canon - How good are the EF-mount to E-mount adapters?

I currently live in the Canon ecosystem, but really like the look of the Sony a7S. But before I switch I want to know just how well I'll be able use use my old EF-mount lenses on my new a7S.

How well do the mount adapters perform?

Are there any converters that would allow me to use my old EF lenses with 100% of the original functionality (auto-focus, IS, EXIF data, etc)?

I would not normally ask, but this isn't the sort of thing I can try out in the store.

Answer

Yes, there are adapters with full function (the pretty expensive Metabones adapter springs to mind) so you'll have aperture control, but the AF performance won't be anything near what you're used to on EOS mount. To quote from their website for the Mk III EOS->E adapter:

Autofocus is supported, with the following known limitations.

Autofocus speed is very slow and inadequate for most moving subjects. The autofocus speed is unfit for professional use for sure, and it would disappoint most enthusiasts. Only Canon-branded lenses introduced in or after 2006 are officially supported. Autofocus may be disabled for older Canon lenses and most third-party lenses, including most Sigma, Tamron and Tokina lenses and all Contax N lenses modified by Conurus. ...

Along with a whole mess of other caveats. So, practical upshot, you'd probably be better off trying to adapt manual-focus lenses that have an aperture ring on them. Much simpler, easier, and pretty much the same (or more) function. And if they're rangefinder lenses, then the size/weight is closer to ideal.

In the mirrorless world, to me if you're a Canon L shooter, if you're willing to sacrifice sensor size for lens selection, you go µ4/3. If you're happy to sacrifice lens selection (and speeed) for full-frame, you go Sony A7 system. And if you're happy with APS-C and fast expensive glass, then you go Fuji X.

The Sony A7 models are particularly problematic, given how there's a lack of full-frame lenses for it from Sony, they tend to be very expensive, and adapted rangefinder lenses--particularly the Leica M wide angles, have questionable performance, quite possibly due to the sensor stack thickness. In addition, to keep size and weight down, most of the full-frame E lenses are f/4, but have f/2.8 L lens prices. Best to make your peace with that before getting an A7, in my book.