Hi Petra you stick the flattener straight into the refactor then work out the recommended spacing (if any) with your spacers then the camera. Usually the spacing is measured from the glass at the back of the flattener to the sensor on the camera that might be recessed into the camera body. Usually between 6mm up to 19mm depending on the camera model. So if you measure your spacing from the camera face you have to deduct that distance. Clear as mud