Brain teaser: Possible mirror problem MF SLR

[DannL]

If I focus through the prism on an object that is, let's us say 30 feet away, and the developed negatives clear show that the field of focus landed approximately 10-15 feet from the camera, is the mirror too close "or" too far away from the view screen (gg)? A lens aperture of f/5.8 was used.

Is it possible for a lens to be so far out of specs that it will focus fine in the viewer, but the focus is not at the film plane? (exluding any posssiblilty that the film plane is not in the correct position.)

Thanks in advance. Dann

 

[bdial]

Most likely it is the focusing screen which is not in the right position. Some cameras use shims or other positioning method to adjust the position of the screen. I don't think the position of the mirror could affect this.

 

[kodachrome64]

The focusing screen could certainly be the culprit. Although mirror adjustments are not usually needed, they can definitely affect focusing. I doubt it is the lens either; at f/5.6 you should have a pretty decent amount of headroom on the focus, so something must be out of alignment I would think.

 

[DannL]

Thanks guys. You hit the mark on what was required to regain control of the focus. After another inspection I had no choice but to remove the focusing screen and fresnel screen and make some adjustments. What a pain in the arse. Since this has a Hasselblad 120 type back, I put a piece of semi-opaque cellophane tape over the viewport hole on the insert film plane. This way I was able to compare the focus at the film plane through the back against the focus on the focusing screen. I had to raise the focusing screen and fresnel about 1.5mm above their original location. I used some sticky backed foam strips cut to about that thickness. I also had to replace the original spacers with something much thinner. Strange stuff. At f/4 we are now within an inch or two difference with a test target placed at 1 meter from the camera. That's much better than before.

I've included an example of the problem. I specifically focused on the tree and the grass was in focus at the film plane (orange arrow).