At least my own report of this difficulty was based on observations made with the real-world sights on my heavily modded Nerf "Deploy CS-6" carbine. The effect is also visible in photographs taken from eyeball perspective of the sight picture from a real rifle.
So be it a toy or a deadly weapon, the effect is really the same: When you focus your vision/lens on either the target or the front post of the iron (or plastic) sights , the rear elements blur out until they are all but invisible.
This is true even with one eye closed, which is why the same burring will happen in photography. With both eyes open, one sees the entire weapon blurred out to transparency (and doubled over to the side as well, though I'd rather not have that in any game for all the clutter it'd make)
Artificially replacing the mesh or textures of the rear sight ring with a blurred out version while aiming (as done by RHS on their M1 Garand) is a perfect compromise between the pixel perfect all-range focus of (non post-processed) computer graphics vs. the fuzzy behavior of the quantum blobs we see as light in real life, as far as the purpose of using iron-sights in a game goes...