Use the force, Luke... (couldn't resist).
Really though, I think you've got it almost right if I understand correctly, although I don't really understand your terminology exactly. There are a couple of things that stand out as not correct, however. Let me walk you through what I do and see if it agrees with what you think is correct.
First, I place my plane of sharp focus by choosing focus points. If I'm just using tilt or swing (or nothing at all), I'll just choose two. If I need both swing and tilt, I'll choose two each for vertical and horizontal. For your example, using just tilt, I'd choose one close and one far. What I don't think you've got correct is the concept of 1/3 of the depth of field toward the lens and 2/3 behind the plane of sharp focus. This works kind of if everything is set up parallel. If not, i.e., if you are using tilts or swings, then things are much different. Think of a tilted plane of sharp focus, for example intersecting a foreground rock and a distant mountain. The depth of field emerges from the plane in a wedge shape, growing greater as the distance increases. So, where the plane of sharp focus is near the camera, the depth of field on both sides of it is rather shallow. At a greater distance, the DoF is much greater, but at both places, it is about equally divided on both sides of the plane of sharp focus.
Therefore, in your example, I'd pick a near focus point about in the middle of the rock and about halfway up the mountain. I'd then set my tilt so that both my chosen points were in focus.
Now, I find the focus spread by finding the most distant things on either side of the plane of sharp focus that I still want sharp. When using tilts (or swings) you have to keep in mind that things on either side of the plane of sharp focus are no longer "closer" and "farther" from the camera! Instead, they are (in the case of tilts) above and below the plane of sharp focus (or right/left when using swings). That means, for example a near object that is below the plane of sharp focus requires you to focus closer to infinity than a near object above the plane of sharp focus. Similarly, the top of the mountain, in our example, is now above the plane of sharp focus and therefore requires more extension to focus on than the base of the mountain, which is below the plane of sharp focus.
At any rate, check all those extremes and find the farthest objects from the plane of sharp focus on either side that you want in focus. Note both of these extreme positions on your camera rail/bed. I use a millimeter scale on my camera beds to measure this distance. The optimum position for the focus then is exactly halfway between these extremes on the camera rail/bed. Notice that we are talking about the physical position of the camera standards here, NOT where you focus in the scene. I don't even worry about where in the scene the final focus is; once I find my extremes, I just position the focus halfway between them. However, just so you know, this does put the plane of sharp focus somewhere close to 1/3 into the scene when the standards are parallel. When you use tilts/swings, it's more complex, but still the plane of sharp focus gets positioned similarly. Still, it's not really necessary to know that, or even check it on the ground glass.
Now, once you've determined your focus extremes, you should know the focus spread in millimeters. The greater the focus spread, the greater the DoF you will need and the smaller the aperture you will need. All this is nicely worked out for you in the article on finding the correct f-stop in the main LF page, here:
http://www.largeformatphotography.info/fstop.html .
You can use the info there to work out your own table or simply use their suggestions. I've been using this method for years and never have a surprise as far as desired DoF goes. I recommend it highly.
You might want to check out the focusing article on the main page too. It's here:
http://www.largeformatphotography.in...-to-focus.html .
Hope this helps,
Doremus
Originally Posted by David Karp
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