Tube lenses. I'm still waiting - or have I missed it?

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ChrisR
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Tube lenses. I'm still waiting - or have I missed it?

Post by ChrisR »

I and others have tried a hotch-potch of lenses as "tube" lenses with infinite objectives.
Some (not I) have tube lenses designed for the purpose, produced by Nikon or Mitutoyo. Fine, but those take considerable effort and money to get connected up.

The hotch-potch set includes many lenses not designed to work at infinity (such as process or copy lenses), general purpose zooms and the "morfanon" which was a happy accident - designed for who-knows-what.
Some which might give performance at the better end of the bunch include fixed focal length (sometimes old, relatively inexpensive ) camera lenses which are of course designed to focus at infinity. By "better" I mean sharper - it's happily the case that the prime lenses also have their entrance pupils in a good place to avoid vignetting. There's bound to be more to it - used as a tube lens, only a small part of a camera lens is used.

So, as a member of the hotch-potch user group, I'm keen to know how much better the "Proper" tube lenses are, compared with the least inappropriate of what I/we, down here, have.

Does anyone have images which directly compare just that - a very good camera lens, with a scope manufacturer's tube lens?

Chris S.
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Post by Chris S. »

Such a test definitely needs to be done. And I keep hoping that somebody else will do it! Conducting it properly will be a lot of work for someone.

Ideally, the test would include a Mitutoyo tube lens, Nikon tube lens, a plain old 200mm prime or two (not the Nikkor 200mm micro), and some of the apo-whatevers that have been shown to produce really nice results. It should test the lenses with objectives mounted as close as possible, and with X-pol, which is much more demanding (as it typically requires some extra distance between decollimating lens and objective, which can bring into use less ideal portions of the decollimating lens). Now that I think of it, it might be good to test with even more separation, for those folks who may want to incorporate an iris.

The tester would likely have to borrow some lenses, but that should be possible for an established forum member--though if borders are crossed, I have no idea how customs would view "loan for testing purposes." One such member and I have been talking about this very thing--either of us would ship our lens to the other for testing, but so far, neither of us has had the time to be the tester. Maybe one of us will, when the snow flies?

Integrating all these lenses for a fair test would be a huge piece of work, at least in my horizontal rig, where the tube lens or bellows supports the camera and objective. Ugh.

It was partly because I didn't want to do this kind of testing that I bought the Mitty tube lens and Edmund Optics integration hardware. In one (albeit painful) stroke, both optical and mechanical issues were solved. (OK, maybe two strokes, since I had my local fabricator mount the assembly in a piece of aluminum angle stock.) But objectively, was it so painful? A lot of Nikon shooters are walking around with a $3,000 camera and three zoom lenses costing $1800-$2400 apiece. If the shooter also does macro, there is a $900 or $1700 macro lens, or both. In this context, is the $600 tube lens (plus $600-$800 for integration hardware) so drastically out of line? Especially when the investment in a range of Mitutoyo objecitves is not inconsiderable? The answer, of course, will depend on the photographer. My point is that may make sense for more us than it might at first seem.

Anyway, I hope somebody (else) will conduct such a test.

jcb
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Post by jcb »

Here is a little vignetting test that of course does not say anything about resolution.
Camera was a Nikon D700 (full frame), the tube lens proxy was a Nikon 4/200mm AI telephoto set at infinity focus and full aperture (tests with a 4/200mm micro Nikkor AFD give roughly the same results), microscope lens was a Nikon CFI plan 10x NA 0.25 WD 10.5mm mounted on a jinfinance 25-52mm adapter screwed on an empty filter mount screwed on the lens filter thread (the empty filter mount is there to protect the lens filter threads).
The lens was aimed at the northern sky, automatic white balance and exposure :

Image

Vignetting appears significant on an evenly lit subject. I'll guess that the evenly lit circle is somewhere near 38 to 40mm (which is quite good for a lens sold as having a 25mm image circle)
The same configuration was tested with a Nikon TC-14A 1.4x teleconverter :

Image

Vignetting seems reasonable. It even seems difficult to get a much better better result as shown by the same configuration with a Nikon TC-201 2x teleconverter :

Image

Vignetting does not seem to improve.
Tests with a Nikon microscope objective CFI Plan 4x NA 0.1 (same serie) shows a slightly worse result without a teleconverter and very similar results with any teleconverter.
Resolution test will have to wait as I have little time to do them now. However, I believe that this little test shows that the optical chain needed to cover a full frame sensor will, in many instances, have to include more that simply a tube lens (of whatever model and make). This might influence greatly the results and make them quite different from those obtained on a smaller sensor and may also vary from one tube lens or proxy to the other.
Hope this helps.
Last edited by jcb on Mon Oct 17, 2011 12:33 pm, edited 1 time in total.

ChrisR
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Post by ChrisR »

is the $600 tube lens (plus $600-$800 for integration hardware) so drastically out of line?
Not really, in context (sigh) I have to admit.
But my Nikkor 200mm f/4 was about $100, in its box!
I take your point about the support problem

Perhaps the question could be split, so more of us could add parts of the answer, by direct comparisons between say, a
Nikon CFN 10x 160/- NA 0.3 and a
Nikon CFM 10x 210/0 NA 0.25 and a
Mitutoyo APO 10x NA 0.28 on a tube lens.
It's indirect, and introduces more variables of course, between the different objectives.

My impression is quite simple, so far. The Mitutoyo isn't as good as either of the others, in terms of sharpness, on an APS-C sensor. But is it just my Mitty?

