank you very much! (I haven't expected it, TBH). Yeah, it also explains why keyed-out hair looks horrid with Alpha Clip and the only way to let hair look naturalistic is to use Alha Hashed. Somewhat I was always thinking that I do keying wrong, but apparently that's why. However, then the whole clip sometimes is transparent in few places, so I have to always cheat by placing perimeter-masked/shrinked Alpha Clip right behind it to get much more forgiving results. :-) Thank you very much!

@Firepal64 but since we are already talking about this topic, there is another related little nitpick I found, which is sort of forgiving, but not always. See, in Eevee, if the focus us point Image as a Plane with Alpha set to hashed (to get it sharp and everything else blurred), yet while all DoF'ed 3D objects are gets blurred properly, still they their edges are razor sharp as long as they overlap the visible parts of Alpha image, and are properly blurred if they are overlapping over transparency of that Alpha image. However, if I set Plane to Alpha Clip, DoF is blurred correctly (but then the image as a plane itself gets then out of focus). Is there any way to fix that or Eevee is too limited at this point?

You're welcome! =)

In T80972#1023987, @Bo Maryniuk (isbm) wrote:

if I set Plane to Alpha Clip, DoF is blurred correctly (but then the image as a plane itself gets then out of focus).

I can't replicate that. I tried Alpha Hashed and Alpha Clip and I don't see a difference.
What Blender version do you use? Can you share a blend file that shows this, maybe?

(Are you perhaps referring to the background DoF blur bleeding onto the foreground?)

Also, I guess this task can be marked as "Invalid", since this is a (seemingly undocumented) known issue of EEVEE DoF.
But I'm not sure Users are allowed to do that x) *shrug*