Undergrad Double slit particle distribution with convex lens?

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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?

 

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You may have overlooked mfb's answer in your earlier thread (https://www.physicsforums.com/threa...n-in-double-slit-when-there-is-a-lens.828239/) - take another look at it. If it's still not clear, continue the discussion there so we only have one thread going.This thread is closed so we can keep the other one going.