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I think the 5x powermate and most barlows have diverging exit rays so entending the light path after the barlow or Powermate increases inage size. I'm sure though that the 2.5x Powermate has converging exit rays so extending the light path actually reduces image size (from 2.5x).
So with the 2.5x Powermate you'll get best image scale with the EP or camera direct into the Powermate. Most other barlows or powermates I think you get more image scale with extension tubes of some sort.
I'm sure some one like DP posted a link or a copy of a Powermate chart that showed all this...
Al.
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