In one double slit experiment, one physicist placed a convex lens before the back screen detector. The individual photons passed through the double slit then through the lens. The individual photons then go through the focal point, then a good distance after they reach the focal point they hit the detector screen. However, they don't produce an interference pattern, instead they produce a particle pattern of the two slits. My question is what happens after the individual photons after the focal point that makes them produce a particle distribution instead of the interference pattern? Does the the focal point after the lens somehow destroy the interference pattern?