According to a Reddit post from a Viltrox representative, the company will soon be launching a front-mounted 1.4x teleconverter, along with a 0.8x wide-angle conversion lens, for mirrorless and DSLR cameras. "With these, you can instantly unlock new focal lengths without changing your camera" says the poster.
Sony Alpha Rumors has a handy list of Sony lenses and what focal lengths can be achieved were you to attach the teleconverter or wide conversion lens. However, take this with a pinch of salt. Any front-mounted lens adaptor will of course be designed for a specific filter thread diameter, so it'll only fit compatible lenses. Furthermore, you probably won't be able to use a front-mounted teleconverter on a lens with a focal length wider than about 85mm, as depending on the physical depth of the converter, it'd likely cause obvious vignetting if fitted to a wide-angle lens. That said, there's little practical reason to use any form of teleconverter on a lens of a short-to-medium focal length, when the aim is to maximize telephoto reach.
It remains to be seen what kind of impact these conversion lenses may have on image quality. I've lab tested bona-fide rear-mounted 1.4x and 2x teleconverters from Canon, Nikon, Fujifilm and Sony for our guide to the best teleconverters, yet even these high-profile offerings still noticeably reduce image sharpness and increase chromatic aberration.
I remain to be convinced that a (presumably) cheap front-mounted teleconverter can provide acceptable image quality and won't detrimentally reduce the performance of the host lens.
We await details of pricing and availability for both Viltrox conversion lenses.
