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Ivanoff corrector


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#1 craig

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Posted 21 March 2008 - 02:35 PM

I've been looking into Ivanoff correctors and wanted to see what the WP people feel about them.

For those who don't know, an Ivanoff corrector is an alternative to a dome or flat port. A dome port does not alter perspective but does alter focus while a flat port does not alter focus but does alter perspective. An Ivanoff corrector does not alter perspective or focus. It consists of two lens elements, an outer element which is a negative power and offers a flat phyisical interface to the water, and an inner positive power element. There is a small air gap between the two elements. Ivanoff correctors should be easy to design but will require large pieces of glass. Plastic is unlikely to be an option (at least on the larger, outer element).

This port design was granted a US patent in 1956 so any patent issues expired long ago. The US patent number is 2,730,014 for those who'd like to look it up. There's is a layman's description of the port design at the bottom of this page.

I've gathered a few things in reading about this port design. First, it has been used routinely in video and motion film applications but has been less used for still photography. I don't know why it is less popular with still but I know that domes are far easier to do and less costly. Second, the port is easier to implement with narrower perspectives. I don't whether that is due to optical problems or simply due to size demands. I know that Hasselblad made one for their 38mm lens at one time and that is both a still and wide angle application. It must be possible, the only question is whether it is financially viable.

I don't know but I'd imagine that wide angle ports for popular video housings are an adaptation of this design. Video cameras frequently have fixed zoom lenses and manufacturers want to offer zoom-through capability.

This port design is sensitive to the nodal point of the lens and must be large enough to avoid vignetting. For that reason there are conflicting design goals. The further the port is from the lens mount the larger in diameter it must be. The larger the port is, the heavier and more costly it will be. Bouyancy could become a major issue. It is unlikely that a port design made for use with a large zoom lens will work properly with a physically small prime lens by way of a shortened extension. I would be delighted to learn otherwise.

An advantage of an Ivanoff design is that it does away with the potential need for diopters. Because it offers a flat face, it also offers the potential for wet-mount filters, either colored ones or ND filters for shooting sunbursts. That assumes, of course, that such filters could be obtained large enough. Finally, such a port could offer an internal filter capability, perhaps even one similar to the Singh-Ray variable ND filter. This could be particularly interesting for sunburst shooting.

I've read recently in the RED video forums that video manufacturers have access to manufacturing capabilities for this type of port system but that it would be costly in small quantities. The RED camera uses SLR lenses and has an APS-C sensor plus it has resolution demands not unlike modern DSLRs. Video guys like 100 degree perspectives just like still guys do.

So, I have a bunch of questions. First, does anyone have practical knowledge of these port systems and what it takes to design and manufacture them? Second, how great an interest would there be in such a port assuming it could adapt to several port systems and support, say, the new Canon 16-35 II and the Nikon 14-24 and 17-35? I assume it could support primes if it could handle these lenses, but I believe a single port design will not have the flexibility of a dome in accommodating lenses. I would assume there would be substantial sticker shock on something like this, probably several times the cost of a Seacam dome.

When you consider how many problems with wide angle optics we have between good dome performance, proper selection and basic availability of diopters, and the increasing inability to even use diopters and filters on the new lenses, you have to wonder how much it might be worth to consider this. With Nikon raising their base ISO by a stop, I'm concerned with the ability to shoot wide angle into the sun, yet suitable lenses are losing their ability to take even one filter much less a filter plus an ND. Imagine being able to use a wet ND gradient underwater. By the way, Ivanoff ports do good splits. :-)

Amphibico manufactured and sold a wide angle port that cost $7000 back in the days of Hi8 and standard DV. With current photo pros routinely spending $8K on a body and another $8K on a housing (ignoring everything else) yet having to accept limitations in the wide angle optics, wouldn't they be willing to consider another $8K on an Ivanoff port if it sufficiently delivered the goods?
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#2 loftus

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Posted 21 March 2008 - 03:48 PM

Sounds interesting, but for every advantage there seem to be as many disadvantages - weight etc.
Could one not design a wet flat lens that fits over a regular dome port?
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#3 Phil Rudin

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Posted 21 March 2008 - 04:22 PM

Hello Craig,

I met Dr. Ribikoff at a shipwreck symposium in Ft. Lauderdale Florida in the early 1980's. He was speaking with Dr. Harold Edgerton, the guy that invented the electronic flash (strobe) and sideband sonar among other things.

I talked with Dr. Ribikoff about this lens design and its main application at the time. The original design was for a camera being used on a submersible at depths of around 6000 feet (1800+ meters). The flat outside design allowed the port to work at such great depths where a traditional curved port would have been impractical.

And yes it was expensive to build.

Phil Rudin

Edited by tropical1, 21 March 2008 - 04:23 PM.


