Tom Kline

Nice report! A basic tenet of underwater photography is that one needs to minimize the water distance between the camera and the subject. Therefore the angle of view of one’s optics needs to be proportional to the subject size and therefore the focal length inversely proportional. A 200mm focal length macro lens will be great for shooting butterflies topside but mainly useful for tiny subjects under water. Aiming and focusing a 200mm will also be a bit more challenging than shorter focal lengths.

I think it is now more important that ever to sign one's works to "humanify" them. Hence a watermark. This can be forged as well but if there is an authorship question the author can be contacted to verify. It is only a matter of time before AI starts adding imperfections - a speck of backscatter here and there for example. Easy to ID species will be less problematic for AI compared to others. I invite people to look at dichotomous keys to see the types of minutia that may be needed for others. Some require looking inside as well as I recall a fish key I used in a class that showed up during an examination that required knowing the peritoneum color.

Tariffs strike again!

I wrote it live so to speak but I did have to scroll up and down quite a bit to see the prev mssgs super keyboard challenged

somehow I screwed up and the text goes beyond the page!

Another data point: I have the book titled Free Diving by Rebikoff translated by Mervin Savill. Published in London 1955, first published in 1952 in French as Exploration Sous-Marine Ch. VII (the last one) is titled Underwater Photography and Filming. He writes a lot about his underwater flash units but not a peep about corrective optics Interesting that Ivanoff lived so long but what did he do from the 1960s onward? Searching his name I came up with a living artist. Another data point; See: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9192/91920P/Afocal-viewport-optics-for-underwater-imaging/10.1117/12.2061445.short Read the abstract!!! He calls them "the Ivanoff corrector lens" Lens is a bit vague since it can be one piece of glass or a whole unit like a camera lens, but for lack of a better English word lens has to be used (Abstracts often have a word limit (based on a lot of experience)). This is from 2014 so relatively recent. At Chris: the Nikonos opinion is from a current web page so would seem to be not too out of date but maybe limted to the experience of the author(s). I remember when the blue housings were released! It was during the 2 years my family lived in the US (Germany before and Japan after) when I bought the Hasselblad issue (house magazine) on it at a local camera store. This was between Sept 1970 and August 1972. So it is about 2 decades post Ivanoff earliest dates from the patent. BTW the first Hasselblad to use the 38mm lens was called the Supreme Wide Angle or SWA. The SLR was the 1000F. I have seen a SWA in person - it had a more primitive shutter. From the early 1950s. The SWC, the later camera, has to be used in the blue housing that takes the corrector lens. SWC/M and newer, no, because the tripod mounting shoe was moved to allow fitting of Polaroid backs.

Happy Thanksgiving! Thank you Alex. Not surprised that Zeiss may have made some improvements, but a doublet (2 elements glued together) for the port would also need to consider pressure resistance. How would hydrostatic pressure affect the glass at the contact surface between the elements? Lens element separation is bad enough in air (have seen Z optics with this problem - both camera and microscope). Thank you Adventurer. The 2003 article is interesting but may be misleading. It would be good to know how the author obtained the quoted material. It seems unlikely that it is from his memory if from 1968!! Rebikoff may have worked with JYC but it is Edgerton that JYC called Papa Flash. JYC is known to have had custom built underwater housings for his motion picture work. If Rebikoff worked with Ivanoff to invent the I-K lens per this article why is Rebikoff not in listed in the patent (other names (Grand and Cuvier) are)? Rebikoff may have outlived Ivanoff so had more of a chance to blow his horn (as well as being the book author). Also note the differences in the figure captions I previously posted. System Ivanoff in 1955 then called Rebikoff correction lenses in 1965! For the same items. R does show the smaller lens (actually two of them) on his stereocam. R may have built the cam but I more likely (his company or his associates) the lenses that are part of the housing. Thank you Davide. The Fathom unit seems to work more like the recent Sea&Sea correction lens as it is designed to work with a dome port but with 3 lenses elements so may be even more expensive. The S&S lens is aspherical and maybe it is doing the same job as 3 that are spherical. Thank you Chris. These lens were first developed by 1955 as that is the date of the earlier book and there are more examples in it. The patent was submitted in 1951 (in France apparently), received by USPO in 1952. Suggests a bit earlier than the 1960s.

