Perfect distance between Sigma LSA and microscope lenses=?

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rjlittlefield
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Post by rjlittlefield »

Big difference, but very confusing.

In the upper right corner, the 50 is much sharper and with less CA. In the lower left corner, the 50 still has less CA but the sharpness is reversed: 10 much sharper than 50. In the lower right corner the 50 has less CA and the sharpness is about the same. (There's nothing in the upper left corner to look at.)

At center, oddly, the 10 is noticeably sharper than the 50. The reason I say "oddly" is that at the center, changing separation should have no effect. If everything is properly aligned and all aberrations are radially symmetric, the same bundle of rays from the objective will go through the same part of the tube lens at the same angles, so you end up with the same image on the sensor.

Since they're not the same, it seems that something is in play besides separation. The 50 looks like it might be a bit motion blurred in an up/down direction. All the lines are sharp on one side, blurred on the other. Either that or you have some asymmetric aberrations in play, so the rotational angles of the lenses matter.

If these were my results, my next step would be to get a target with detail everywhere in the frame, and run another set of tests to see what parts of these results are even repeatable.

By the way, how are you illuminating these? In your earlier results, I noticed that the mid-to-dark grays have quite a bit of pixel noise. Lens tests are best shot at lowest ISO and with flash, to kill noise and vibration as much as possible.

--Rik

Adalbert
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Post by Adalbert »

Hello Rik,

Thank you for your analysis!

The test-photographs have been taken by using of the flash and with ISO=100.

My last test-subject looks like that:
https://www.ebay.com/itm/Cross-Dot-0-01 ... 3cf5ea9b22

What do you think about this one?
https://www.ebay.com/itm/Microscope-Eye ... 0705.m4780

BTW, my current setup:
Image


BR, ADi

ChrisR
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Post by ChrisR »

While looking at the calibration grid you found, I saw this strange thing:
https://www.ebay.com/itm/Microscope-Eye ... 5731.m3795

Image
Scaling from the monitor screen, the whole patch of squares is about 4mm across.
The smallest squares, #23, are about 160µm square, having 9 lines & gaps therefore about 9.4µm thick.

I don't think I've seen those before. Interesting, but maybe not too useful!
Chris R

Adalbert
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Post by Adalbert »

Hi Chris,
I have already seen that, you found. It is not bad :-)
But what about a square (1mm*1mm) with two diagonals (printed by the laser printer) for the LU Plan 20x ?
BR, ADi

rjlittlefield
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Post by rjlittlefield »

Adalbert wrote:BTW, my current setup:
Perhaps there is some distortion in the photo, but as shown, I have some concerns about possible misalignment:

Image

Bellows are very convenient, but they are not very good at maintaining precise parallelism between the front and back standards. I would check with machinist's calipers to be sure.

The apparent misalignment between objective and tubes is more puzzling. For that I would check by rolling the tubes + objective on a flat surface and seeing if the objective wobbles.
what about a square (1mm*1mm) with two diagonals (printed by the laser printer) for the LU Plan 20x ?
A rectangle with 3:2 aspect ratio would be better, since that could hit all four corners.

But I suggest considering less highly structured solutions. in my own targets, the most useful part has turned out to be the "light gray" background, which the printer and photocopier have turned into a well distributed array of toner dots. See Tube lens tests on D800E full frame for illustration. An even more dense and uniform array of dots would be better, but that will give you the idea.

