Soft focus is usually camera/lense related. I have yet to see a "Back" which is soft focus! As the sensor chip is a semiconductor, it is absolutely flat on its surface, nor can any curvature creep into the process, hence centre or edge "out-of-focus" conditions never occur.
With the camera body and lense it is a different story.
. The body can off registration with the mirror
. The body can have off registration with the back
The best DIY that I have tried, is to get some thing like a "Lense Align" focus at the centre and find where the focus is on the back. Then adjust the focusing screen to synchronize with the back. An incline steel/wooden scale marked set up at an incline (towards and away from you), will also do admirably in this case.