The string trick is very reliable. I put a knot in the string, focus while the subject is holding the string taut to their nose, and then I can use the string to check focus after inserting the film, removing the darkslide, and cocking the shutter. The other end of the string is tied to a fixed point on the tripod. You can shoot 8x10" and larger at a wide aperture, and just about every frame will be dead on.