FUJIFILM "X-Mount" Maximises Lens Performance.
Specifically designed to maximise the mirror less design of the body the X-Mount has a short flange back distance of just 17.7mm. This means the rear lens elements are as close as possible to the sensor. The wide opening allows the lens to be mounted deeper within the body - up to 7.5mm (approx) from the mount surface - reducing the back focus distance of each lens to the minimum possible, thus achieving high resolution all the way to the edge of the image.