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Am I getting this right?
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cube-tube



Joined: 10 Oct 2017
Posts: 67
Location: Durham, NC

PostPosted: Thu Jun 28, 2018 2:15 pm    Post subject: Reply with quote

perdu34 wrote:


I may be getting the wrong end of the stick here but I would think this is a really bad idea for macrophotography.


Just noticed what you said now. Those are some interesting points... I do not believe that specular highlights are an issue with confocal because the light source is large and coming from many different angles. With light sheet maybe, depending on how the sheet is set up. With 'real' light sheet, a laser is used to create a microscopically thin slice of light, but for our purposes, the sheet could be as big as half of the depth of the subject and still at least partially function by preventing light from hitting parts of the subject that we aren't trying to photograph in those frames.

I'm not totally sure what you mean by separating the input light from the reflected light, because it is definitely possible to use a beamsplitter to illuminate the subject in full color, through the objective. It is true that dichroics don't really apply, because there is no fluorescence involved.
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Thu Jun 28, 2018 3:13 pm    Post subject: Reply with quote

Alan Wood wrote:
I remember a light sheet system, probably 1970s or 1980s, long before digital, stacking, lasers and confocal, that was used to photograph insects.
...
I wish I could remember how it was done. I have not been able to find any references.

The subject was moved perpendicularly through a thin plane of illumination, on which the camera was focused.

See "Constructing a Scanning Light Photomacrography System" by Ted Clarke.

Quote:
There were amazing photos in magazines, sharp from head to tail but with odd perspective.

The perspective was orthographic, same as a telecentric lens would give.

I expect the odd appearance was mostly because of the large depth of field and the fact that the published pictures were mostly things like head-on views of flies. If the subjects had been oriented obliquely then the perspective would have looked more natural, but the presence of unlit concavities would have made the limitations of the approach more obvious.

--Rik
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JohnyM



Joined: 24 Dec 2013
Posts: 321

PostPosted: Sun Aug 05, 2018 8:32 am    Post subject: Reply with quote

rjlittlefield wrote:

There is another consideration that also affects how far away from telecentric is acceptable. That consideration is the amount of scale change there is within the DOF slab at each focus position. With a truly telecentric lens, magnification is perfectly flat as a function of distance. But when a non-telecentric lens is forced to stack as if it were telecentric, the magnification in a focus stacked image becomes a sawtooth curve that on average is flat but that varies systematically within each focus step. As a result, features that are seen in focus at the near and far boundaries of adjacent steps are seen at two different scales in those two source images. It should be clear that if those scales are too much different, then there must be some sort of artifact in the stacked image.

The acceptable tolerance for this consideration depends to some extent on the stacking method that is being used. But for the sake of discussion let's say that the scale change within each focus step should be no more than 1 pixel across the width of the frame. Then if we're using a sensor that is 5000 pixels wide, we want a scale change of no more than 1 part in 5000 per focus step. Again considering that same objective, we conclude that the focus step should not be larger than 42mm/5000 = 0.0084 mm. This is quite a lot shallower than the nominal DOF of 0.055 mm for an NA 0.1 objective, so again we conclude that the Nikon CFI BE 4X would not be a good choice to use for faking telecentricity.

In contrast, I have just now tested a Mitutoyo M Plan Apo 10X NA 0.28 objective as having a scale change of about 1 part in 15,000 with a step size of 10 microns. Since the nominal DOF of this lens is even a little shallower than that, clearly the Mitutoyo 10X can be treated as telecentric with no problems at all. Similarly the Mitutoyo M Plan Apo 5X NA 0.14 shows about 1 part in 9000 at 40 microns step, so again, no problem treating that as telecentric.

--Rik


Sorry for digging this thread.
As i understand, this "fake telecentricity" is affecting stacking, but not stiching?

I had problems with stiching images from CFI Be Plan 10x @7x FF sensor.
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rjlittlefield
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PostPosted: Sun Aug 05, 2018 9:27 am    Post subject: Reply with quote

JohnyM wrote:
I had problems with stiching images from CFI Be Plan 10x @7x FF sensor.

When you had those problems, did you have scale correction turned off in the software?

If so, then the stitching problems probably came from X/Y shifts that were introduced by either 1) physical shifts in the focus stepping mechanism, or 2) computational shifts caused by the alignment method being misled by how subjects changed their appearance as focus shifted.

The only guaranteed way to get perfect stitching is to turn off computational alignment altogether and shoot the stacks with a setup that physically maintains the proper alignment between frames and between stacks.

In this last case, the effect of using a not-really-telecentric lens will be only in the stacking, where you may get slight misalignments of features on the boundaries between focus steps, due to optical scale change within each focus step.

--Rik
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JohnyM



Joined: 24 Dec 2013
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PostPosted: Sun Aug 05, 2018 5:13 pm    Post subject: Reply with quote

I didnt turned off scale correction or XY corrections. Next subject i'll run with those disabled. Thanks.

Another question: If telecentricity, beyond certain magnification and aperture does not have an impact on the image, due to shallow DOF, why such lenses exists?
Ie: Nikon MM or Mitutoyo 10x / 20x / 50x / 100x

Also, could you elaborate a little on your method of determining "faked telecentricity" according to entrance pupil of the lens? Im really struggling to understand telecentricity.
You divided subject-entrance pupil distance by sensor resolition, to determine how shallow DOF should be to treat lens as telecentric.
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rjlittlefield
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PostPosted: Sun Aug 05, 2018 10:02 pm    Post subject: Reply with quote

JohnyM wrote:
Another question: If telecentricity, beyond certain magnification and aperture does not have an impact on the image, due to shallow DOF, why such lenses exists?
Ie: Nikon MM or Mitutoyo 10x / 20x / 50x / 100x

The impact at high magnification is minor, but maybe measurable.

More important, suppose you are a manufacturer. Which do you think is the better approach?
1) Provide a higher magnification lens in the same series as the lower, where telecentricity really is important, or
2) try to convince your customers that it doesn't matter.

I'm pretty sure that I would go with option 1.

Quote:
Also, could you elaborate a little on your method of determining "faked telecentricity" according to entrance pupil of the lens? Im really struggling to understand telecentricity.

Think about simple perspective. The apparent size of an object varies inversely with its distance from the center of perspective. That center of perspective is the entrance pupil. So, the relative scale change within DOF is roughly proportional to DOF/(distance from subject to entrance pupil).

One key characteristic of a telecentric lens is that scale does not change at all, depending on distance from lens to subject. Another key characteristic is that the entrance pupil is at infinity.

These two characteristics are consistent with thinking about scale change through perspective, because DOF/infinity = zero.

Does this help?

--Rik
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