Using Infinity-corrected Objectives Without a Tube Lens

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Macro_Cosmos
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Using Infinity-corrected Objectives Without a Tube Lens

Post by Macro_Cosmos »

Hi, got this done quickly.
Lighting was inconsistent but it is conclusive enough in my opinion. I was physically holding the light guide because I did not want to go through the trouble to mount it on a post.
I messed up the magnifications a little, the images with no tube lens are slightly magnified.
Tube lens is a fluorite, or semi-apochromatic, custom thing. The only tube lens I still have access to. German wafer showcased.
Be ready for a surprise.

First, 2x objective. Pretty poor either way but without a tube lens, what even is this?
Image

5x objective. I would not call the results without a tube lens unusable. The centre is indistinguishable. Corners are bad as I have said before.
Image

20x objective. Say hello to the most spherical aberration you might have seen.
Perhaps it is due to my stupidity, but deviating away from the exact magnification should not induce that much spherical aberration, surely? I have a 10x objective with a similar numerical aperture (NA) which I constantly use without a coverslip. Perhaps the objectives are less tolerable to deviations in tube length.
Image

To finalise with speculations.
My guess is very low magnification objectives will absolutely not work, 1x and 2x types. They are not even as good as macro lenses, which is unfair since such a large field of view must be packed into a small objective and made parfocal with higher magnification ones. Histo-pathologists love 2x objectives, they have their important place in saving lives.
Then at higher magnifications, commonly due to the higher correlated NA, spherical aberration will make them unusable. They are designed to work with a relay lens focused to infinity.
So, at an acceptable NA (<0.3) and magnification, letting go of the tube lens could yield great results, with centre resolution being more or less unimpacted.

Not 100% related, here is a chart illustrating the relation between NA and spherical aberration. I think it is applicable to what we are seeing here.
Image

Someone with the Mitutoyo 10x and 7.5x could give this a go. The 10x with 0.28 NA falls comfortably under 0.3.

Thanks.

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