I've had a similar interference pattern occur when using a filter on my EF300/4 (non-IS), which has an almost flat front element. Back when I was a young man with a full head of dark hair, we would look for interference fringes when checking optical flats and precision gauge blocks under a monochromatic light. Neuro mentioned 'Newton's rings'....that's a phenomenon of similar cause, in the context we used it was an indicator that cemented lens elements were beginning to separate. We'd re-cement them with Canadian Balsam, nowadays replaced for the most part with different adhesives. This thread has conjured up memories of the smell of hot Canadian Balsam and hours of rebuilding Navy binoculars....a pleasant memory. Thanks!