Is flange focal distance of a camera system measured from the front edge of the lens mount to the film plane, or the back edge of the lens mount to the film
![optics - How can I increase the flange focal distance (back focal length) of DSLR lens? - Photography Stack Exchange optics - How can I increase the flange focal distance (back focal length) of DSLR lens? - Photography Stack Exchange](https://i.stack.imgur.com/jvzLX.png)
optics - How can I increase the flange focal distance (back focal length) of DSLR lens? - Photography Stack Exchange
![terminology - What exactly is focal length when there is also flange focal distance? - Photography Stack Exchange terminology - What exactly is focal length when there is also flange focal distance? - Photography Stack Exchange](https://i.imgur.com/V1WySTT.gif)
terminology - What exactly is focal length when there is also flange focal distance? - Photography Stack Exchange
![Re: About flange focal distance: Photographic Science and Technology Forum: Digital Photography Review Re: About flange focal distance: Photographic Science and Technology Forum: Digital Photography Review](https://cdn.photographylife.com/wp-content/uploads/2012/07/How-Phase-Detection-Autofocus-Works.png)