Sony A-mount and E-mount systems
Sony α series interchangeable-lens digital cameras are currently produced in two categories, each of which uses a different lens mount and different types of lenses: A-mount cameras with advanced translucent mirrors, and compact E-mount cameras that don’t use reflex mirrors at all. Despite their remarkable compactness and portability, E-mount cameras feature sensors that are capable of delivering image quality on a par with A-mount cameras.
In addition to overall size, the main difference between A-mount and E-mount lenses is their “flange back distance.” The flange back distance is the distance from the rear of the lens to the image (sensor) plane. Since many A-mount cameras are traditional SLR designs that have a reflex mirror between the rear of the lens and the sensor, they need to have a flange back distance that allows space for the mirror. E-mount cameras, on the other hand, do not have reflex mirrors and therefore can be designed with a much shorter flange back distance, and consequently smaller lenses overall.