What’s working:

So I have a basic tile map that is 25 tiles across and 15 tiles high. The tiles themselves are 16x16 pixels making the full size of the tilemap 400x240 in pixels. On a display that is always assumed to be 1920x1080, that 400x240 tilemap is scaled up by 4.8 x 4.5 in order to fill the screen (with some negligible distortion).

Problem:

The above works for a 2D projection, however, I want this arrangement to exist within a 3D projection, such that other tilemaps can be drawn at different levels of depth, in front of the main tile map and/or behind it.

Most importantly, I’d like to be able to flexibly experiment with the depth of field, (and possibly the positions of the near and far planes), while still having the primary 400x240px tile map set in a fixed position and not have it distort as depth of field is being changed. Also, being able to “zoom in” to different scales than 4.8 x 4.5 would be a good feature, if I wanted to add focusing effects on certain parts of the action on the tile map.

As an example, I found this picture online and, as an analogy, you might imagine the z plane is in the position where the dimensions are always fixed at 1920x1080 regardless of the change in depth of field, near and far planes:

{"name":"613166","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/6\/d\/6df9ab1a817e10d51c3d8f2a49935e4f.jpg","w":405,"h":203,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/6\/d\/6df9ab1a817e10d51c3d8f2a49935e4f"}

Hmm, not sure if I understand correctly.
I think I hacked together something related to have animation effects (rotation around X / Y axes) in lgui tests / demo:

https://github.com/frank256/lgui/blob/develop/src/tests/lguitest.cpp#L254

Note sure if I did it the "correct" way, but basically it puts the usual 2D screen on a plane on z=0 within a frustum so that you can draw as you would in 2D, however, different Z coordinates lead to some (slight) perspective effects.

It's been a while, would need to wrap my head around it again.

Ok, yea this actually works quite seamlessly!

Yea this is basically exactly what I'm looking for. I thought I'd have to figure it out myself but your code worked for me out of the box and the comments were very nice.

I only had to tweak the w / h variables to 1920x1080, and then I only needed to change the Z_DIST_FACTOR to expand/contract the focal length depth effect. I did have to add the depth rendering flags to get the depth buffer:

      // before display creation:
      al_set_new_display_option(ALLEGRO_DEPTH_SIZE, 32, ALLEGRO_SUGGEST);

      // before renders:
      al_set_render_state(ALLEGRO_DEPTH_TEST, true);
      al_clear_depth_buffer(1000);

Not only did it work well, it also worked seamlessly with the different camera control strategies that I had in place. So zooming in/out, changing the camera position to lock to different "rooms", etc, in the game integrated perfectly.

Thank you