Adding "spacer" between an infinite objective and tube lens

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mjkzz
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Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

This is probably very basic knowledge and has been discussed on this forum, yet, sometimes, it is rather important to re-visit some fundamentals.

Objective is a genuine Mitty 5X MPlan APO, it is an infinite objective. Two images were captured, one with the Mitty mounted as close to the tube lens (Raynox DCR-150) as possible, the other was captured with 35mm extension tube between the Mitty and the tube lens (Raynox DCR-150). Overlapping the two images with "difference" used in composition between layers, it shows that there are no (noticeable) change in magnification. Most of differences are due to focus (seems to me)

Finally, making it a bit complicated (vs basic), what are other advantages or disadvantages by doing this? I personally do not know this :D But I remember a post here claiming the Mitty 5X works better with some extension added, not sure if that is true, but since it does not affect magnification, I have been using it that way for many images.

Image captured with "normal" (ie, as close as possible) setup
Normal.jpg
Image captured with extended spacer between infinite objective and tube lens
EXTENDED.jpg
Difference
DIFF.jpg
Normal setup
normal_lens.jpg
Extended setup
extended_lens.jpg

Adalbert
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Re: Adding "spacer" between an infinite objective and tube lens

Post by Adalbert »

Hi Peter,
In my case, the distance between the lens and the Raynox is 50 mm.
viewtopic.php?p=245808#p245808
This results from the fact that there is a mount and an iris between them.
BTW, yes, I use the iris but mostly with Mitu 50x / 0.55
Best, ADi

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

Cool. But, I used to use M42 extension tubes personally, they have some disadvantages. 1. Inside diameter is too small, making it difficult to flock it with thicker material (ie, none-paint) 2. most M42 extension tubes I can find have smooth interior, very reflective, hence the flocking. I get a lot of haze if not flocked. 3. With thicker flocking material and on a full frame, I get vignetting.

So I eventually switched to larger diameter tubes with knurled inside to reduce internal reflection. It worked so well, I do not have flock the inside anymore which also saves me from fibre (those black velvet like material with glue backing) falling onto the sensor.

Adalbert
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Re: Adding "spacer" between an infinite objective and tube lens

Post by Adalbert »

Hi Peter,
I use the light trap from protostar:
https://www.fpi-protostar.com/hitack.htm
in the following way:
I_F.jpg
Best, ADi

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

wow, very cool and seems effective, no internal reflections.

Back to subject :D
BTW, yes, I use the iris but mostly with Mitu 50x / 0.55
Best, ADi
Is this why you use Iris to block those out of focus points? As per Rik in another thread.
It's important to realize that the space behind the objective is "infinity" only for the perfectly in-focus subject plane. For any other part of the subject, the same space is not infinity because for those parts, the light rays do not form parallel bundles.

It's also important to remember that placement of the aperture has no effect on the position of perfectly in-focus points. Placement of the aperture affects only the position of blur circles for out-of-focus points, and it is precisely those points for which the "infinity" space is not that at all.

--Rik
Since those out of focus points do not form parallel rays, that means the longer and narrower the extension tube (between objective and tube lens), the better? Or like ADi did, add an Iris . . .

I got one Iris and I should give it a try.

Adalbert
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Re: Adding "spacer" between an infinite objective and tube lens

Post by Adalbert »

Hi Peter,
Is this why you use Iris to block those out of focus points? As per Rik in another thread.
not only.
I use the iris on every lens to find the subjects of interest. When I close the iris,
I increase the DOF (decrease the resolution) and can locate the subject faster.
Then I open the iris (increase the resolution) for the stacking.

But only at Mitu 50x / 0.55x do I close the iris a bit for stacking as well.
This removes some reflections.
With other lenses I have, this is not necessary.

Best, ADi

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

Thanks ADi. Clever ideas.

Just did a quick and dirty test with a disk with a hole in the center, discovered something fundamental and interesting. Wanted to continue with an iris but could not find the right adapter ring for it. Adapters are coming, but unfortunately, I will be back in crazy mode of another project :D, but will find time to finish this, really interesting stuff.

M42 to M42 Iris
P1030111.jpg

Lou Jost
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Re: Adding "spacer" between an infinite objective and tube lens

Post by Lou Jost »

Robert has tested this issue extensively, as did Nathanm earlier. Robert's results are available on his website, closeuphotography.com, which I highly recommend. The ideal distances turned out to depend very much on the exact lens in question and on the desired format (some set-ups improved full frame corners but decreased center sharpness, making APS images worse), unfortunately. Longer distances are better for some kinds of microscopy because you can fit an illuminator in the space between objective and tube lens.

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

Lou Jost wrote:
Mon Oct 18, 2021 12:00 pm
Robert has tested this issue extensively, as did Nathanm earlier. Robert's results are available on his website, closeuphotography.com, which I highly recommend. The ideal distances turned out to depend very much on the exact lens in question and on the desired format (some set-ups improved full frame corners but decreased center sharpness, making APS images worse), unfortunately. Longer distances are better for some kinds of microscopy because you can fit an illuminator in the space between objective and tube lens.
Thanks Lou, great info.