--Rik

Adalbert
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Post by Adalbert »

Hello Rik,
I have tried to print a rectangle :-)

Tested with the LU Plan 10x / 0.30
10mm
Image
https://farm5.staticflickr.com/4747/259 ... 747c_o.jpg

50mm
Image
https://farm5.staticflickr.com/4655/259 ... 1842_o.jpg

BR, ADi

ChrisR
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Post by ChrisR »

They look very close to me. Good test, thanks Adi.
Chris R

lonepal
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Post by lonepal »

What do you think about this?
( I am using this value.).

http://www.photomacrography.net/forum/v ... c&start=22

The best distance is 76,5mm according to this document.
Regards.
Omer

Chris S.
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Post by Chris S. »

lonepal wrote:What do you think about this?
( I am using this value.).

http://www.photomacrography.net/forum/v ... c&start=22

The best distance is 76,5mm according to this document.
Omer, the above represents a widespread misunderstanding of Mitutoyo literature. What Mitutoyo meant was that for optimal results, the objective should be placed within 76.5mm of the tube lens. Not at 76.5mm of the tube lens. If I recall correctly, Mitutoyo also provided a formula to help users who wanted more space than this estimate vignetting. They did not mean to imply that distances less than 76.5mm were sub-optimal. This misinformation seems to be based on an unfortunately-brief word choice in a Mitutoyo diagram or two, the need to dig into small print to understand what was really meant, and a writer at Edmund Optics who got it wrong. (Edmund Optics is a generally excellent source of information--this was unusual for them.)

Mitutoyo has expressly stated that shorter distances are just as good as 76.5mm. I can confirm this based on personal experience with a Mitutoyo tube lens and many Mitutoyo objectives.

Cheers,

--Chris S.

lonepal
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Post by lonepal »

Thanks for explaining Chris!

But it also says ''If the distance is too short, it may cause vignetting; if it is too long it causes dim image and dark images.''

Then what is the minimum distance?

I also used the Mitty 5X on a raynox as close as possible but there was no vignetting (APSC).

There is no clear information about this.

Bytheway I have no problem about the distance but this matter should be clarified I think :)
Regards.
Omer

rjlittlefield
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Post by rjlittlefield »

Adalbert wrote:Hello Rik,
I have tried to print a rectangle :-)

Tested with the LU Plan 10x / 0.30
The new target is very nice.

The results are still troubling.

Upper left, quite similar.
Lower left, much sharper at 10 mm.
Lower right, much sharper at 50 mm.
Upper right, much sharper at 50 mm.
Center quite similar.

The bit about 50 being sharper in upper right but 10 sharper in lower left is the same as in your earlier tests.

The lower right corner is not consistent with earlier test, where the sharpness was about the same with the two extensions.

At this moment, my feeling is that quibbling over 10 mm versus 50 mm is maybe a bit pointless, given that the answer seems to depend on where and when you look.

For that same reason, I would be not at all inclined to generalize these results to anybody else's system.

--Rik

rjlittlefield
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Post by rjlittlefield »

lonepal wrote:But it also says ''If the distance is too short, it may cause vignetting;''
...
Then what is the minimum distance?
There is no such minimum distance. The person who wrote those words was apparently confused, because the statement is simply wrong. In this sort of system, vignetting is caused by making the distance too long, not too short.

(In other optical systems, for example coupling a camera to a microscope eyepiece, having not enough distance can indeed cause vignetting. It is a matter of matching the exit pupil of one lens with the entrance pupil of the next. With a microscope objective and a tube lens, the pupils are matched best at short distances; increasing the distance makes the mismatch worse, until finally it becomes bad enough to cause vignetting.)

--Rik

Chris S.
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Post by Chris S. »

Omer,

I'd respond to your query, but Rik has done a splendid job already. I completely agree with him. :D

--Chris S.

lonepal
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Post by lonepal »

Rik and Chris;

Thanks for the explanations, that makes sense.

I understand the distance does not matter unless it is too long to cause vignetting.

For the optimal distance, we need Adi's test to finish :)
Regards.
Omer

Adalbert
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Post by Adalbert »

Hi Rik,
”machinist's calipers”
I only have one like that:
https://www.ebay.com/itm/SHINWA-Vernier ... Sw-EBZrLqQ
but it is too short. So, I cannot measure the whole distance at once :-( (more than 200mm between the camera and the lens)

BR, ADi
Last edited by Adalbert on Tue Jan 23, 2018 7:26 am, edited 1 time in total.

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