First, here is a short checklist that covers most situations:When I'm shooting a stack, what's the best way to step focus? Should I turn the focus ring, or use a focus rail or slide? Does it matter if I move the subject instead of the camera? If my lens is on a bellows, there are even more options. So many choices -- what's best??
- Don't agonize over the decision. It usually doesn't matter much.
- Use the longest lens that's convenient and gives you the resolution and field of view you want.
- Use the focus ring if you can make fine enough movements.
- Otherwise use a focus rail to change the subject-to-lens distance, leaving the lens set at a fixed focus.
- At higher magnifications, you'll need something like a screw driven table or a microscope stage.
If you're happy with just accepting the recommendations, then stop reading now and skip to later posts in this thread that will show you how to put hardware together.
To understand the recommendations, it's easiest to start with a bit of theory.
Stop down your lens, look into the front of it, and you'll see the diaphragm that forms the aperture. The apparent position and size of that aperture is called the "entrance pupil". The entrance pupil is usually somewhere around the middle of the lens, but depending on lens design it can be nearer the front or farther back, even behind the camera in rare cases.
The center of the entrance pupil is also the center of perspective for each frame in your stack.
You want perspective to change as little as possible between front and rear of the stack. With simple subjects, this is not a big issue. But if your subject has parts that overlap each other, then changing the perspective can change the overlap. If the change is too big, it can produce defects in the stacked image.
Roughly speaking, the change in perspective depends on the distance the entrance pupil moves while focusing, divided by the total distance from entrance pupil to subject. A small movement combined with a large distance is good.
Using a longer lens increases the distance.
The other recommendations are a tradeoff between minimizing movement of the entrance pupil and getting fine uniform focus steps.
Ideally, the entrance pupil would not move at all. However, this usually requires strange setups that have other drawbacks.
So, as a matter of practice you most commonly choose between:
- Option 1) Change the lens extension or focus ring while leaving the camera and subject in fixed positions.
- Option 2) Leave the lens extension fixed while moving the camera and lens as a unit (or equivalently, move the subject).
Edit 8/21/2013: to tweak thread title