What's the difference between a lens with a Sony A mount and a lens with an E mount?

Although the history of lenses is quite complex, the difference between lenses is easy to explain. The A-mount lenses are made for digital SLR cameras by Sony, and the E-mount lenses for the mirrorless cameras. Besides, an E-mount lens fits both on mirrorless cameras with an APS-C sensor and a mirrorless camera with a full-frame sensor. The A-mount lenses are suitable for 1 type of image sensor, APS-C or full-frame.

Choose an E-mount lens if you have a Sony Alpha mirrorless camera. In that case, it doesn't matter which type of image sensor you have, because the lens fits both on a full-frame and an APS-C camera. In the end, you'll choose based on the quality of the lens. Are you looking for a lens for your Sony SLR camera? You'll need an A-mount lens. This lens will only fit on the type of image sensor specifically for that camera. This is almost always an APS-C sensor.

There are lens mount adapters available that make A mount lenses suitable for Sony E mount cameras. That means you can use an A mount lens on a Sony mirrorless camera with the help of an adapter. This is very useful if you've purchased a new Sony mirrorless camera, but still have several lenses for a Sony SLR camera.