Focus stacking by moving only the camera (not the lens)?

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Beatsy
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Focus stacking by moving only the camera (not the lens)?

Post by Beatsy »

Sometime in the dim and distant past, I read an article or post that stated the "best" way to alter focus when stacking was to use a bellows and move only the camera. That's all I remember. I don't know what kind of lenses it applied to either, or how it would apply, and thinking about that has made me wonder if I mis-remembered. Could it work with infinity objectives (or their tube lenses)?

FWIW, logically separating camera and optics like this would have a lot of practical benefits for me and the idea has rattled around in my head for years. Rig layout and stability during adjustment could be improved and sharing cameras with other uses would be far easier (less disassembly that disturbs settings). I've been hitting these problems more often with current activities. Coincidentally, I have a good friend with a well equipped workshop who's run out of projects and wants something to tinker with. If this works, it would need some nicely crafted bits to make it "proper". It would be inconsiderate of me to not let him have a go at 'em, don'tcha think? :-)

Trouble is, I've searched and can't find a reference to this (so maybe I *am* making it up in my head). Have I gone mad, or is this a thing? Can someone point me at a doc or discussion? Ta.

Enoplometopus
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Enoplometopus »

Beatsy,

I remember having seen it in products of the German company Novoflex: They have a special bellows unit („Castbal Pro“) operated with their stacking unit („Castel Micro“) that seems to do just what you mentioned. I have tried it a while ago but didn‘t get along with it. Probably the objectives I use (like Mitu 5, 10, 20) don‘t fit to that technique.

https://www.novoflex.de/de/produkte/mak ... micro.html

Best regards,

Daniel
Best regards,

Daniel Knop/Enoplometopus
My website:www.knop.de
Instagram: danielknop10

Doppler9000
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Doppler9000 »

Nathan Myhrvold followed this approach, and fabricated a nice set up. He wrote a fair bit on focusing “by rear”.

https://www.photomacrography.net/forum/ ... 25&t=34619

https://www.photomacrography.net/forum/ ... 77#p213077

Rik put together a table...

https://zerenesystems.com/cms/stacker/d ... versusrail
Last edited by Doppler9000 on Mon Apr 05, 2021 8:46 am, edited 2 times in total.

rjlittlefield
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Re: Focus stacking by moving only the camera (not the lens)?

Post by rjlittlefield »

Beatsy wrote:
Mon Apr 05, 2021 2:17 am
or is this a thing? Can someone point me at a doc or discussion?
It's definitely a thing. See discussion at https://zerenesystems.com/cms/stacker/docs/troubleshooting/ringversusrail . The key point is that focusing by moving only the camera guarantees that there is no change of viewpoint (center of perspective) from one frame to another. So, this focusing by "bellows rear" movement totally eliminates some kinds of smearing artifacts that arise when the lens-to-subject distance is varied.

The technique is most applicable for magnifications surrounding 1:1. For very low magnifications, such as focus stacking your backyard, you might as well use the focus ring on the lens because there won't be any significant change in viewpoint anyway. For very high magnifications, say 10X and above, DOF per frame is so shallow that again, there won't be any significant change in viewpoint within the in-focus slab. There's another problem at high magnifications, that the required amount of rear bellows movement becomes too large because of Z-axis elongation of the subject in the image. (Depth elongates by the square of the lateral magnification.) But in the middle range, it is both feasible and effective, hence the description "Ideal" in the page I linked.

--Rik

Lou Jost
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Lou Jost »

Another problem with rear standard focusing at high magnifications is that the lens will be operating away from its design point for much of the session. A high NA objective is designed for certain specific distance from lens front to subject. When you use the rear standard for focusing on a three-dimensional object, you will sometimes be using the objective to focus on nearer or farther distances than it was designed to do.
Last edited by Lou Jost on Tue Apr 06, 2021 5:59 am, edited 1 time in total.

ray_parkhurst
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Re: Focus stacking by moving only the camera (not the lens)?

Post by ray_parkhurst »

My most recent system configuration, intended for ~2x mag stack and stitch, is convertible between camera+lens and camera-only stacking. I've done quite a few camera+lens SnS, and so far just a few camera-only. What I've seen so far is that the camera-only stacking gives an undesirable amount of magnification change across the stack, which in my case is quite a bit shallower (<0.5mm) than what others may be shooting. For the camera+lens stacks I have been able to turn off scaling (as one would do for telecentric optics) and the resulting distortions are manageable by the tiling program. My first few camera-only stacks were also done with scaling off, but tiling was much more difficult and resulted in visible stitching errors. Turning scaling on reduces sharpness, so I'm loathe to do it. This is now a prime driver for me to go with a larger sensor to minimize the number of tiles and hopefully improve the tiling errors without having to do scaling with camera-only stacking.

Beatsy
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Beatsy »

Thanks for the comments everyone. This does actually look very promising for 1x to around 10x (and hopefully just a little higher - maybe 12x-15x too). Definitely something I'm going to try with Mitties and a little 35mm scanner lens (I like the idea of preserving lens perspective as it could help with some compositing ideas I've been musing about too...

