In the double slit experiment, when you measure which slit your photon passes through, it will land in the corresponding location on the photosensitive film at the end of the apparatus. If, however, you don't measure which slit it goes through, an interference pattern will appear on the film. If the film is sensitive enough to show two distinct blobs vs an interference pattern, why does it not constitute a measurement of which slit the photon went through? I.e. why is there ever an interference pattern?