I found a couple more links while searching the web: https://g6yb.com/g3ynh/photography/articles/wconv_intro.html Here is the important text: "Rebikoff-Ivanoff corrector, in its basic form, consists of a plano-concave front element with a magnifying element behind. It eliminates the optical distortion caused by a flat air-water boundary. The lens elements can be mounted in their own underwater housing, allowing the rear surface to be placed close to the flat port of an underwater camera housing. If the intervening water layer is thin, the external lens corrects for the distortion caused by the port." Interesting is that it can be used as a wet lens. How similar is this to existing wet lenses? Another is this: https://www.seafriends.org.nz/phgraph/film.htm Here is the important text: "Rebikoff-Ivanoff correction lens Demitri Rebikoff was one of the pioneers in underwater photographic equipment in the era 1940-1980. He designed an underwater correction lens that is also a wide angle converter, based on an inverted telescope. Because it does not change the camera's focus, it is said to be a-focal. As shown in the diagram, this correction lens consists of a negative lens as port and a flat positive lens placed 30mm further towards the camera. The lens can be put together from the parts supplied by a technician for eye glasses. It has a number of attractive advantages, not the least that it can be used both above and under water. It is also used as a 'wet-mate' underwater attachment. The Rebikoff 'port' is often used for underwater television cameras, but it is disappointing for still cameras, and cannot match the sharpness of the Nikonos lenses." Cutting and pasting brought in the figure. Note the change in order of the names for both sources. Was this arbitrary? Nikonos lenses are better???!!!!!!!

I second Davide! I am curious about: Rebikoff fronts glas uw correctors Ivanoff-Rebikoff corrector ports What is the difference and please provide citations? I ask because Rebikoff used inconsistent nomenclature in his books. I have his 1955 and 1965 books published in the US. They are very simmilar even using some of the same pix. See attachment. From what I know Ivanoff wrote the patent and Rebikoff actually built them. Apparently early in France and then in the US.

FYI, I typically see a jump in the remaining battery power of one bar (on the unit's display) about an hour or so after ending a shoot (when I am home) and the housing is still cold to the touch even during the summer. This goes for both Li and Ni type batteries - cameras and strobes.

This is why I have mostly used gripped bodies here in Alaska: Nikon one-digit and Canon 1d models. Even with them I have had sudden death with the batteries (after a lot of shots).

Great find and read Chris! A 33% increase seems valid. I suspect it is more noticeable when DOF is somewhat greater. For example if DOF is 100mm, 133 mm might be noticeable. However if DOF is just 1 mm, 1.33 might not seem like much of an improvement.

Thanks for the pointer Alex, I had to look it up!!! On the page above Fig 6.13 is point #4 (a list of the correction port properties): "4. The depth of field is increased about a factor of 2 over a plane port with the same camera lens and aperture." This sounds to me more a benefit of the de-magnification effect (1.33x eliminated) compared to a plane port. Mertens has a long discussion on depth of field and gets into loss of aperture due a plane port, compensating by changing distance, etc. There is also Fig. 6.17 that would be much better in color as there are 8 lines if I counted correctly. It would have been more interesting if Mertens had compared to a dome port as well. Based on the caption in Fig. 6.14 the examples of I-R port in the book (Fig. 6.13 same swimming pool) were shot with a 21mm Super-Angulon lens (made by Schneider for E. Leitz), f/4 and f/3.4 max aperture versions made by then; as well as M (Leica Rangefinder camera) and mirror lock-up versions (for the Leicaflex as the Leica SLRs were named back then)). These lenses are of the non-retrofocus type not too dissimilar to the 38mm lens your port was designed for.

I concur with your skepticism! In my experience focus is critical even with fisheye lenses which have a lot of DOF - (focusing evident because of focus breathing which is visible when changing pix quickly such as in the Lightroom "film strip"). Also interesting is that the port can do 120°!! This means that a 14mm should be possible. Main requirements seem to be the need for a front filter thread (so my old Nikon 14/2.8 lens is out) and a non-extending lens for either focus or zoom. He also answered my initial question which was related to the obvious modularity of the port.

Thanks for posting the link to the previous thread which I had missed. I looked up the references in that thread. Note that the description in the Ivanoff patent indicates two lenses named 1 and 1'. Shown as well in the figures (but a and b in Fig 5). If your Ivanoff port does not contain both of these lenses it is incomplete. Lens 1 (or a) is the replacement for a dome port but only curved on the inner surface, the front surface being flat. To accommodate the curvature the front port is thick at the edge is hence a negative lens. The correction lens (1' or b) is a negative lens, thicker in the middle.