Joseph S. Wisniewski wrote:
OK, say we're using the 63mm Luminar, on a bellows, with your four thirds camera. And we're stalking a small flower, approximately 40mm high and wide, so we need about 0.25x magnification, and start out focused at the front of the flower, 78.75mm from the subject to the front node of the lens, 315mm from the rear node to the sensor. Out flower is roughly 20mm deep. Now, to move the subject focal plane that first mm (from 78.75mm to 79.75mm) we move the rear standard forward 15.04mm, to get a rear node to sensor distance of 299.96mm.
The next slice, 80.85mm, comes a little cheaper, we only move the rear standard 13.3mm to get a 1mm front focal plane shift. By the time we reach the last slice, 97.75mm from the focal plane, we're only moving the rear standard 3.4mm to get each 1mm of front shift. So if you want uniform slices through the subject, you have to move the rear standard in a highly non-linear fashion.
We also note that the magnification has increased from 0.25x to 0.55x, so we're making an ugly crop. Which means that we should have started this sequence of computations from the back, and we have to use a different lens, because the luminar won't let us move the front focus plane 20mm forward when we start at 78.75mm. There's only 15.75mm of motion before the subject is one focal length from the front node and magnification goes infinite.
I'm still not sure what you mean by "ugly crop". I can see where the total depth of field available on lens that don't focus to infinity may be smaller than the subject, but this should just be an artistic challenge
I'm still experimenting with the workflow and here's what I'm currently doing:
1. Decide on the maximum magnification.
2. Set the entrance pupil to the rotation point.
3. Focus on the closest object and note the bellows draw position.
4. Focus on the furthest object.
5. Determine the number of images in the stack. Part of this is to decide whether to shoot the entire stack at the increment of the maximum magnification or vary it to keep the number of images to a mininum.
6. Shoot the first frame from back to front so the closest focus is the last shot.
7. Rotate the head to the next position allowing a 20% overlap. Note the amount of rotation.
8. Shoot the stack.
9. Reset bellows to position noted in step 3.
10. Rotate the head using the same amount of rotation as step 7.
11. Repeat 8 through 10 until done.