Originally posted by JamieWakeham I'm getting that at the closest focus (1m), the actual plane of sharpest focus is perhaps 2mm behind the point that the screen indicates, and at 15m, it's maybe 5cm in front.
Welcome to the Pentax Forums!
The above is your focus plane (aka plane of focus). If you shim, you will be aligning your new screen to the focal plane in the camera. Whether you can improve on your current adjustment depends heavily on what you are using for shims. The gradations with the Pentax shim set are quite small (0.05 mm in the range of 0.15mm to 0.55mm). If you are using the plastic shims from focusingscreens.com, the options for adjustment are less fine.
Since you are seeing front focus, you will need a somewhat thinner shim than what is currently in place. If there is no shim, there is nothing you can do. Be aware that your existing shim, if any, fits behind a second frame between your screen and the bottom face of the pentaprism. Any replacement belongs there.
As for calibration, I would suggest a fast 50 (f/2 or wider) and a distance to target of about 20x the focal length. The important thing is to be in that range where there room for fine control (near minimum focus distance) and that the lens have a long focus throw. While the Lens Align is popular on this site, any flat high contrast target will work. What is important is that the target provide a clear vertical line to work against and that the plane of focus is parallel to the focal plane.
Actual focus evaluation goes like this:
- Make your best effort at manual focus using the magnified live view, this is your standard
- Without touching the focus ring, switch to the optical viewfinder
- If needed, adjust the focus to align the focus split image
- Record "0" for none, "-" for nearer, or "+" for farther on a sheet of paper
Repeat the above steps 10 times, defocusing before each time. A preponderance of results (4 or more out of 10) indicates either no change or an adjustment up or down.
If you add or subtract a shim, repeat the above to confirm.
Fun, eh?
Steve