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Andy,
Depending on spacing (for Barlows) the magnification will be as advertised ie x2, x2.5, x3 etc.
This means your f7.5 scope would end up f15 (1200mm fl), f18.75 (1500mm fl), f22.5 (1800mm fl) - much more than just a "stop" (That's one reason I went for the TMB x1.8 Barlow on my f6 system)
Again most Barlows are 1.25" or 2" fitment and may or may not have a T thread for camera connection.
My Powermates are 1.25" and 2" fitment but do have a T thread adaptor.
If the Barlow is (or can be) positioned closer to the camera than design distance, the magnification will be reduced. NB This doesn't apply to Powermates which have the same magnification at any/all distances.
Hope that helps
Last edited by Merlin66; 13-09-2017 at 02:18 PM.
Reason: detail added
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