Petrochemist
Mu-43 All-Pro
No, vignetting is a darkening of corners, a teleconverter spreads the image so should reduce vignetting from the lens.I have one and notice no loss (do you mean vignetting?) on APS frame. Used with as wide as 28mm SR-mount lens.
I was quite specific that it was FOV (Field of View) that is lost. A 50mm M42 adapted to Nikon will give a FOV nearer to a 55mm lens. Not a huge difference but worth knowing about. @barry13 is quite right that the image will also be slightly darker (around half a stop) and can have other aberrations introduced.
Many of these adapters sell for $20-30 so you can hardly expect the optical element to be high quality!
Like all teleconverters they enlarge the image from the lens, making any flaws in that more noticeable too - but with the degree of enlargement here that's probably not very significant.
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