## Reality check

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

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4odonates
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Joined: Sun Oct 30, 2022 12:43 pm
Location: Fairfax County, Virginia

### Reality check

Let's say you're using a standard macro lens to photograph relatively large subjects, by macro standards. If you focus on the farthest point on the subject, take a photo, and then use a macro rail to move the rig from back-to-front (without changing focus), I maintain what you're really doing is "mapping" the camera sensor in increments equal to the step size. Let me try to be clearer.

The maximum number of steps for your focus stack should be based upon step size relative to the dimensions of the camera sensor. The actual distance between the back and front of the subject doesn't really matter. What matters is the height of the camera sensor -- the maximum distance the carriage of a focus rail will travel is directly related to the dimensions of the sensor.

Am I right? Is there a scenario when/where my theory would be wrong?

Walter
Last edited by 4odonates on Sat Mar 18, 2023 4:15 pm, edited 1 time in total.

rjlittlefield
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### Re: Reality check

I'm really curious...

How does this line of thought relate to the recent long conversation that we had about calculating step size?

--Rik

4odonates
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Joined: Sun Oct 30, 2022 12:43 pm
Location: Fairfax County, Virginia

### Re: Reality check

Let me start by saying how much I love your DOF Calculator, Rik! Access to that tool was/is a game changer for me.

In my last post, I'm not saying I discovered a backdoor way to determine step size. Here's the backstory.

I decided to create a focus stack using my Fujifilm X-T5 and Fujinon 80mm macro lens. I chose an aperture of f/8. I used a step size of 50 microns, as recommended by your DOF Calculator. The subject was a coin that nearly filled the picture frame. I never thought about how many steps it would take to cover the subject from back-to-front, that is, until about halfway through the project when I realized the finished stack would be in the hundreds of photos! Did I mention I'm using a manual focus rail? That's insane!

Afterward, I did some back of the envelope calculations. The digital sensor in the X-T5 is 15.7 mm high, or 15,700 microns. 15,700 microns/1 * 1 step/50 microns = 314 steps. Turns out that's about the same number of photos I shot to cover the entire subject.

More recently, I shot two more focus stacks using the same camera rig. This time I chose an aperture of f/11. I used a step size of 800 microns. 15,700 / 800 = 19.6 steps -- around the same number of photos I shot for each stack.

Coincidence? I don't think so. I thought back to a simple video I created for a blog post intended to help a friend visualize what goes on during focus stacking and why we do it.

In the video, I used red focus peaking to highlight the areas of the subject that are acceptably in focus. As the focus rail moved from back-to-front, the areas in focus moved from top-to-bottom of the photo frame. That's what I meant when I said "mapping the sensor."

Walter

P.S. - If you read some of recent blog posts then you will see I'm using Helicon Focus to create my focus stacks. Full disclosure: Many years ago I was provided with a complementary copy of Helicon Focus in exchange for a product review in my blog.

At that time, my macro photography gear was so poor that I realized it would be unfair to evaluate the performance of Helicon Focus. Now that I have better gear and better tools (like your DOF Calculator), I'm using the focus stacking software I have and it works fairly well. That is, when it works. It seems like it either works or it doesn't.

All of that being said, I'm convinced Zerene Stacker is the best focus stacking software available currently. Hope to get a copy soon.

JKT
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### Re: Reality check

4odonates wrote:
Sat Mar 18, 2023 4:14 pm
In the video, I used red focus peaking to highlight the areas of the subject that are acceptably in focus. As the focus rail moved from back-to-front, the areas in focus moved from top-to-bottom of the photo frame. That's what I meant when I said "mapping the sensor."
This one is easy ... and is outside the dof calculation formula. Your coin is not parallel to the focus plane or not orthogonal to the optical axis - whichever way of saying the same thing is more clear to you. The number of steps needed to move the focus plane from one edge of the coin to the other should be added to the number of steps given by the calculator.

rjlittlefield
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### Re: Reality check

Walter,

Thanks for the backstory.

Quickly summarizing, here is what I hear you saying:
• Step size comes from the calculator
• Number of steps = depth of subject / step size
• (Observation) Number of steps = height of sensor / step size
So then, a bit of algebra reveals the magic:
• Height of sensor = depth of subject
You've observed a spurious correlation, caused by the size of the subject space in the few cases that you've considered.

If you considered a wider range of situations, you'd find that the sensor height rule doesn't work well at all.

JKT wrote:
Sun Mar 19, 2023 12:41 am
The number of steps needed to move the focus plane from one edge of the coin to the other should be added to the number of steps given by the calculator.
No addition is needed if the calculator is used as intended. The key input is the total depth of field to be captured in focus, measured along the optical axis, taking into account all tilts as well as dimensions of the subject.

--Rik

JKT
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### Re: Reality check

rjlittlefield wrote:
Sun Mar 19, 2023 8:37 am
JKT wrote:
Sun Mar 19, 2023 12:41 am
The number of steps needed to move the focus plane from one edge of the coin to the other should be added to the number of steps given by the calculator.
No addition is needed if the calculator is used as intended. The key input is the total depth of field to be captured in focus, measured along the optical axis, taking into account all tilts as well as dimensions of the subject.
That would be the true if the angle was planned and considered in the calculation. Based on the question I would assume that was not the case here.