Range finder black out with slow lens

I HAVE A MANUAL FOCUS CAMERA WITH SPILT IMAGE FOCUSSING SCREEN, WHEN USES WITH TELEPHOTO LEN 300/F4 + X2 TELECONVERTOR = F8. THE RANGE FINDER ALWAYS BLACK OUT AND VERY DIFFICULT TO FOCUS THE SUBJECT (MOSTLY BIRDS). HOW DO YOU SOLVE THIS PROBLEM?

If you have a body with interchangeable focusing screens try to get a plain matte (if possible laser) screen without focusing aids. With a 300 mm lens the depth of field is shallow enough to enable you to focus precisely without them. If you do not have such a camera try to focus on the area surrounding the focusing aids and then recompose your picture.