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Re: [OM] WTFocus Stacking`

Subject: Re: [OM] WTFocus Stacking`
From: Martin Walters <mwalters1440@xxxxxxxxx>
Date: Sat, 16 Jul 2022 22:01:57 -0400
Thanks, Moose, for the clarifications. I couldn't find any real detail for FS on the 5-II, likely because it's an older camera. Looking at the stacked photos again, it seems that I did focus on the nearest point, probably because I was not using the smallest focus rectangle.

It's clear, however, that the focus in the second photo in each of my FS sequences moves towards the camera (and goes out of focus). After that, the next photo reverts to the initial focus. Subsequent photos show the focus gradually moving away from the camera.

Yes, focus bracketing would probably be better, but I don't have any software to do that. I see that Wayne has identified a couple of freeware options, which I may try.


On 2022-07-16 4:03 p.m., Moose wrote:
On 7/16/2022 7:44 AM, Martin Walters wrote:
I'm late to the focus stacking (FS) discussion. I finally decided to try FS with my E-M5-2 and the 12-100. Works fine, at least on a tripod.

First: my dumb question.

As FS moves both towards and away from the camera, where should I set the initial focus point?

You seem to be confusing recent posts about FS on later cameras, in the M1 series, with your E-M5 II. They work differently. The E-M5 II does not do the closer/further dance of the later ones. They also handle up to 13 slices, a clue to the limitations of 8.

As I wanted to shoot things several inches deep, such as big Dahlias, I set up a still life target and did some experimentation. My results agree with this DPR post. It was as a result that I stopped trying to use FS and went to Focus Bracketing, merging later, in post. I just wasn't getting what I wanted with FS - as a fussy pixel peeper.

I chose a point somewhere near the middle of the subject (an echinacea flower, which has at least 2 cm depth). The eight frames did cover the full flower. So, while this worked fine, would it be better to start at the nearest point in focus?

With your camera, start with the closest point. If using FB, with large numbers of slices, focus a little closer than closest point. Other than when testing, I work sans tripod. I focus on closest point using small point AF, then lean back a little bit, before pressing the button all the way.

The is purely empirical, as a result of missing too many actual closest points. I also only use Focus Differential of one. As the DPR post says, setting of 3 may be ideal - on a tripod. Standing in a narrow, dirt path through a rice paddy, while on the way to something else, thus with limited time, shooting an insect that will soon move, and in other similar situations, I'm shooting with a long FL, often 400 mm, hand held. There's no way I don't sway fore and aft, if only a tiny bit. By using the lowest Focus Differential, I greatly increase the chances of covering the whole depth thoroughly.

Here's four examples, using that technique, with an E-M5 II. <http://galleries.moosemystic.net/MooseFoto/index.php?gallery=Olympus_List/Stacks> It works! I'll add some more, as I think of it.

Here's a DPReview post offering an explanation of how FS works, particularly as it relates to lens aperture/DOF and the focus differential setting. Quite interesting. In my case, I did not see any lack of focus anywhere in the stacked version, with using F/4. I did not try F/8 or something like that, though a single shot at F/8 was not sufficient to have the entire flower in focus.


"Ran an experiment last night to find out what is really going on with the focus stacking in my EM-1 ii.  I was under the mistaken belief that the focus differential setting was unaffected by the aperture setting. WRONG. I was under the impression that one would not get soft focus in the composite image when using a high focus differential setting as long as one also used a large f stop. WRONG.

So, what I was actually looking for was the minimum aperture I should use at each focus differential setting within the Focus Stacking feature.  IOW, I expected that maybe at a focus differential of 1, you could use f/2.8, but at a focus differential of 5 or 7 or 9...somewhere f/2.8 would fail to cover the gaps between the 8 shots the focus stacking feature takes. I wanted to know at what point f/2.8 fails to cover the gaps. I wanted to know the same for f/4 and f/5.6 and f/8. Turns out I was asking the wrong question.

Here's what I found. At f/2.8, the highest focus differential one can use and not have soft spots between the frames and in the final jpeg is 3.  A f/4, the highest focus differential one can use is 3. At f/5.6, the highest focus differential is 3.  At f/8, 3. At f/11, 3.  Noticing a trend here?  But wait, higher f stops mean greater depth of field, right?  Why aren't the higher f stops covering the gaps?  Because as you increase your f stop number, the camera increases the space between the shots!!!!!

Yup! Exactly what I found to be true (and posted here, ages ago). It's as though their engineers determined the minimum CoC required for "sharp" and adjusted slice separation with aperture to deliver that.

So, the difference between using f/2.8 and f/11 at a focus differential of 3 is the total depth of field covered.

Exactly so. For a deep subject, set a small aperture. The right choice for an in-camera FS with only 8 slices. Unfortunately, they carried the idea over into FB.

Remember, though, that 4/3 sensors start to suffer diffraction softening by f11. I wouldn't go past f8. According to the old rules, even f8 is into diffraction limiting, but practice says it ain't so.

If you go to a focus differential of 4, you will start to see some minor soft spots in the final product. Go to a focus differential of 5 and the soft spots interspersed become more apparent. The higher you go on the focus differential scale, the larger and more apparent the soft spots get...regardless of aperture. What the aperture does is increase or decrease the total depth of field.

I thought of them as "waves of focus".

So, it finally dawned on me that what Olympus seems to be doing here is relating the coverage as much to the number of shots stacked, as with aperture. It may well be that stacking 8 shots works well with a focus differential of 3 or less. If you want to use a focus differential of 4, you'd better stack at least 9 frames . If you want to use a focus differential of 5, better stack 10 shots. If you want to use a focus differential of 2, you may only need to stack 7 shots.  As I don't have a good focus stacking software, I was not able to test this theory. The EM-1 does do focus bracketing up to 999 frames.  I'm guessing that is overkill even when you set the focus differential to 10.

Bottom line...I did the testing and now I know. For in-camera focus stacking, the magic focus differential is 3.  Use the aperture to adjust the total depth of field. If you can't get enough depth of field even with a high aperture...then you need to switch to focus bracketing and take a lot more frames, which allows you to use a higher focus differential and still not get soft spots (I call them spots, but they are really more like strips).

One other note, the place to aim your focus when using the focus stacking feature is at 1/5th of the total distance from the close edge you want in focus.  So, you want your total depth of field to be 5 inches, focus 1 inch deep from the close edge of that 5 inches you've targeted (4 inches from the back edge)."

Maybe for FS, with its limited number of slices. For real world, FB, hand held, with E=M5 II, focus on the nearest point.

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