Some scales with Nikon plan apo 20X

[JH]

Some scales with a Nikon plan apo 20X 0.17 NA 0.75. on 210 mm tube length, 2.5X PL lens. Zerene Stacker, Adobe LR and PS.

I was surprised that I could get OK pictures of butterfly scales even though I used the lens in a way that it was not made for.


Regards Jörgen

[rjlittlefield]

These do look pretty good. Part of the explanation may be compensating errors. I'm guessing that your 20X lens is designed for 160 mm tube length. In that case you're using it with a tube that is longer than expected, but a cover slip that is thinner than expected. Those errors are in the right direction to partially compensate for each other. I haven't tried to compute how much the compensation would be, however.

--Rik

[carlos.uruguay]

Very nice

[Pau]

...In that case you're using it with a tube that is longer than expected, but a cover slip that is thinner than expected. Those errors are in the right direction to partially compensate for each other. I haven't tried to compute how much the compensation would be, however.

Do you have any source on how to calculate it?
Does focusing the tube lens other than to infinite produce a similar effect with infinite corrected objectives?

[JH]

Thanks Carlos and Rik

I did not use a coverslip, and the lens is intended for 160mm tube length.
I agree with Pau, it would be interesting to know how to calculate the change in tube length needed to compensate for a change in cover glass thickness.

Regards Jörgen

[rjlittlefield]

No, I do not have a source and offhand I do not know how to do the calculation. Probably the brute-force technique for calculating ray paths that I use for other situations would work OK, but I think this one is a little more complicated than anything that I have already set up.

--Rik

[ChrisR]

How did you get light to it - I imagine you had very little working distance?

[JH]

How did you get light to it - I imagine you had very little working distance?

This would be a fantastic lens if it had a WD of just a few more mm. The first picture is diffused LED-ligh, the second is EPI light.

Regards Jörgen

[Charles Krebs]

They do look good. Normally a 20/0.75 160/0.17 would look very soft if used with no cover slip.

Almost certainly the longer "tube length" helps to keep the image quality to a good level.

You don't see the method of altering tube length to correct for coverglass mentioned much these days since the microscopes have no provision to do this (and newer objectives that might need it badly have correction collars). So I dug through some of my old microscope books written at a time when this was a viable technique. One had a chart that is interesting to look at (but there is no methodology given as to how it was derived):

(from "The Microscope, A Simple Handbook", Conrad Beck, 1921 downloadable from http://ia700400.us.archive.org/24/items ... 00beck.pdf
Also check out his more "advanced handbook" https://archive.org/details/microscope02beckuoft )



Since the objectives are referred to by their focal lengths I've also included a chart from earlier in this book. Here you can see that the "8mm" is about what we would describe as 18.5/0.50 objective



It is interesting that after adjustable tube length microscopes where no longer readily available this was rarely considered a very useful technique to make certain aberration corrections. But with the myriad configurations we use here, and with the instant feedback possible with digital photography, I can envision set-ups where this approach can be put to good use. JH has shown us a good example here!

(BTW... if this is the Nikon 20/0.75 160/0.17 objective, then the working distance is 0.64mm... when used at the "proper" tube length.)

[Pau]

Charles, thanks for posting the tables and references.

As you say this was mainly useful with old instruments with regulable tube lenght (I have used them many years ago). Of course changing the lenght also changes magnification significatively.

[Marek Mis]

The third image is very nice !
I appreciate very interesting discusion.

Marek

[JH]

Thank you Charles !

Very useful information - gives me a reason to check back on my amber to see if I can get further into the amber with a shorter tube lens.

Thanks Marek, I agree with you about the interesting discussion - one of the reasons that this forum is so interesting to follow and participate in.

Regards Jörgen

[rjlittlefield]

Does focusing the tube lens other than to infinite produce a similar effect with infinite corrected objectives?

Yes, I think that should work the same. In this case, focusing the tube lens to some finite distance will cause the objective to focus closer, like increasing the tube length of a finite setup. If you're using a setup with bellows or extension tubes, then you can also use an extension shorter than normal to make the tube lens focus "beyond infinity". That has the same effect as decreasing the tube length of a finite setup.

to see if I can get further into the amber with a shorter tube lens.

This may work, but I'm not sure how much more depth you can get. In any case, be careful that "shorter tube lens" means a tube lens that is used on a shorter than normal extension, so that it focuses "beyond infinity" as noted above. If you just swap in a shorter tube lens, say 135 mm instead of 200 mm, but still focused at infinity, then you will only be affecting the magnification and not the amount of aberration.

--Rik