The Sony FE 2.0x Teleconverter renders the FE 70-200mm f/2.8 GM OSS lens, as a 140-400mm f/5.6 lens. The dedicated design maintains full communication between the lens and E-mount mirrorless cameras, including auto-exposure metering, autofocus, and Optical SteadyShot image stabilization. Additionally, this teleconverter also features a dust- and moisture-resistant construction, matching the lens, to benefit working in harsh environmental conditions.
It does what it's supposed to, without impacting the performance of the 70-200, except the reasonably small reduction in light transmission you'd expect from one of these. Images remain sharp, and auto-focus performance isn't badly impacted. Of course, you still have to take into consideration lighting conditions, but starting with a bright lens helps things all the way around. Although I'm partial to prime (fixed focal length) lenses, it has been wonderful seeing what a truly great 70-400mm zoom can do.