High end zoom lens for tube lens. Vignetting?

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curt0909
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High end zoom lens for tube lens. Vignetting?

Post by curt0909 »

I currently use Mamiya 645 prime lenses as a tube lens for photography with infinity objectives. I was wondering if anyone has experience using a high end zoom lens as an adjustable tube lens. I'm considering a Canon 100-400mm II L. At any given zoom length its much sharper than my Mamiyas. I'm concerned that it may vignette on my full frame sensor. Many thanks in advance!

Chris S.
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Post by Chris S. »

Curt,

I think you're right to be concerned. So far as I know, every zoom lens so far tested vignettes when not zoomed at or close to its maximum focal length. This is not because of the quality of the zoom lens, but the apparent location of the entrance pupil (too far back, when zoomed in). And this is on APS-C sensors, let alone 35mm full frame.

BTW, while very different from the Cannon 100-400x you asked about, I did once test a Mamiya-Sekor C 100-200mm f/5.2 W zoom for converging Mitutoyo objectives. Since this lens was designed for a 6x7cm sensor, I hoped it would cover an APS-C sensor even when zoomed in. As reported here, it didn't.

--Chris S.

curt0909
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Post by curt0909 »

Hi Chris,

Thanks a lot. That's helpful. I'm surprised the Mamiya didn't cover it. I was hoping for a one lens solution. I guess I'll be sticking to my Mamiyas and teleconverters for higher magnification.

Chris S.
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Post by Chris S. »

Hi Curt,

I wasn't surprised that the Mamiya vignetted, as the too-far-back--entrance-pupil issue so far seems to be universal among zoom lenses. This said, I was still disappointed--a 100-200mm zoom converging lens would seem perfect, if it worked.

Have often wondered if a zoom to suit our needs could be designed? And if so, could it be produced at a cost we could afford? Knowing that lens design involves trade-offs, we could trade off things that a regular zoom-lens consumer could not: The lens need only focus at infinity; it need not have a wide aperture; focus-stackers could accept more curvature of field; perhaps the lens wouldn't need terribly high resolution, as many good converging lenses don't seem to exhibit this. It might even be acceptable for the lens to be varifocal, rather than true zoom (parfocal), so long as visual markings/mechanical clicks were present to let us match infinity focus to various focal lengths.

On the other hand, such a lens would have to have the entrance pupil sufficiently far forward to not vignette with microscope objectives; would need to exhibit little or no chromatic aberration; and would need a decently-sized high-quality image circle to cover at least full-frame 35mm DSLR sensors. Ideally, it would either remain of constant physical length and be mountable in ring-clamps, or have a sufficient portion of its length fixed and ring-mountable so as to provide for robust integration.

A number of forum members are knowledgeable about lens design (I'm not). If any such person cares to weigh in, I for one would be very interested in the reply.

--Chris S.

Pau
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Post by Pau »

Mitutoyo makes or made a zoom tube lens, although I don't recall any test of it, only some info at http://www.savazzi.net/photography/mitutoyo_fs-60.htm
Illumination through the eyepieces becomes very dim when using the zoom tube lens at 2x. In practice, I would say that 1.5-1.6x is the maximum practical zooming for comfortable visual observation through the eyepieces. In addition, zooming at 2x halves the effective N.A. of the objectives on the image side and brings all objectives into diffraction territory when used with a 12 or 16 megapixel sensor
So it seems to be 200-400mm, not the best range for most of us

Of course it's doable
Pau

Lou Jost
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Post by Lou Jost »

I've been testing my few zoom lenses as tube lenses for objectives but also as tube lenses for reversed camera lenses. The Panasonic 100-300mm MFT lens did not vignette even at 100mm when a 70mm lens was reversed on it. It did vignette badly at anything much lower than 300mm when a Mitutoyo 10x was mounted on it.

curt0909
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Post by curt0909 »

Thanks for the info. The other options for variable magnification I've considered would be a Zeiss Optovar(axioline preferably) and Nikon/Optem Optizoom. Both of these would still need to be used in combination with a tube lens.

