## Focus stacking by focus ring

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

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barnack-bill
Posts: 36
Joined: Tue Mar 29, 2016 1:55 am

### Focus stacking by focus ring

I understand that this is preferred over using a rail but how do you determine the steps to get regular spacing? Most focus rings have a non linear (logarithmic?) scale and focus by wire lenses follow some complex rule in which focus change depends on speed of focus ring adjustment as well as absolute rotation?

At least with a rack the advance per revolution of the adjusting wheel is constant.

Is it just estimation and experience?

ChrisR
Posts: 8671
Joined: Sat Mar 14, 2009 3:58 am
Location: Near London, UK
Does the lens extend like it's on a simple thread?
If so you can treat it like a thin lens, allow for thickness (it doesn't change with the focus for a simple sort of lens) put the numbers into Excel and make a table.
If there's a way to make a "linear" plot from that I don't know it. You may be able to draw a useful graph with right dimensions. I wanted something similar for the movement of the "rear standard" but gave up with the graph/scale and used the table, of millimeters

For a lens I have wrapped paper round, copied the marks, put the paper under a travelling microscope to make accurate marks, (you could just imterpolate) , and put the paper back. I used a yellow Post-it.
A radial lever would make the movements much easier, but with the lens I was using I couldn't find a good way to fix one because the ring was narrow.

The numbers I came up with were near to even degrees of turn iirc, which makes sense if you consider ye olde DOF marks on lenses - fixed width, per aperture.

Or you can just eyeball the subject it in expanded live view.
Chris R

rjlittlefield
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Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
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There are a lot of interesting aspects to the general issue of stepping focus. Regarding ring versus rail in particular, first take a look at http://zerenesystems.com/cms/stacker/do ... versusrail for some explanation of why it's better to use the focus ring at low magnification (much less than 1:1). I wrote that page as a direct result of seeing experienced photographers seriously misuse their rails. The page also has a quick summary of other methods of stepping focus, which may be a bit of an eye-opener.

Now, you asked specifically "how do you determine the steps to get regular spacing?"

Just identify where you want to start and end, on whatever control you have for focusing, then divide that interval into N equal increments.

That will usually produce focus steps that are almost equally sized in image space, that is, back behind the lens where the sensor is.

If you're working at low magnification, those equal focus steps at the sensor definitely will not translate into equal focus steps at the subject. If that bothers you, then I encourage you to relax and get comfortable with the idea. Despite what is often said, focus stacking does not really benefit from equal steps at the subject. The reason is that focus stacking doesn't deal with the subject; it works only with the images.

To help understand this, consider the extreme example where you're focus stacking from say 4 inches out to 100 feet. At 4 inches, you might need a focus step of only 1/2 inch in order to get enough DOF overlap. But continuing to step by 1/2 inch all the way out to 100 feet would be silly. Instead, parts of the subject that are farther away from the lens experience greater DOF in subject space, so it's appropriate to make bigger steps out there. (If you slog through the math, it turns out that the appropriately bigger steps in subject space turn out to be equal steps in image space.)

You also asked, "Is it just estimation and experience?" To that, the answer is basically "yes". Using a rail, you know exactly how much the lens-to-subject distance changes, so you can match that against any of several DOF formulas to calculate how many frames you need. But with focus motor control, I have not seen even a single case where there's a published spec about how far one "small" or "medium" or "large" increment in the focus control actually changes the focus. So pretty much all you can do is to determine by experience that "with lens X, at aperture Y, it takes step size Z to get adequate overlap". With old lenses that have DOF marks, the manufacturer sort of bundled their experience for you, but with new lenses you have to figure it out for yourself. BTW, there's also no guarantee that different control software will use the same concept of step size even for the same camera and lens. Focus control by internal motor is a package deal that depends on the lens, the camera, and the controlling software all combined.

Does this help?

--Rik

barnack-bill
Posts: 36
Joined: Tue Mar 29, 2016 1:55 am
Rik, thank you. I had seen the link in another post which is what prompted me to raise the subject here, particularly with flower/butterfly applications in mind. The advice you both have given is appreciated.

I'm a Sony user so clever focusing programs are not available. Depending on the lens I may have mechanical or electronic (focus by wire) focusing and in the least helpful case, internal focusing with focus -by-wire so it is going to be good old guess and hope I suppose.

Thanks again

Bill

rjlittlefield
Posts: 23937
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:
I'm a Sony user so clever focusing programs are not available. Depending on the lens I may have mechanical or electronic (focus by wire) focusing and in the least helpful case, internal focusing with focus -by-wire
In that case, remember that all this stuff is a matter of tradeoffs.

The difficulties that prompted the ring-versus-rail page were things like somebody shooting a necklace on a slanted board using a wide-angle lens close up on a rail, and then wondering why the rendered output had glitches.

Lots of people have successfully used rails to stack flowers and butterflies.

It's just a matter of having the ratio of depth to working distance be small enough to keep those perspective changes under control. The usual rule of thumb is that you'll be OK if the stack depth is less than 1/10 the distance from subject to entrance pupil. Less is better, check to be sure, and so on. What you can get away with depends on several factors that make it difficult to predict.

Anyway, if turning the ring is difficult or awkward, then do try a rail and see if it works OK.

--Rik