‘Alpha’ is the generic brand name used by Sony for its interchangeable lens cameras. This can be confusing because Sony makes cameras in two types – SLT (single lens translucent) and mirrorless models. Both are Alphas, but the Alpha A9 (mirrorless) and Alpha A99 II (SLT) are entirely different cameras with different lens mounts and lens ranges. Sony’s SLTs use Alpha A-mount lenses, while the mirrorless models use E-mount lenses.
Sony
SLT (single lens translucent)
SLT cameras are made by Sony as a kind of hybrid of the regular digital SLR design and the always-on live view of a mirrorless camera. They do have a mirror in the body, but it doesn’t flip up and down when you fire the shutter. Instead, it has a translucent surface so that the image can pass straight through to the sensor on the back of the camera.
The point of this design is that the fixed mirror can reflect light back on to the camera’s fast phase-detection autofocus sensor even while the image is being composed using a live view image on the rear screen or in the electronic viewfinder.
The downside is that you get all the bulk of a DSLR without the clarity of an optical viewfinder. Sony still makes its SLT models but with improvements in sensor-based hybrid autofocus technology, most expect it will soon transfer all its efforts to its mirrorless models.
A-mount (Sony)
The lens mount used by Sony’s Alpha SLT cameras. Because these cameras have a mirror in the body, even though it’s a non-moving one, the rear of the lens is further from the sensor, so Alpha mount lenses are physically different to Sony’s E-mount lens range. You can use Alpha lenses on an E-mount camera with an lens adaptor, but not the other way round.
E-mount (Sony)
This is the name of the lens mount used by Sony mirrorless cameras. Regular E-mount lenses fit its APS-C format cameras, like the Sony A6500, while FE lenses fit its full-frame mirrorless cameras, including the A7 series and Sony A9. Sony also makes A-mount lenses for its Alpha SLT cameras, but these are not the same.