enricosavazzi
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Post by enricosavazzi »

I remember I tested a Sigma 100-300 mm f/4 zoom as tube lens on APS-C, and it did not vignette at any FL. This is a largish, heavy zoom with internal zooming and focusing. I no longer own it though.
--ES

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Post by Chris S. »

enricosavazzi wrote:I remember I tested a Sigma 100-300 mm f/4 zoom as tube lens on APS-C, and it did not vignette at any FL. This is a largish, heavy zoom with internal zooming and focusing. I no longer own it though.
Very interesting, Enrico! I didn’t recall this test, but it’s the first zoom I’ve heard of that doesn’t vignette in a range much below 200mm with Mitutoyo objectives. If this is the lens you tested, my first reaction is ouch--$1500. But when compared with the $725 price for a Mitutoyo MT-1 tube lens, which has a single focal length of 200mm, a zoom-capable integration based on a converging lens roughly twice the price might be rational.

Do you recall which Mitutoyo objectives you tested it with? Particularly, were any of them low magnification, such as 2x? In my experience, these show vignetting more dramatically than higher mags.

Also, did you test with any significant separation between zoom lens and objective? For example, with enough room for cross-polarization?
Pau wrote:Mitutoyo makes or made a zoom tube lens. . . it seems to be 200-400mm, not the best range for most of us.
Pau, thanks for bringing up this item. Mitutoyo still seems to offer this in their catalog, page 14. Of course, the lens is buried inside the microscope itself, and apparently not available separately. And as you say, most of us would like something in the 100-200x range, rather than 200-400x. But these quibbles are, I think, beside your point.
Pau wrote:Of course it's doable.
This is more to your point, yes? That if this zoom lens can be made to not vignette when converging Mitutoyo objectives, another zoom lens more to our needs—say 100x-200x could also be made.

I know little of lens design, but ask if it truly follows that if a 200-400x converging lens can be made, so can a 100x-200x. Does it?

One naive question that occurs to me is based on the fact that both a 200-400x zoom and a 100-200x zoom would drag a Mitutoyo objective away from its designed intent (fill a 24mm-diameter field of view with a high quality image, when converged with a 200mm prime). But a 200-400mm zoom would increasingly crop the image as it moves away from design intent, which seems forgiving of edge aberrations such as vignetting. On the opposite, a 100-200mm zoom would work exposing more of the image circle as the objective is dragged away from its design intent. If so, might a 100-200mm converging lens be much harder to design than a 200-400mm one?

I would love to be wrong on this! :D

--Chris S.

enricosavazzi
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Post by enricosavazzi »

Chris S. wrote:Very interesting, Enrico! I didn’t recall this test, but it’s the first zoom I’ve heard of that doesn’t vignette in a range much below 200mm with Mitutoyo objectives. If this is the lens you tested, my first reaction is ouch--$1500. But when compared with the $725 price for a Mitutoyo MT-1 tube lens, which has a single focal length of 200mm, a zoom-capable integration based on a converging lens roughly twice the price might be rational.

Do you recall which Mitutoyo objectives you tested it with? Particularly, were any of them low magnification, such as 2x? In my experience, these show vignetting more dramatically than higher mags.

Also, did you test with any significant separation between zoom lens and objective? For example, with enough room for cross-polarization?
That is the exact Sigma lens. My test was something like 3-4 years ago, and I don't remember the details, but likely it was with the 10x M Plan Apo, which was my first Mitutoyo objective. I mounted it right at the front of the lens without any space between, using just a single adapter I happened to find on eBay.

Given the high length and weight of this lens I never used it in practice as a tube lens, and after I got a Mitutoyo FS-60 microscope with zoom tube lens there wasn't much of a point any more. At the time I was only interested in getting higher magnification than the objective's nominal, rather than lower. The 20x M Plan Apo was the last of those objectives I bought (after the 2x, 5x and 10x), and I never took the jump to 50x or 100x.
--ES

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