This is the delayed-choice quantum-eraser experiment of Kim diagram described on Wikipedia https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
Why do I need a lens in front of the
D0? Why we should focus photons? The classic Double-slit experiment does not require a lens.
What pattern will we see on
D0 if we remove the lens from the experimental setup? Why don't we see interference fringes?