How do I choose macro rail step size for focus stacking?
Asked 1/5/2018
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I'm using a StackShot automated macro rail, which moves in micrometres, and I want to know how far to move the rail between shots when focus stacking. Is there a formula or practical way to calculate the step size so each frame overlaps the previous depth of field slightly?
My setup is a 60mm macro lens with Kenko extension tubes (12mm, 20mm, and 36mm).
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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Is there a formula where I can enter in X Y and Z to get the amount of um the rail should travel between each shot so I only slightly overlap for each depth of field slice?
Since a rail moves the entire camera forward or backward without changing the focussed distance, you just need to know what distance you want between shots. If you know how much total depth of field you're getting with each shot, you'll probably want to make your layers bit less than that. Using a depth of field calculator can help, although your macro rings may complicate that.
Here's an example: Using a 60mm lens on a full frame Canon 5D, and a distance to subject of 20cm, f/4 gives you a depth of field of 0.19cm. So you might want to space your layers around 0.1cm.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
8y ago
0
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A practical approach is to base the rail step on the depth of field in each shot. Since the rail moves the whole camera/lens assembly, you choose a movement distance that is a bit smaller than the depth of field so adjacent frames overlap.
So the workflow is:
- Estimate the depth of field at your actual magnification/aperture.
- Set the rail step to somewhat less than that value.
A depth-of-field calculator can help, though extension tubes make the exact numbers less straightforward. As a simple example from the answers: with a 60mm lens on full frame at 20cm and f/4, the depth of field is about 0.19cm, so a step around 0.1cm would give overlap.
In short: there isn’t a single universal X-Y-Z formula from the provided info; use your effective depth of field and choose a step smaller than it.
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