My SQA does not have an instant return mirror. The mirror lowers once the shutter is cocked. I use the SQA tripod mounted and also use the mirror lock-up - speed is not important to me! The lenses for the SQ have the shutter mounted in them, but the shutter speed is set on the camera body. I think you are probably referring to the Mamiya where the shutter is in the camera body. In this case, if you were using a leaf-shutter lens then the focal plane shutter would be opened on B/T and then the exposure made using the lens shutter. Hope this helps?