As a followup to his initial test of the new 70-200mm f/2.8 GM G Master lens, Patrick Murphy Racey takes a look at the new 1.4X and 2X teleconverters designed specifically for the new G Master lens.  

"I got a chance to check out the two brand new Teleconverters from Sony for the FE70-200mm f/2.8 GMaster lens. The 1.4X is tiny. It's the smallest 1.4X I've ever seen and you hardly notice it when it's attached to the α7R II. Best of all, I cannot see any performance loss in either initial AF acquire or in tracking speed and accuracy vs. the lens attached directly to the camera without it. The 2X is also excellent and is still smaller in size than even the Canon 1.4XIII, and also enjoys great speed tracking but it's acquire is just a step behind the 1.4X. Both are killer new additions to Sony Alpha."

In addition to his evaluations of these teleconverters, Murphy-Racey gives a pretty thorough explanation of how teleconverters work in general and the inherent advantages mirrorless cameras have over DSLRs when you're using teleconverters.