#4 craig

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Posted 21 March 2008 - 06:14 PM

Yes, weight is an issue. In the larger video rigs the port will have supplementary flotation. Those ports, though, have wide angle adapters built into them and have many more lens elements. Think of an Ivanoff port as a large flat port with unusually thick glass and a big, matched diopter behind it. Not as heavy as those big video ports.

Yes, I've read that the ports were often used for deeper diving where pressure was a challenge. I've read that Ribikoff once had a 105 degree port manufactured so it is apparently practical. The outer lens element doesn't have to be as thick as it would need to be for greater pressures, but the larger the required diameter the thicker the outer element will be on the edges. Higher refraction glass would ease that problem, but my understanding is that those types of glass are more vulnerable to scratching and not well suited to outer elements. The inner element could use that type but it's not the heavy part.

Fathom should have the ability to design one of these easily and could probably answer all these questions without much effort. Wish I had friends there. :D

It would be impractical to build a flat assembly on the outside of a dome when the angle of view of the lens is over 100 degrees and the dome is 23cm. By the time you got it flat you'd have a huge poster for a filter! Molding a spherical filter would be more practical but who knows anyone that can do that with sufficient optical quality? Of course, none of that addresses the diopter issue or the edge softness problems either.
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#5 Alex_Mustard

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Posted 21 March 2008 - 07:13 PM

Hey Craig,

I have a Carl Ziess Ivanoff for the Hasselblad SWC. I am happy to provide you with some measurements when I get home from this trip.

Alex

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#6 Paul Kay

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Posted 22 March 2008 - 02:14 AM

Craig

I am again talking to optical designers to see if we can come up with an economic alternative to using concentric dome ports. As far as I can ascertain, the Ivanoff corrector had to be matched up to each specific lens (or focal length, first principle point, etc.) and suffered from several problems - the glass/plastic used (which shouldn't be a problem today as far more and higher refractive index glasses and plastics are available today), the specific set-up (limiting each corrector to a specific lens) and size - for a large field of view size became resrictive - not so bad on an ROV, but tricky on a hand held system.

Today there may be far more viable alternatives - non-concentric domes utilising an aspheric surface for example, which would have been prohibitively expensive in the 50s. Also it might be possible to mold ashperic domes today - a technology which is becoming better all the time. Lastly, there are other optical solutions which may be worth looking into. An Ivanoff corrector, or modification of, might be a possibility but there are other, potentially more viable options to explore.
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#7 craig

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Posted 22 March 2008 - 09:48 AM

The Ivanoff patent isn't especially helpful to the layman but it's clear that at least some of the restrictions are as you say, Paul. How restrictive they are is what I would fear, since we probably won't collectively get interest in one that only works with a single lens. My interest in the design stems from some of its attractive features and it's relatively simple nature. It would have to offer superior IQ, though, and broad enough appeal to be viable.

I share your interest in alternatives to current dome ports. I believe that the ports we use are the weak link in our equipment at this point and I wish there was more active interest in pursuing technical improvements. If there were resources to study regarding engineering details of these systems I'd invest time in learning them. I find the Ivanoff design sexy because of the diopter and filter solutions it offers, but what I really want is the same as you---better edge performance with wider angles. Of course, I want that with zooms as well.

In the video world there was very slow uptake by certain manufacturers with port designs that allowed full zoom-through and good focusing. I hope the need for better port solutions isn't greeted with the same reluctance now. If we do nothing else, I'd ask that we express to our vendors and our manufacturers that we want to see port systems that make the best of the great lenses and bodies we now have access to. I'd like to think that there are enough of us with the interest and the resources to justify a superior solution should we be confident that we would get it. Companies like Fathom do significant engineering work to develop complex products that ship in low volumes. I wonder if it could be as simple as telling the right people that we need these products made.

I'm impressed that we can shoot wide angle at f/11 and up by cranking ISO when we need to (and I think that's a very clever hack) but we really need a better answer than that.
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#8 divegypsy

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Posted 26 March 2008 - 03:49 PM

I spoke with Ribikoff at DEMA 1 in Miami year ago about the corrector lens. He told me it was critical that the distance between the elements attached to the front of the camera lens and the element in contact with the water had to remain constant. At that time virtually all wide-angle lenses extended outward as they were focused. He suggested keeping the distance fixed and letting the whole camera body move backwards and forwards (on rails?) in the housing during focusing. So I lost interest.

With today's internal focus designs, at least some lenses (Nikon's 18mm?) might work in a normal housing. But they might have to be fixed focal length lenses as zooms usually change their length when zooming even if they are internal focus.

But still a very expensive proposition to get high quality glass elements made in the quantities of a few discerning underwater photographers who might have the money and willingness to pay for such a project.

Fred