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Optical aberration or a possible stack artifact?
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cmagno



Joined: 08 Oct 2012
Posts: 48
Location: Porto, Portugal

PostPosted: Wed Jun 19, 2019 4:34 pm    Post subject: Optical aberration or a possible stack artifact? Reply with quote

I would like to submit for evaluation of the forum a situation that I have come across in some images that I produce and that is also possible to see in images of other users.
The problem occurs in parts of the image where you can see much brighter points than the background that surrounds them.

In short:
    - It can happen anywhere in the image (center, corners, ...).
    - It resembles a coma, always presenting itself in the shape of a cross.
    - Apparently, none of the images that make up the stack presents a similar problem.


I will share with you one of my last examples where this effect is observed, a photograph of an amber inclusion.

Picture taken with a very simple setup: Nikon D850 in Full Frame Mode and Lomo 3.7x NA 0.11 with a 125mm extension from base lens to sensor plane.
Final Zerene Stacker PMAX image without any kind of post processing, where the affected area is marked:




Cropped area (100%) with some artifacts marked an example, you can see quite a few more, and there are other in various locations of the image:



In the next animation, we can see the evolution of the stack and suggest that the effect is due to a "movement or stretching of the focused / bright area" between the images:



----
EDIT: In doubt, I ended up replacing the link with an image with less stacks in order to comply with the 300kb rule and so it is immediately available showing what is intended.

Does anyone have an idea of what might be happening?
If it is not a problem exclusively from my optical setup, can I do something to avoid or minimize it?


I hope I was clear in my exposition.

All comments, suggestions or explanations are welcome.

Best regards,
Carlos Magno


Last edited by cmagno on Thu Jun 20, 2019 2:47 pm; edited 1 time in total
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ChrisR
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PostPosted: Wed Jun 19, 2019 5:09 pm    Post subject: Reply with quote

I think it's coma/astigmatism which, as you focus through the point, gives blur which goes from an out of focus (eg)north-south elongated blob to one at (eg) east-west.

Pmax will grab all of what looks like subject detail, hence the crosses.
My first try would be to retouch with straight Dmap output. It may need substacks of small numbers of frames, to pick from.
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rjlittlefield
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PostPosted: Wed Jun 19, 2019 8:57 pm    Post subject: Reply with quote

I agree, except I would narrow the aberration to astigmatism. As far as I know, coma will not produce the tangentially elongated bar that forms the cross.

In any case the defect is normally not seen at image center. You're getting it here because you're shooting through dense medium that is not perfectly flat and homogeneous.

--Rik
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Lou Jost



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PostPosted: Wed Jun 19, 2019 9:12 pm    Post subject: Reply with quote

A partial solution is to look for lenses that are designed to shoot through a thick medium. This includes reversed Micro-Four-Thirds lenses (designed to shoot through 4mm thick glass), reversed digital camera lenses (designed to shoot through varying thicknesses of glass depending on model), G Plan Apo Mitutoyo lenses (designed to shoot through 3.5mm glass), LCD Plan Mitutoyo objectives (designed to shoot through 1.5mm I think), some Reichart objectives (designed to shoot through 1.5mm quartz oven window), etc.
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enricosavazzi



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PostPosted: Thu Jun 20, 2019 2:40 am    Post subject: Reply with quote

It is not necessarily an aberration caused by the lens and/or the thickness of the amber/copal matrix surrounding the fossil (although they remain possible).

The alternative is aberrations caused by uneven curvature of the surface of the amber/copal block. They are ground and polished, but not to the standards of optical surfaces. To avoid this problem one can immerse the material in a liquid of the same refraction index and shoot through the surface of the liquid (held flat by surface tension, although not in proximity to the edges of the container).
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Peter M. Macdonald



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PostPosted: Thu Jun 20, 2019 3:04 am    Post subject: Reply with quote

It is a simple matter to try immersing the amber in fluid to see if Enrico is correct. I rather suspect that he is.

The easiest and cheapest liquid with a very similar refractive index to amber, at least here in the UK, is to use Johnson and Johnson Baby Oil. The citrus scented version is quite pleasant on the nose if you are working with amber for prolonged periods.

Another, closely related possibility, is that the lens which you are using is designed for use with a cover glass. Not sure if that is the case with the Lomo which you are using. In that case, it is best to give it what it is looking for. Place a cover glass on the specimen. Hold it in place with a small blob of KY Jelly. It also has a very similar refractive index to that of amber.
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enricosavazzi



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PostPosted: Thu Jun 20, 2019 3:41 am    Post subject: Reply with quote

Thinking about photography of amber inclusions, there must be scientific literature about this. A quick search found the following, but likely there is more:

https://www.researchgate.net/publication/249558145_Photography_of_amber_inclusions_in_the_collections_of_National_Museums_Scotland

Edit: some information and references also here:
http://www.paulselden.net/uploads/7/5/3/2/7532217/earthsciencereview2017.pdf
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Lou Jost



Joined: 04 Sep 2015
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PostPosted: Thu Jun 20, 2019 6:08 am    Post subject: Reply with quote

Quote:
Another, closely related possibility, is that the lens which you are using is designed for use with a cover glass. Not sure if that is the case with the Lomo which you are using. In that case, it is best to give it what it is looking for. Place a cover glass on the specimen. Hold it in place with a small blob of KY Jelly. It also has a very similar refractive index to that of amber.


That might help take care of the flatness problem but will only increase the spherical aberration problem, since there is already at least a 0.17mm thick layer of glasslike material (amber) over the insect.

