Debunking the high NA / coverslip correction myth

[Macrero]

I have noticed there is a common belief that high NA, coverslip corrected objetives are virtually unusable on camera without using a coverslip.

While it is true that coverslip correction in high NA objectives do impacts IQ seriously, the lack of coverslip can be successfully compensated by "re-focusing" the tube lens.
I have no formula for that, in my experience it depends on the tube lens, but usually adding some extension does the trick.
((This might (and most likely won't) work with objectives corrected for thick coverslip.))

As a picture is worth a thousand words, here is an example with the Nikon Plan Apo 20/0.75 + Repro Master 9.25/213, focused to compensate for the lack of coverslip, at abour 23X on a MFT sensor.

Salamis anteva scales:



Bigger: https://images2.imgbox.com/35/9b/8NbzkKPH_o.jpg

Sure, it's a tricky to use optics, but resolution wise, you can hardly get any better at that magnification.
And objective is obviously more than "usable".

Best,

- Macrero

[WojTek]

Hello Macrero,
absolutely fantastic photo!
I have the lens too and I love it.
BTW, I don't believe in stereotypes :-)
Best, ADi

[Pau]

The image looks excellent and the technical aspect is most interesting. (And also illumination is exquisite, even more for small WD objective)

I would like to know more data:

- Is the objective refocused? (from the normal infinite design position, 1mm WD) If so, closer or farther of the sample? (my bet is that it is refocused closer although farther would be more desirable) How much?

- When you say

+ Repro Master 9.25/213, focused to compensate for the lack of coverslip, at about 23X

Are 9.5 the F number and 213 the focal length or...? What's the magnification it gives when focused to infinite with the PA20/0.75?

Thanks in advance

[Macrero]

Thank you, Adi!

in my book seeing is believing, no matter what theory says.

At the price those optics are sold on the second-hand market, they are an absolute steal, if you're ok with the short WD.

Best,

- Macrero

[Macrero]

The image looks excellent and the technical aspect is most interesting. (And also illumination is exquisite, even more for small WD objective)

I would like to know more data:

- Is the objective refocused? (from the normal infinite design position, 1mm WD) If so, closer or farther of the sample? (my bet is that it is refocused closer although farther would be more desirable) How much?

- When you say

+ Repro Master 9.25/213, focused to compensate for the lack of coverslip, at about 23X

Are 9.5 the F number and 213 the focal length or...? What's the magnification it gives when focused to infinite with the PA20/0.75?

Thanks in advance

Thank you, Pau,

yes, those are the f/number and the focal length. With the 213 mm tube lens focused at infinity magnification on sensor is 21.3X.
To compensate for the lack of coverslip, the tube lens was "long-focused" (if that makes sense) by adding bellows extension, hence the higher magnification.

Best,

Macrero

[Lou Jost]

It is really interesting to know that changing estension can help correct for the lack of coverslip. Could you compare your picture with a photo taken through a coverslip and an inifinity-focused tube lens??

[Macrero]

It is really interesting to know that changing estension can help correct for the lack of coverslip. Could you compare your picture with a photo taken through a coverslip and an inifinity-focused tube lens??

I dont't have coverslips at hand and given my setup it would be hard to perform such test, sorry.
But I doubt result would be much (if any) better.
It would certainly be quite messier to work with though.

[WojTek]

It is really interesting to know that changing estension can help correct for the lack of coverslip. Could you compare your picture with a photo taken through a coverslip and an inifinity-focused tube lens??

Hello Lou,
I have taken some scales on the slide with cover glass using the Nikon Plan Apo 20x/0.75
viewtopic.php?p=289905#p289905
Best, ADi

[WojTek]

At the price those optics are sold on the second-hand market, they are an absolute steal, if you're ok with the short WD.

Hi Macrero,
I totally agree with you, this is the best bargain ever, ever :-)
Best, ADi

[Lou Jost]

It is really interesting to know that changing estension can help correct for the lack of coverslip. Could you compare your picture with a photo taken through a coverslip and an inifinity-focused tube lens??

I dont't have coverslips at hand and given my setup it would be hard to perform such test, sorry.
But I doubt result would be much (if any) better.
It would certainly be quite messier to work with though.

Thanks very much. I think your photo with coverslip has much more contrast and more resolution than your photo without, but the "without" (no coverslip but with compensating tube lens refocus) photo is still very good.

[Sym P. le]

OK so my Plan APO can do that, but can I?

[JH]

Nice picture - beautiful colours!
Regards Jörgen

[Beatsy]

Great picture and proof of the concept. I understand that older (ancient) finite optics microscopes allowed you to adjust the tube length to compensate for coverslips and correct for SA. Same principle I guess.

[Macrero]

Thank you all!

Great picture and proof of the concept. I understand that older (ancient) finite optics microscopes allowed you to adjust the tube length to compensate for coverslips and correct for SA. Same principle I guess.

Yep, apparently the same principle is also applicable in infinity systems / optical tube length.

If I had to choose between no-coverslip 20X/0.40 and coverslip-corrected 20X/0.40, I would definitely choose the first one.
Now, between 20X/0.40 NC and 20X/0.75 cs-corrected, I'd go for the (much) higher res optics, especially if it's sold for virtually peanuts.
The short WD is not a problem for me, it might be for others though depending on what they're photographing.

[rjlittlefield]

Great picture and proof of the concept. I understand that older (ancient) finite optics microscopes allowed you to adjust the tube length to compensate for coverslips and correct for SA. Same principle I guess.

Yep, apparently the same principle is also applicable in infinity systems / optical tube length.

The combination of an infinity objective plus its tube lens is very much like a finite objective.

But beware the terminology: once you change the tube length, then that area behind the objective is no longer "infinity space" because it consists of diverging or converging rays, depending on which way you've changed the tube length.

Macrero, how did you determine the best amount of added extension? Did you explore how much tolerance there is in the amount of added extension?

--Rik