I am a visual person but to certain extent -- I prefer to see binary results, ie, either yes or no. And to get that, I would design a setup to achieve that. For example, to check an optic is NOT telecentric, I need to check images sizes when the SAME set of images stacked from front and back (reversed). In this case, you really do not need a reference optic. When comparing two optics on NOT telecentricity, compare how much difference in sizes (produced from same set of images for the same optic), if one optic produces more difference, it is worse in term of telecentricity. For near telecentric optics, deeper stack is needed. The comparison do not need to have SAME scene because we are comparing images stacked from the optics themselves. But same lighting, same stacking depth and preferrablely same subject, as well as keeping the scene the same, are highly desired. With these in mind, I can draw a conclusion that one is worse than the other in terms of telecentricity. How much worse? I do not know, but it is worse, it is a binary answer and to get this answer, the setup is much less strict in terms of keeping all parameters the same - there is NO way my equipment can produce quantitative result as absolute value and draw conclusion based on that.

I greatly admire Robert's patience and skills, also greatly appreciate his effort making all these available to us. There are a lot of numerical results, like image quality rating, curvature etc. I personally might be able to tell the difference between sharpness of 8 and 9, but not 8.3 and 8.7. Furthermore, maybe some more tests on 3D subjects where most of stackers deal with. So far, most of Robert's tests are planar, only deal with focused (or near focused) points. But like Rik pointed out, when out of focus stuff gets involved, things could be very different and after all, this is why we need focus stacking.

Anyways, thanks for the info, it looks like I won't get the adapters anytime soon.

RobertOToole
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Re: Adding "spacer" between an infinite objective and tube lens

Post by RobertOToole »

mjkzz wrote:
Mon Oct 18, 2021 6:05 pm

This is probably very basic knowledge and has been discussed on this forum, yet, sometimes, it is rather important to re-visit some fundamentals.

Objective is a genuine Mitty 5X MPlan APO, it is an infinite objective. Two images were captured, one with the Mitty mounted as close to the tube lens (Raynox DCR-150) as possible, the other was captured with 35mm extension tube between the Mitty and the tube lens (Raynox DCR-150). Overlapping the two images with "difference" used in composition between layers, it shows that there are no (noticeable) change in magnification. Most of differences are due to focus (seems to me)

Finally, making it a bit complicated (vs basic), what are other advantages or disadvantages by doing this? I personally do not know this :D But I remember a post here claiming the Mitty 5X works better with some extension added, not sure if that is true, but since it does not affect magnification, I have been using it that way for many images.
I can answer this from experience since I just ran something like 10 tube lenses each in about 4 or 5 configurations, including tube lens to objective, all with the Mitutoyo M Plan APO 5x 0.14, and all within the last 2 weeks!

In my recent experience the distance from tube lens to objective is more of aberrations and corner shading, not any focal length change as you mention. This is easy to explain since in this space, the rays are parallel.

I actually point this out in the test notes from my Tube Lens Test from 2018 (https://www.closeuphotography.com/tube-lens-test) where you can find this info in the notes for each tube lens tested. #-o

The DCR-150 was best with minimal space, I mention this in the test I posted last week. https://www.closeuphotography.com/200mm-tube-lens-test
mjkzz wrote:
Mon Oct 18, 2021 6:05 pm

......I greatly admire Robert's patience and skills, also greatly appreciate his effort making all these available to us. There are a lot of numerical results, like image quality rating, curvature etc. I personally might be able to tell the difference between sharpness of 8 and 9, but not 8.3 and 8.7. Furthermore, maybe some more tests on 3D subjects where most of stackers deal with. So far, most of Robert's tests are planar, only deal with focused (or near focused) points. But like Rik pointed out, when out of focus stuff gets involved, things could be very different and after all, this is why we need focus stacking.

Okay then, feel free to forget about the answer above (joking). So why are you asking for advice from others on this in all seriousness?


Best,

Robert

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

@Robert, cool, greatly appreciate your work.

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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

OK, managed to put the iris on the Raynox DCR150. It is crazy, with QV and Mitty 5x, stopping down the iris actually made them hypercentric, and be honest, first time I have seen this effect. With a known non-telecentric objective, the difference between stacked forward and reversed actually gets smaller when iris is at its minimum, of course a lot of vignetting [edit] and reduced sharpness.

Looks like too much work, comparing all kind of objectives, put the iris at different locations, etc, etc, so I am going to give up :D

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

Here is a MUST SEE video (to me) while playing with iris (thanks to ADi) placed in the "infinity space". A "MUST SEE" video if you have not seen what hypercentric effect is :D

https://youtu.be/PtChcgA_cac

Please do not subscribe as it is personal account and it is just a temporary video holder.

RobertOToole
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Re: Adding "spacer" between an infinite objective and tube lens

Post by RobertOToole »

mjkzz wrote:
Mon Oct 18, 2021 10:31 pm
Here is a MUST SEE video (to me) while playing with iris (thanks to ADi) placed in the "infinity space". A "MUST SEE" video if you have not seen what hypercentric effect is :D

https://youtu.be/PtChcgA_cac

Please do not subscribe as it is personal account and it is just a temporary video holder.
Didn't expect that! Interesting!

Hypercentric, I think Edmunds sells lenses designed for that. Also Pericentric, the edges go the opposite way, they look out so you can see 360 degrees with pericentric lenses.

Best,

mjkzz
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Re: Adding "spacer" between an infinite objective and tube lens

Post by mjkzz »

OK, here is how it looks. Did some quick experiments, it seems the closer the iris to the tube lens, the more hypercentric effect. Played a bit with other parameters, all affects how the system response. Too busy for this now, but maybe later I will document more on this experiment, I believe it is rather interesting and a way to understand optics better. Or maybe somebody with more patience can continue :D
HyperSetup.jpg

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