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Well if I'm understanding it correctly each ep will have a slightly different focus point when seated in the coma corrector so they will have different amounts of coma correction whether that be over or under. That's why spacers probably wont help you because unlike a camera there is no rigid backfocus shoulder you can measure from. So in order to get each ep set optimally you would get one ep at the right distance that gives good correction then once it's also in focus you can put in any other ep in and once that is in focus it will also have good correction. That's why people parfocalize eps, it means all you have to do is adjust focus, no need to set distance.
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