Rik: I'd forgotten about depth of focus (not field) being subject-DoF squared. Sounds bad, but in the focus range given I think it could be a benefit in that I don't need a rail that does sub-micron steps when using pushed-down objectives with silly-shallow DoF. I have one bodged setup which can do it, but two, more stable, setups that max out (or min out) at 1 micron. Big steps of the camera producing much smaller steps in the object focal plane would be useful with my current kit. Very handy for high res, lo mag work. Assuming I've understood that correctly.

Ray: Ref stitching, I was thinking about that the other day in the context of testing a new wide-angle lens on a pano head. I assume Mitties (and any objectives really) have a nodal point just like regular lenses. Could it be possible to rotate the whole system (rail, camera and objective) around this nodal point to frame the various pano-tiles rather than just shifting the subject (or rig) in x and y? Assuming it works, then in combination with moving camera and not the lens for stacking, this would provide parallax free tiles and thus avoid stitching errors. Right? This is total theory, I haven't tried it, and have yet to think of a simple way to find the nodal point (easy with wide-angle lenses and huge DoF but not so obvious how to do it with the razor thin DoF of objectives). But it looks feasible, so something I plan to look into at some point, but I'd be interested to see examples if it has been done before.

Thanks again all - I'll report back here with any developments...

ray_parkhurst
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Re: Focus stacking by moving only the camera (not the lens)?

Post by ray_parkhurst »

Beatsy wrote:
Mon Apr 05, 2021 10:30 pm
Ray: Ref stitching, I was thinking about that the other day in the context of testing a new wide-angle lens on a pano head. I assume Mitties (and any objectives really) have a nodal point just like regular lenses. Could it be possible to rotate the whole system (rail, camera and objective) around this nodal point to frame the various pano-tiles rather than just shifting the subject (or rig) in x and y?
The stitching software I use (ICE) has selections for different panning methods such as "planar", "planar with perspective", etc to account for these methods. I doubt it would make any difference to my work given the relatively shallow topography of my subjects, but if you were doing something more complex like a bug or whatever then a perspective-based method might do better than pure planar on things like legs and antennae. Implementation would not be for the faint of heart! The entire stacking axis would need to be tilted in pitch and yaw.

I suppose the problem I described before, ie change in magnification with camera-only stacking causing stitching errors, might be solved with a workflow change. I do stacking of the individual tiles, then tile the panorama, so each tile has variable-magnification elements. Maybe tiling each focal plane, then stacking the resulting images would give a better result, but that is yet another daunting workflow I would want to avoid.

rjlittlefield
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Re: Focus stacking by moving only the camera (not the lens)?

Post by rjlittlefield »

Beatsy wrote:
Mon Apr 05, 2021 10:30 pm
have yet to think of a simple way to find the nodal point
The "nodal point" that you're talking about is actually the center of the entrance pupil, the so-called "no-parallax point" for panorama stitching. This is also the viewpoint = center of projection for normal perspective.

The best way I know to locate that point with a microscope objective is to lock the optics, vary the distance from lens to subject, measure change in magnification versus change in distance, then do the geometry to calculate where the entrance pupil must be in order to produce the observed ratio of changes. For example, if you observe a change in magnification of 1.0:1.0001 (one part in 10,000), for a change in distance of 5 microns (0.005 mm), then you know that the entrance pupil must be located 10,000 * 0.005 mm = 50 mm away from the focus plane.

The change in magnification is just the Scale component of Zerene Stacker's alignment information (File > Save Other > Save Registration Parameters). For a highly detailed target, changes in scale can be measured down to a small fraction of a pixel.

A good reference discussion for this is at https://www.photomacrography.net/forum/viewtopic.php?t=39677 , "On the sensitivity of Zerene Stacker scale measurements". I strongly recommend to read that before running any experiments, since there are some aspects of technique that can be unexpectedly important.

--Rik

Macrero
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Macrero »

Last week I was experimenting with that "system" in the 1.5X - 30X+ magnification range. It was fun. There are several advantages in theory, but in practice I did not find any significant advantage over the "traditional" method, and I found it quite a bit more uncomfortable to work with.

Try it and decide for yourself, in my opinion it's not worth the hassle.

- Macrero
https://500px.com/macrero - Amateurs worry about equipment, Pros worry about money, Masters worry about Light

Lou Jost
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Lou Jost »

One thing to note is that acquiring images by moving the whole camera or the rear standard can prodiuce a more accurate image than using the lens focusing ring, which typically changes perspective/magnification much more than the other two methods as the stack is acquired.

Macrero
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Macrero »

Easiest way (the one I used) to try before spending time, effort and money on a good setup, is removing the front part of the bellows and mounting it on a holder/bracket, leaving the rear part with the camera on the rail. Sorry I didn't take pics, but I hope it is clear enough.
https://500px.com/macrero - Amateurs worry about equipment, Pros worry about money, Masters worry about Light

Beatsy
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Re: Focus stacking by moving only the camera (not the lens)?

Post by Beatsy »

Yes, I have a very nice old bellows languishing on a shelf - so that was the plan for first experiments. But I'm still a few experiments deep with other things at the mo - so on the list for near future...

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