The ultimate goal of on-screen computer graphics is photorealistic rendering, in which case modelling the camera lens is the only option that makes sense.
If you want to aim for eye-realistic rendering, it belongs to VR mod: there you can use eye tracking to detect focus distance for the eyes.
Overall, I personally don't like the idea of Depth of Field effect in TDM, and I suppose most of people won't.
But yeah, you can probably use g_testPostProcess cvar to add a full-screen effect, using _currentRender and _currentDepth as input texture, and writing custom shader.