Judging high-NA infinity-corrected objectives

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Post by Chris S. »

Macro_Cosmos wrote:The unrelated problem here would be the availability of such tube lenses. Plenty 200mm tube lenses can be found, but I've never came across dedicated 400mm tube lenses. I'm sure they do exist somewhere.
One dedicated 400mm tube lens is the Mitutoyo MT-2. This can be integrated into a macro rig using much the same parts one would use for the 200mm Mitutoyo MT-1 tube lens. (For the parts list I use for the MT-1, see The Bratcam gets a Mitutoyo tube lens).

By the way, I'm not advocating purchase of an MT-2. Though I could easily add an MT-2 to my rig as an option, I've never seen much reason to do so. But as you suggest, the lens most definitely exists. (For that matter, my Mitutoyo FS-70 microscope head has 100%-200% zoom capability, which implies that it contains a 200-400mm zoom tube lens.)

--Chris S.

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Post by Lou Jost »

Regardless of the availability of dedicated tube lenses, it is worth remembering that our DIY substitutes work very well; in fact I suspect the majority of us don't use dedicated tube lenses.

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Re: Judging high-NA infinity-corrected objectives

Post by mjkzz »

Lou Jost wrote:With the recent glut of 20x 0.75 Nikon objectives, it might be worth reminding people of how special it is to have high NA in a low-power objective.

The resolving power of an objective on the subject depends only on the NA, not the magnification of the objective. The standard 200mm tube focal length is not optically anything special, and any other tube focal length will still preserve the resolution on the subject. The lower power infinity-corrected objectives differ from higher-power infinity-corrected objectives mainly in their bigger image circles at any give tube focal length.

That means you can use the 20x 0.75 objective at 40x or even more, using a 400mm tube lens, with no degradation relative to a purpose-built 40x 0.75 used with a 200mm tube lens. This 20x pushed to 40x should outperform 40x 0.65 objectives.

So for a given NA, a lower-power infinity-corrected objective is much more useful than a higher-power one. You can use it at the lower magnification and you can also use it at higher m without facing empty magnification..

By the way, a good 400mm tube lens is the old manual Nikon ED-IF f/5.6 lens.
wow, nice, thorough explanation of all, thanks.

however, something I still could not grasp -- chromatic correction. See, an APO obj focuses lights of some different major spectrum from a point on subject onto the same point (or close enough) on the image plane at designed magnification. If magnification is stretched by the mean of longer tube lens, what is happening to chromatic correction? Would it be wore, not affected?

Or maybe for us without expensive equipment, it is not observable and we can ignore it.

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Post by mjkzz »

or put it the simple way: if I have an infinite 4x achromat, stretched to 8x by the mean of longer tube lens, would it get worse in terms of chromatic correction (or un-correction if that is a word).

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Post by Lou Jost »

Using a longer tube lens gives exactly the same image as a shorter tube lens (if both lenses were perfect), just larger. So the CA of the objective is blown up along with the rest of the image.

So a 100mm tube lens on MFT and a 200mm lens on FF give the same image with the same amount of CA in relation to the frame width. But yes, if you are comparing long vs short tube lenses on the same frame sizes, the CA would be more apparent on the more magnified image.

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Post by mjkzz »

Lou Jost wrote:Using a longer tube lens gives exactly the same image as a shorter tube lens (if both lenses were perfect), just larger. So the CA of the objective is blown up along with the rest of the image.

So a 100mm tube lens on MFT and a 200mm lens on FF give the same image with the same amount of CA in relation to the frame width. But yes, if you are comparing long vs short tube lenses on the same frame sizes, the CA would be more apparent on the more magnified image.
ah, so you are saying that with same sensor, same obj on longer tube lens would cause more apparent CA problem, right? If that is the case, how would a 20x 0.75 objective perform better using a 400mm tube lens? I thought CA is a rather glaring factor when comparing obj's, maybe I am too green with this matter.

As for different sensor size comparison,
So a 100mm tube lens on MFT and a 200mm lens on FF give the same image with the same amount of CA in relation to the frame width.
I understand the same image size, but in terms of chromatic correction, is your statement that, they get same amount of CA, a fact or something you think is true?

As far as I know, though you get same size image, the optical path light travels is very different for MFT with 100mm tube lens vs FF with 200mm tube lens. Most chromatic correction magic heavily depend on the optical path so that lights of different freq will converge at designed point along the optical path.

So I am not sure if your statement is empirical based on your observation as you deal with this stuff on daily basis, ie, what you think, or something that can be supported by theory. I THINK chromatic correction magic of each objective are guarded trade secrets, so exactly how they are implemented, the formula that creates glass material might be really different.

So please point me to some reading material to back your statement (MFT + 100 vs FF + 200 are same in terms of chromatic correction). Thanks in advance.

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Post by mjkzz »

since I mentioned material, I think making an objective has two parts, science and craft (engineering). Or maybe more if you include luck :-)

I believe the science part is well understood, it is the craft part, making the science part happen is key and is heavily dependent not only on how it is made, but also how it is used.

