The expression “we marry out lenses but date our cameras” is common in photographic circles. It’s because photographers keep their lenses for a very long time, often for life, while changing the camera bodies they attach them to every couple of years. The advent of mirrorless cameras has only strengthened and reinforced this statement because DSLR owners no longer have to divorce their lenses if they choose to move to a mirrorless brand of camera. Using a ‘Lens Adapter’ means that they can keep all their favourite lenses while changing the brand of camera they are mounted to. This is a revolution in photography because photographers used to be locked into brand loyalty because it was usually economically unviable to witch brands. The ability to retain your lenses when moving to mirrorless is one of the contributing factors to the enormous success of the Sony A7 series of cameras. The focal range and performance of the lens remains the same as the lens adapter simply positions the lens at the correct distance from the sensor of the mirrorless body – basically filling the gap left where the mirror of the Single lens Reflex Camera would normally sit.
Cheap and technically simple or ‘dumb’ adapters allow the photographer to connect a non-Sony lens to their Sony E-Mount body but the electronic communications between the lens and the camera body are usually lost. The photographer using a dumb adapter has to manually focus and adjust the aperture on the lens (if available). Information about the Lens Aperture that is normally passed to the EXIF data of the file through the electronic contacts on the lens is often missing.
Sony’s smart adapters
When Sony released the A7 and A7R cameras their were just three full-frame or FE lenses in the system. Sony, however, also released their LA-EA3 and LA-EA4 adapters so that owners of the DSLR and DSLT A-Mount cameras could use their existing lenses on the new full-frame E-Mount cameras. The LA-EA3 is a technically simple adapter that does not support Auto Focus but does record the EXIF data and allows the Sony user to adjust the lens aperture using the camera. The LA-EA4 adapter was a lot more sophisticated and has a built-in translucent mirror and a Phase Detect AF system. This provided the A7R camera with Phase Detect AF and became the adapter of choice for most A7 owners using the first generation of A7 cameras. The latest E-Mount cameras such as the A7II, A7RII, A6300 and A6500 have sophisticated on-sensor Phase detect Auto Focus so no longer need the Phase Detect AF (PDAF) of the LA-EA4 adapter. In fact the PDAF of the recent cameras is vastly superior to the one found in the LA-EA4 adapter. As a result of the recent advances in technology the LA-EA3 adapter has become the adapter of choice for Alpha users who own A-Mount lenses. I still use the SAL 70-400 G OSS II lens on my A7RII in conjunction with the LA-EA3 adapter. The only downside of this setup is that Eye-AF and Lock-On AF are not supported. This means that you must keep the subject in the original focus area that you first started tracking your subject in order to achieve consistently sharp images.
Lens Adapters have become increasingly sophisticated
Adapter that provide autofocus for manual focus lenses
This adapter provided autofocus functionality for manual focus Leica lenses attached to Sony E-Mount cameras. Techart also provide options to adapt non-Leica-M lenses to the Teacart PRO Adapter (effectively double adapting lenses to be mounted on Sony mirrorless bodies). Here is a link to their website for more information: http://techartpro.com/product/techart-pro-leica-m-sony-e-autofocus-adapter/
An independent review of this adapter can be found here: https://johnleathwick.wordpress.com/2016/11/19/the-techart-pro-leica-m-to-sony-e-adapter-an-enduser-perspective/