对于可更换镜头照相机而言，接环系统的法兰距（也称为焦平面距离）(英文:flange focal distance)就是指相机机身上与镜头接触的金属环到成像介质（感光组件或底片）之间的距离，通常用毫米表示。不同的接环系统有不一样的法兰距。法兰距互相符合的情况下，镜头只要对焦成功就能获得清晰的照片。

在把某个镜头通过转接环安装到其他接环系统的机身上的情况下，法兰距是考量的因素之一。若要制造一枚不需要纠正镜片而又能让镜头无限远聚焦的转接环，那幺镜头的法兰距必须长于对应镜头接环的设定法兰距，以腾出空间来安装转接环。因此长法兰距的相机系统可用的镜头就越少，法兰距短的也就越多。

For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), or register, depending on the usage and source) of a lens mount system is the distance from the mounting flange (the metal ring on the camera and the rear of the lens) to the film plane. This value is different for different camera systems. The range of this distance which will render an image clearly in focus within all focal lengths is usually measured in hundredths of millimeters and is known as the depth of focus (not to be confused with the similarly named depth of field).