Je kan een ander matglas plaatsen waarmee je wel handmatig kunt scherpstellen.
Split-Image
The split-image focusing screen is probably one of the most useful manual focusing screens. A split-image focusing screen has a split circle in the middle. Usually the split is Horizontal. But there are vertical and diagonal focusing screens out there as well. In fact, for some camera systems, there is a “cross-split” type that has both vertical and horizontal splits.
The split divides your image in half. If your subject is not in focus, the two images does not line up. With the horizontal split, one half of the circle is shifted to the left, while the other half is shifted to the right. By aiming the circle on your subject's edge, it is very easy to see whether the edges line up. Instead of looking at whether the image is blurry, adjust the focus until the edge lines up, then you'll know the subject is sharp.
http://www.camerahacker.com/Tools/Viewfinder_Screens.shtml