Optimal step sizes for macro photography using a rail

JimKasson · Mar 8, 2025

I've done a lot of work on how to set up the step sizes when you're using the focus bracketing features built into the GFX cameras.

https://www.lensrentals.com/blog/2023/02/how-your-cameras-focus-bracketing-system-works/

Now I'm doing macro work at greater than 1:1 magnification, and I need to do something similar so I can pick step sizes when using my Cognisys rail.

I created this graph:

b7fe7a9818ad45098ab7dc91d232ae4b.jpg.png

You will note that the right step size is fairly insensitive to magnification about a magnification ratio of about 1.5:1. Why is this? It's because the effective f-stop is varying with magnification.

This is for the Rodenstock 105mm f/5.6 HR Macro Digaron that I'm using wide open.

I'd appreciate it if some mathematically inclined person would check my numbers.

Jim

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Pd printer · Mar 9, 2025

It looks that you vary the distance of lens to subject which changes perspective whereas in camera focus bracketing with most modern macro lenses keeps the distance of lens (front) to subject constant. Last year I experimented with various setups with my 907X and found that focus stacking in both Zerene and Helicon gives less artifacts when the distance to subject stays constant as both software can easily compensate for change in magnification both not so much in perspective (or any other movement).

I am using a Swebo bellow/rail with back standard movement but the step size has to be compensated by a factor m squared so much larger step sizes at higher magnifications which will be more problematic at higher (>2x) magnification but then the issue with change of perspective will be small as step size is small.

JimKasson · Mar 9, 2025

Pd printer said:
It looks that you vary the distance of lens to subject which changes perspective whereas in camera focus bracketing with most modern macro lenses keeps the distance of lens (front) to subject constant. Last year I experimented with various setups with my 907X and found that focus stacking in both Zerene and Helicon gives less artifacts when the distance to subject stays constant as both software can easily compensate for change in magnification both not so much in perspective (or any other movement).

I am using a Swebo bellow/rail with back standard movement but the step size has to be compensated by a factor m squared so much larger step sizes at higher magnifications which will be more problematic at higher (>2x) magnification but then the issue with change of perspective will be small as step size is small.

In my experience, back standard focusing is the best way to go for three dimensional subjects at magnifications from 1 to about 0.1. The reason for that is the perspective issue that you've mentioned. However, not all equipment supports that. At magnifications beyond 1, in my experience the subjects are not sufficiently three dimensional that there are perspective issues with moving the whole camera assembly or the whole subject.

JIm

Crispy_Bee · Mar 9, 2025

For 1.5x and beyond it looks quite good - though for lower magnifications the step size is quite conservative.

For example:

1x at f5.6 = 251 micron DOF so with a 35% overlap that'd be 163 micron
0,75x at f5.6 = 341 micron DOF so with a 35% overlap that'd be 222 micron
0,5x at f5.6 = 564 micron DOF so with a 35% overlap that'd be 367 micron

And I'm using a CoC value of 0.002 which is already very very small, as is the 35% overlap. But one shouldn't go overboard, it's not necessary to use an even smaller CoC value or an overlap of 50% or more, that might actually reduce the image quality if the stacking software has to deal with very complex yet repetitive structures and has to deal with too many images covering the same area multiple times.

JimKasson · Mar 9, 2025

Crispy_Bee said:
For 1.5x and beyond it looks quite good - though for lower magnifications the step size is quite conservative.

For example:

1x at f5.6 = 251 micron DOF so with a 35% overlap that'd be 163 micron
0,75x at f5.6 = 341 micron DOF so with a 35% overlap that'd be 222 micron
0,5x at f5.6 = 564 micron DOF so with a 35% overlap that'd be 367 micron

And I'm using a CoC value of 0.002 which is already very very small, as is the 35% overlap. But one shouldn't go overboard, it's not necessary to use an even smaller CoC value or an overlap of 50% or more, that might actually reduce the image quality if the stacking software has to deal with very complex yet repetitive structures and has to deal with too many images covering the same area multiple times.

My simple-minded takeaway is that, beyond 1.5 magnification, I can use the same step size, which simplifies my life.

Crispy_Bee · Mar 9, 2025

JimKasson said:
Crispy_Bee said:

For 1.5x and beyond it looks quite good - though for lower magnifications the step size is quite conservative.

For example:

1x at f5.6 = 251 micron DOF so with a 35% overlap that'd be 163 micron
0,75x at f5.6 = 341 micron DOF so with a 35% overlap that'd be 222 micron
0,5x at f5.6 = 564 micron DOF so with a 35% overlap that'd be 367 micron

And I'm using a CoC value of 0.002 which is already very very small, as is the 35% overlap. But one shouldn't go overboard, it's not necessary to use an even smaller CoC value or an overlap of 50% or more, that might actually reduce the image quality if the stacking software has to deal with very complex yet repetitive structures and has to deal with too many images covering the same area multiple times.

Click to expand...

My simple-minded takeaway is that, beyond 1.5 magnification, I can use the same step size, which simplifies my life.

first of all: you don't have a simple mind so there's no simple minded takeaway possible

but if it helps with my calculator I get the following numbers for f5.6 using a 35% overlap:

1,33x: 125 microns

1,50x: 113 microns

1,75x: 101 microns

2,00x: 92 microns

2,25x: 85 microns

2,50: 80 microns

2,75x: 76 microns

3,00x: 72 microns

I guess the question you might want to ask yourself is whether it makes sense to use the lens beyond 2,5x or if upscaling and cropping is a better solution, there's not a lot more to see beyond an effective aperture of f20 on a 100MP 44x33.

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Optimal step sizes for macro photography using a rail

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