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It's worth mentioning that the backfocus of the imaging train should be taken into account when looking at the magnification specs, both TV barlows and powermates will change magnification the further you sensor is from the flange.
By way of example, at 55mm backspacing, a 4x Powermate and a 3x barlow both deliver 4.2x.
They have a little graph on their site that explains this, but it's only a factor for imaging.
Markus
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