i had the same issue, the documentation stated a 55mm distanced to the chip from the film plane. i measured it with a set of verniers and found that with a cannon 350d i could leave the ring on, if you remove the ring the top bit still leaves a gap anyway.
Having checked my images i found the edges to be still slightly elongated but an improvement over before (12inch newt) may experiment with the distance but currently not a great concern.
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