Q.G. gave a very good and accurate response to your question, as usual.
As he wrote, the screen needs to be at exactly the correct height to guarantee correct focus. And, while most Hasselblad technicians have the proper tools to insure that the screen is at the correct height, it is possible for you to adjust the screen properly by using the following method:
Mount the camera body on a sturdy tripod. Then, attach a lens to the camera, and set the lens to the infinity setting. A lens with a longer focal length works best ... either a 150 or 250mm lens.
Point the lens to a distant object, at least 100 yards away. (Like the edge of a building or an antenna).
Turn the four screws that hold the frame for the screen clockwise, until they bottom out.
CAUTION: Don't force the screws. They are very small in diameter, and forcing the screws can break them.
Look at the image on the screen. It will probably be out of focus.
Now, turn each of the screws 1/2 turn counter-clockwise and look at the image on the screen.
Repeat this proceedure until the image on the screen is in sharp focus. Once the image is in sharp focus, the screen is as the correct height.
NOTE: For those of you who want to install an after-market screen (Brightscreen or Beattie), you would use the same proceedure as above to adjust the height of the screen.
David S. Odess
Factory trained Hasselblad technician
www.david-odess.com