And I believe it is the craft part that differentiates a well made APO lens vs a cheap Chinese clone (if they are even clones). Two years ago, I spoke to some Chinese supplier about their APO objective, they sounded knowing it all, yet their stuff IS so crappy even as a beginner in extreme macro, I could not give them credit for. I even posted the result here but nobody paid attention. I also wrote a bog about it, too.

So what I am saying is, if it looks to be true, it might not be due to what you think. So about your statement of MFT+100 vs FF+200 and CA, do you have something analytical?
Last edited by mjkzz on Sun Jan 20, 2019 9:21 pm, edited 1 time in total.

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Post by Lou Jost »

No, it's just my thinking and my experience. I'm basically assuming the tube lens is not contributing to the CA but just magnifying.
"how would a 20x 0.75 objective perform better using a 400mm tube lens?"
That's not what I said. Sorry if I wasn't clear somewhere. What part of what I said made you think that?

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Post by mjkzz »

Lou Jost wrote:No, it's just my thinking and my experience. I'm basically assuming the tube lens is not contributing to the CA but just magnifying.
"how would a 20x 0.75 objective perform better using a 400mm tube lens?"
That's not what I said. Sorry if I wasn't clear somewhere. What part of what I said made you think that?
That means you can use the 20x 0.75 objective at 40x or even more, using a 400mm tube lens, with no degradation relative to a purpose-built 40x 0.75 used with a 200mm tube lens. This 20x pushed to 40x should outperform 40x 0.65 objectives.


The "no degradation relative to . . ." and higher mag made me think.

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Post by mjkzz »

Lou Jost wrote:No, it's just my thinking and my experience. I'm basically assuming the tube lens is not contributing to the CA but just magnifying.
OK, so it is what you think, or your empirical observations, or assumptions. I thought they are facts or at least backed by some analytics.

Like I posted just before yours, I think the craft part is key and when you have highly crafted equipment, I believe it is better to use it as it is designed.

This is my experience and personal opinion, of course.

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Post by Lou Jost »

"This 20x pushed to 40x should outperform 40x 0.65 objectives."
Here I compared these two objectives at 40x; the 20x (NA=0.75) + 400mm should have better resolution than the 40x (NA=0.65) + 200mm, based on the difference in effective aperture.

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Post by mjkzz »

Lou Jost wrote:So a 100mm tube lens on MFT and a 200mm lens on FF give the same image with the same amount of CA in relation to the frame width. .
I think a while back we had discussion about NA and I pointed out that NA is an intrinsic property of an objective in an attempt to clear some misconception of NA. I have read your research paper at crazily abstract level and it shows how scientific approach you usually go about things. But this statement is somehow lack of scientific approach, if not totally a misconception, lacking some understanding of optical design. Given the fact that I am an total ignorant when it comes to optics and optical design, this is a glaring one to be rectified.

Anyways, I have high regard about your posts, full of inspirations and all, and I think you can rectify that statement so that people can still trust what an important figure say here.

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Post by mjkzz »

Lou Jost wrote:
"This 20x pushed to 40x should outperform 40x 0.65 objectives."
Here I compared these two objectives at 40x; the 20x (NA=0.75) + 400mm should have better resolution than the 40x (NA=0.65) + 200mm, based on the difference in effective aperture.
Whatever, the over all tone in the original post sounds like favoring stretching an objective is better.

Anyways, resolving power is partially affected by chromatic correction -- if a white dot on subject is NOT a white dot on the sensor anymore, what good is the resolution? Even the calculation of resolving power assumes a particular (averaged) spectrum, that is where, fundamentally, changes the concept of resolving power.

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Post by mjkzz »

More thinking about what you said, I start to think there are more fundamental flaws in your statements, I now really think you just came up with those statements without any factual or analytical backing. The statement is contradicting itself.

I hope this is to be rectified as it took me this long to come to such conclusion, maybe this is because I have ZERO knowledge of optical design. But for those unknowing readers, this could mislead them to think the same way.

Image

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Post by Lou Jost »

What part of that do you think is self-contradictory? It would be helpful to me if you explained what was the contradiction. Thanks. The statements are based on my (non-analytic) understanding of how infinity optics work, and my experience using my objectives and coupled lenses on different formats from MFT to APS to FF (I use all three sensor sizes). I'd love to see a more technical discussion from someone like Rik.

Re-reading what you quoted there, I should probably not have used the word "exactly", which is a very strong word. But I regularly use tube lenses from 90mm to 400mm with my objectives, and I do not notice a general degradation of the image due to tube lens focal length beyond what you would expect from the different magnifications of the image, though of course no real tube lens is perfect, so some are better than others.

Edited to add this: if you look through this forum, you'll see many beautiful photos by people using tube lenses of different focal lengths than 200mm, both lower and higher, with excellent results.
Whatever, the over all tone in the original post sounds like favoring stretching an objective is better.
I don't understand how you got that from what I wrote.

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