Hi Greg,
This is essentially true. A TC magnifies the central portion of the image frame by the mag factor of the TC. However, some TCs (I think most, but can't really say for sure) actually allow the lens/TC combination to focus a little closer than the Minimum Focusing Distance of the lens alone, so the multiple to the mag ratio can actually be a bit higher. I've found, for example, that my P F 1.7x Auto Focusing Adapter on a 1:1 dedicated macro, gives me an actual mag ratio of @ 1.9 : 1 at MFD instead of the expected 1.7 : 1.
Don't forget that multiplying the FL with a TC also reduces the amount of light to the VF and sensor by the same factor as the TC is rated, so with a 1.4x, you lose a full stop and with a 2x you lose two full stops, so usability is reduced as max aperture gets slower.
Scott
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