But anyway, the Lomo does not expect a coverslip, as far as we know. On the other hand, the Lomo NA is low enough that the presence or absence of a cover slip should not matter.

How deep in the amber is the insect? That's relevant to understanding the source of the aberrations.
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cmagno



Joined: 08 Oct 2012
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Location: Porto, Portugal

PostPosted: Thu Jun 20, 2019 4:26 pm    Post subject: Reply with quote

Thanks for all comments.

I should, however, like to add the following, I agree that this effect may be aggravated by the fact that it is an amber inclusion, but it is quite possible to see the same behavior in "normal" studio insect macro photographs.

On the other hand, I have found that using an iris, the effect can be substantially reduced by increasing the depth of field.

ChrisR wrote:
I think it's coma/astigmatism which, as you focus through the point, gives blur which goes from an out of focus (eg)north-south elongated blob to one at (eg) east-west.

Pmax will grab all of what looks like subject detail, hence the crosses.
My first try would be to retouch with straight Dmap output. It may need substacks of small numbers of frames, to pick from.


rjlittlefield wrote:
I agree, except I would narrow the aberration to astigmatism. As far as I know, coma will not produce the tangentially elongated bar that forms the cross.

In any case the defect is normally not seen at image center. You're getting it here because you're shooting through dense medium that is not perfectly flat and homogeneous.


The effect varies as you go deeper:


In this case, as you can see in the image on the left, it starts by being vertical and then having a horizontal drag.
In a first evaluation, I would say that the cross as the final effect would be the result of the information collected in all intermediate states.


Lou Jost wrote:
A partial solution is to look for lenses that are designed to shoot through a thick medium. This includes reversed Micro-Four-Thirds lenses (designed to shoot through 4mm thick glass), reversed digital camera lenses (designed to shoot through varying thicknesses of glass depending on model), G Plan Apo Mitutoyo lenses (designed to shoot through 3.5mm glass), LCD Plan Mitutoyo objectives (designed to shoot through 1.5mm I think), some Reichart objectives (designed to shoot through 1.5mm quartz oven window), etc.


Thanks for the suggestion. At the moment I do not have any such lens to do tests. I will do a search and try to recover one in good condition.

Lou Jost wrote:
How deep in the amber is the insect? That's relevant to understanding the source of the aberrations.


The inclusion is about 4mm deep. The amber is exceptionally transparent allowing a good field of view. However, the density is there, it is a fact.

Regards,
Carlos Magno
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cmagno



Joined: 08 Oct 2012
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Location: Porto, Portugal

PostPosted: Thu Jun 20, 2019 4:28 pm    Post subject: Reply with quote

enricosavazzi wrote:
Thinking about photography of amber inclusions, there must be scientific literature about this. A quick search found the following, but likely there is more:

https://www.researchgate.net/publication/249558145_Photography_of_amber_inclusions_in_the_collections_of_National_Museums_Scotland

Edit: some information and references also here:
http://www.paulselden.net/uploads/7/5/3/2/7532217/earthsciencereview2017.pdf


Thank you for the shared documentation.
All the information on this matter is a precious help.

Kind regards,
Carlos Magno
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Lou Jost



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PostPosted: Thu Jun 20, 2019 4:32 pm    Post subject: Reply with quote

Yes, that's astigmatism. Sometimes it can be reduced by using DMap (in Zerene) or the equivalent in other stacking programs.

4mms is a lot. It is also almost exactly the Micro Four Thirds sensor cover stack, so if you reversed one of those lenses on top of a telephoto lens, making a paper aperture between them, you might eliminate it, if it is due to the medium.
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cmagno



Joined: 08 Oct 2012
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PostPosted: Thu Jun 20, 2019 4:34 pm    Post subject: Reply with quote

Peter M. Macdonald wrote:
It is a simple matter to try immersing the amber in fluid to see if Enrico is correct. I rather suspect that he is.

The easiest and cheapest liquid with a very similar refractive index to amber, at least here in the UK, is to use Johnson and Johnson Baby Oil. The citrus scented version is quite pleasant on the nose if you are working with amber for prolonged periods.


I know the approach but never experienced it for the simple reason that I did not know what fluid I could use to make the experiment. Your suggestion in using J & J is a great idea.

The only disadvantage I see in the immediate is that this technique is more suited to a vertical setup, which is not my case.

I have to think of a solution because the immersion of amber will solve other problems, such as photographing inclusions on curved surfaces.

Best regards,
Carlos Magno
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Lou Jost



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PostPosted: Thu Jun 20, 2019 4:36 pm    Post subject: Reply with quote

If you can't make a vertical system, can you use a first-surface mirror at 45 degrees?
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cmagno



Joined: 08 Oct 2012
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Location: Porto, Portugal

PostPosted: Thu Jun 20, 2019 4:40 pm    Post subject: Reply with quote

Lou Jost wrote:
4mms is a lot. It is also almost exactly the Micro Four Thirds sensor cover stack, so if you reversed one of those lenses on top of a telephoto lens, making a paper aperture between them, you might eliminate it, if it is due to the medium.



Can you give me the example of such a lens to know exactly what to look for?

Thanks in advance,
Carlos Magno
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Lou Jost



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PostPosted: Thu Jun 20, 2019 5:00 pm    Post subject: Reply with quote

All Micro Four Thirds lenses, also the older Four Thirds lenses, which have longer working distances but not such a wide variety. Google them.

But first check to see if the medium really is the problem, and try DMap.
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