Chris S.
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Post by Chris S. »

jcb wrote:Resolution test will have to wait as I have little time to do them now.
JCB, you actually got less vignetting on the D700 than I'd have expected.

As for resolution, I think a key part of the test Chris R proposes is that the various "tube" lenses be tested on the same subject under the same conditions--identical lighting, photographer, support setup, etc. Otherwise, it's not really possible to make objective comparisons.

Cheers,

--Chris

jcb
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Post by jcb »

@Chris S.
Sorry, part of the text was lost in the posting (message above was edited to restore the lost part).
My point was that, depending on the sensor you use, you may have to add some glass behind the tube lens and this glass may be different for different tube lenses making comparisons even harder.
It might be easier with smaller sensors but we are expecting much more image circle than those systems were designed to deliver so I would not be surprised if all do not cover the frame in the same way.

Charles Krebs
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Post by Charles Krebs »

There is no way I would even consider trying to do a "comprehensive" test! ](*,) :roll: ](*,)
Far too many variables and possible combinations.

But it would not be too time consuming to try the Nikon 10X infinity on my 200/4 AF Micro Nikkor and compare it to the same shot done with the "official" Nikon tube lens. This would at least provide some type of comparison as far as "makeshift" versus "official". (I know some have not been too excited about results with their 200mm Micro-Nikkors used as tube lenses, but the few time I tried it my results looked quite nice.)

I've said it before and I can't help but continue to think this way....
The microscope manufacturers make some incredibly exotic objectives, and some of them have prices that would make your heart stop beating! :shock:
These guys are good. I just can't imaging that they would offer tube lenses that would, in any way, compromise the optical results from these lenses. (If this were so, I think you would find them offering several "grades" of tube lenses). So to me the initial question is... can a camera lens pressed into service as a tube lens match the performance of the "official" one?
(However, you all know what they say about assumptions.... )

stevekale
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Post by stevekale »

From a newbie....

Isn't the point of the infinity objectives that they get to pack the fancy stuff into the objective and have a "simple" tube? Surely, any top quality prime or zoom (L-series Canon lenses for example) should do a great job?

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Post by rjlittlefield »

stevekale wrote:Isn't the point of the infinity objectives that they get to pack the fancy stuff into the objective and have a "simple" tube?
No, the point of infinity objectives is that their output consists of pencils of light whose rays are all parallel, not converging in a cone to focus at a finite distance. That parallelism makes it possible to stick additional components like filters and beam splitters into the light path, without degrading the image by adding aberrations. It's still the designer's choice whether to do some corrections in the tube lens. Nikon and Mitutoyo choose to do all of their corrections in the objective, which makes those objectives play nicely with ordinary camera lenses except for the issue of mismatching apertures.

By the way, I've never seen a formal statement by either Nikon or Mitutoyo that says they do exactly all their corrections in the objectives. Empirically that seems to be the case, but there could still be some slight corrections in their own tube lenses that would make them a hair better than any substitutes.

--Rik

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Post by Chris S. »

stevekale wrote:Isn't the point of the infinity objectives that they get to pack the fancy stuff into the objective and have a "simple" tube?
No, Steve, simple tubes are the domain of the "finite" lenses, some of which do, indeed, do everything in the objective. The reason for the infinites is that the distance between objective and eyepiece can be varied much more, to permit additional optical gizmos to be placed in between (more useful to some microscopists than most photomacrographers).
stevekale wrote:Surely, any top quality prime or zoom (L-series Canon lenses for example) should do a great job?
Possibly, but not "surely"--which I think is the point of Chris R's post and Charlie's comments. Like Charlie, I strongly suspect that using anything other than the official tube lens is likely to give something up. The questions are, "How much? And in what situations?" If those questions were answered, individual photographers could make better choices about what approach is better for their needs.

Until a comprehensive test is done, I don't think we'll know.

--Chris

stevekale
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Post by stevekale »

Ok perhaps I was reading too much into Nikon's statement:

"Correction for optical aberration in infinity systems is accomplished either through the tube lens or the objective(s). Residual lateral chromatic aberration in infinity objectives can be easily compensated by careful tube lens design, but some manufacturers, including Nikon, choose to correct for spherical and chromatic aberrations in the objective lens itself."

which I took to mean that the Nikon tube lenses were simpler rather than complex. Given a Canon L series lens is top of game from worthy competitor to Nikon in optics, I was reading that there wouldn't be too much for the Canon tube lens to screw up.

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Post by Charles Krebs »

Steve,

You need to remember that our "needs" are very, very different from the needs of research microscopy. Fluorescence microscopy is a huge force in research these days. That (and to some extent older complex lighting methods like DIC) can really benefit from the characteristics of an infinity optical system mentioned above. If there were no need to place optical components between the objective and intermediate microscope image I can't see what there would have been any movement away from "finite" optical systems.

With finite microscopes, the optics were typically designed to utilize correction in both the objective and eyepiece. Nikon broke away from this in the mid 70's with their "CF" designs. (At least "chromatically"... there may have been some field flattening accomplished in the various eyepieces). But the newer infinity systems have not broken away completely from that method. Zeiss and Leica still use the tube lens for some chromatic corrections, while Nikon and (current) Olympus infinity systems do not. Arguments can be made for both approaches, but in a microscope system both approaches can offer superb results.

But when we grab an infinity objective and start using it with camera lenses as the "tube lens" it is likely best to stick with ones that do not look for additional chromatic correction in the "official" tube lens.

EDIT: Wow... I leisurely typed my reply to Steve and after posting it saw that plenty of others were also typing away at the same time. (And as usual, faster at typing and thinking than I am! :wink: )

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