How to determine the optimal interval size

Shooting in macro mode, techniques, tips & tricks
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MacroIko
Posts: 1
Joined: 18.06.2020 01:17

How to determine the optimal interval size

Post by MacroIko » 18.06.2020 02:11

Hello everyone,

now that I have had a chance to play with Helicon Remote and Helicon Focus a little more, I want to share my process for determining the optimal step size between images - meaning using the least number of pictures (prolonging the life of your shutter) while producing a sharp image without focus bands between.

First my setup: I am using a Canon M50 with a Canon F4 70-200mm L lens, and a Raynox 350 Macro lens & a LED Ring Light. Together, these sit on a WeMacro Rail.

I have established that each step in Helicon Remote corresponds to an approximately 0.1mm advance of the rail system (provided, I only lock in a starting point for the beginning of the stack). I then estimate the depth in millimeters needed, and start with a relatively low number of images, for example 1 image every 2 millimeters of depth. For example, for the Dobsonfly attached, I estimated 5cm total depth, which would result in 25 shots @ Interval size 20. My first run was at 70mm, F11, and EV-1. Of course the F-stop greatly influences your depth of field and the number of exposures needed.

Using Method B of Helicon Focus you can see a white on black inverse focus map during the second pass. If your focus depth is too shallow you will see dark bands between each layer. To remove these bands I had two options, either increase the number of pictures and reduce the Interval, or increase the F-Stop to 16. The uploaded examples include for examples of the inverted map, 100 shots @ Interval 5 (F11), 50 shots @ Interval 10 (F11), 25 shots @ Interval 20 (F11), and the final one with 25 shots @ Interval 20 (F16). The last one was then used for the final image.

Once you have established the optimal Interval size for each Focal Length and Aperture, you can write these down for future reference. This Interval size will not change as long as you maintain the same aperture and focal length.
collage-inverse-band.jpg
(5.41 MiB) Not downloaded yet
dobsonfly-edit-small.jpg
(4.39 MiB) Not downloaded yet
Hope this helps someone.

-Iko

waldenk
Posts: 16
Joined: 15.02.2015 19:17

Re: How to determine the optimal interval size

Post by waldenk » 22.06.2020 15:20

interesting post and helpful info.
question: in extreme macro work, -effective- f-stops can be quite large...
i.e. if you’re using extension tubes, large magnifications etc, stopping down can put you into the f45, f64 range, which is well into diffraction territory.
for this reason, tho my setup is a little different (nikon Z7 + laowa 25mm 2.5-5x macro + stackshot), i’m trying to shoot as wide open as i can.
and i’m using laowa’s DOF chart, which shows DOF for all f-stop/magnification combinations...then stepping about 75% of the DOF for each shot.
of course my stacks are deeper, but i —think— this is the path to maximum sharpness (i.e. shooting fairly wide open vs stopping way down)
i’ve not done any careful testing yet however.
do you...or anyone in the forum...have thoughts/guidance/experience on this?
thanks for sharing your work:)

BobStone
Posts: 44
Joined: 03.06.2017 10:13

Re: How to determine the optimal interval size

Post by BobStone » 22.06.2020 21:37

You should use the sharpest aperture and let the stack build the DOF you need. For Nikon Z cameras and the Nikon 105mm, the camera's shift setting of 4 work well at 1:1 magnification even wide open. (I did a lot of tests!) Smaller steps for larger magnifications of course. Use the electronic shutter so you don't have to worry about mechanical shutter count.

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Catherine
Posts: 405
Joined: 29.04.2019 22:38

Re: How to determine the optimal interval size

Post by Catherine » 22.06.2020 21:41

Hello, and thanks for sharing your results!
MacroIko wrote:
18.06.2020 02:11
I have established that each step in Helicon Remote corresponds to an approximately 0.1mm advance of the rail system
This value is configurable in Helicon Remote preferences - Stackshot, by default it's 0.05 mm.
MacroIko wrote:
18.06.2020 02:11
Using Method B of Helicon Focus you can see a white on black inverse focus map during the second pass.
This is a great way to spot any discrepancies in your stack: if you know that the subject in your frame is continuous along the Z axis, then the reconstructed depth map should also be continuous, and any visible discontinuity / irregularity means that something went wrong.
And you can quickly view the depth map at any time in Retouching (by clicking the "Show depth map" check box).
waldenk wrote:
22.06.2020 15:20
i’m using laowa’s DOF chart, which shows DOF for all f-stop/magnification combinations...then stepping about 75% of the DOF for each shot.
Sounds perfectly fine and close to optimal. As long as you're certain that true DOF more or less matches the calculated value that you're using as guideline, 75% of that is a good interval for shooting a focus stack. If you want to be super safe, make it 30% overlap, so 70% step.

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