There is apparently an experimental solution to solve the lens positioning problem within a dome-port. I found a publication (see below) from a team that calibrates photogrammetry cameras for deep-sea rovers at an oceanographic research institute. Perhaps it is already well-known in this community – but I figured I’d share it anyways. Note: This is not about choosing the right dome size, curvature etc. – just the lens position relative to the dome.
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The idea is quite straightforward: The goal of the dome-port is to avoid refraction by permitting the light to traverse the water-glass-air interfaces at a 90° angle – there should be no difference between air and water then. This is only possible when the lens is in the perfect position, hence our dilemma. If the position is not optimal, there will be refraction (grossly oversimplyfied: the dome will act a little bit like a flat port). Now the trick is to TAKE A SPLIT SHOT of a straight object that enters the water. If the lens is well positioned, there will be no refraction and the lines continue perfectly. Otherwise, the lines will be displaced and slightly enlarged or reduced in size.
In the publication they had the camera on a screw-mount and could adjust its position (and hence the lens) „live“. For a housed „normal“ camera one would vary the port extensions until the best possible image is obtained.
I tested this with my newly acquired (second-hand) Athena Fisheye dome on an Olympus Pen housing with the Rokinon/Samyang 7,5 mm fisheye. This is probably a sub-optimal setup because the dome was designed for the Panasonic 8mm lens which is longer than the Samyang. The image clearly shows that the lines are displaced a little bit. It’s best if you zoom in to the air-water-interface on the right, which is nice and „flat“. According to the publication, I should thus vary the lens position = spacer length. If I understood the publication right, then a displacement towards the outside (as in my test-shot) translates to shortening the spacer (which I can’t – there’s none) whereas a displacement to the inside indicates that a longer spacer is required. I don’t own a spacer and thus cannot try to aggravate the issue either for further demonstration.
This seems like a simple test to perform once you have the lens, the dome and a collection of spacers at your disposal. It is unfortunately not helpful if you are trying to figure out what to order. But perhaps with the help of friends or at a workshop this can be of interest.
Certainly my representation of the physics here is oversimplified, certainly there are many more things to consider - I am not claiming to be a specialist. But experiments don't need to be absolutely perfect as long as they are informative and useful.
Publication:
Mengkun She, Yifan Song, Jochen Mohrmann and Kevin Köser: Adjustment and Calibration of Dome Port Camera Systems for Underwater Vision (), part of Lecture Notes in Computer Science, https://doi.org/10./978-3-030--9_6
(you need a subscription for this, but try Google with Authors / Title before you pay…)
It happens right at the air-water-interface. The goal is to find the extension length that will lead to a minimal offset in how the line continues under water vs. in air. If you have no offset at all, then your lens renders exactly the same image above and below the water line. This happens only when the lens and the port are optimally arranged with respect to each other.
I have probably sent people on the wrong track when I wrote about „stright lines“. Please excuse my lack of eloquence, I am not a native English speaker. Correctly stated, the observed effect here is the lateral offset of straight lines entering the warter. I have cropped the critical regions from the nice images provided by interceptor121 (thanks again) and annotated it accordingly - the test is essentially like aligning an inkjet print head (and this comes from the publication I cited, not my imagination).
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The optical laws have been derived through simple experiments and vice-versa we can verify optical geometry with such tests. Of course everyone is welcome to suggest additional approaches to complement and/or replace it, I am not at all saying that this is a mandatory exercise. I also think that having the port in the ideal position by no means implies that it’s the right port for the lens in question. It may be possible to derive this from the rest of the images, but I have no idea how and that is explicitely not the topic of this post.
I have said what I wanted (and by now more than once), so I will stop posting in this thread.
Cheers,
Klaus
1 hour ago, Klaus said:It happens right at the air-water-interface. The goal is to find the extension length that will lead to a minimal offset in how the line continues under water vs. in air. If you have no offset at all, then your lens renders exactly the same image above and below the water line. This happens only when the lens and the port are optimally arranged with respect to each other.
I have probably sent people on the wrong track when I wrote about „stright lines“. Please excuse my lack of eloquence, I am not a native English speaker. Correctly stated, the observed effect here is the lateral offset of straight lines entering the warter. I have cropped the critical regions from the nice images provided by interceptor121 (thanks again) and annotated it accordingly - the test is essentially like aligning an inkjet print head (and this comes from the publication I cited, not my imagination).
null
The optical laws have been derived through simple experiments and vice-versa we can verify optical geometry with such tests. Of course everyone is welcome to suggest additional approaches to complement and/or replace it, I am not at all saying that this is a mandatory exercise. I also think that having the port in the ideal position by no means implies that it’s the right port for the lens in question. It may be possible to derive this from the rest of the images, but I have no idea how and that is explicitely not the topic of this post.
I have said what I wanted (and by now more than once), so I will stop posting in this thread.
Cheers,
Klaus
Sorry you simply do not get it
Any error will result into the part in water go wider in both cases longer or shorter. The line of the water is because it was not still there is nothing to be derived.
If you bought the article you linked you would see that they use a checkered chart so that they can observe the distortion
An incorrect position of the dome of 2 cm results in an error of 1-2 degrees this is too small to be measured without a proper test rig
If you want to know if the extension was too long or short you would need a pattern similar to those used for lens correction and this would only work with a perfectly corrected lens and a totally aligned rig
To recap
Fisheye lenses are distorted impossible to determine if you are long or short using this method, however normally pulling out until it vignettes is good enough assuming you have an hemishphere
Rectilinear wide angle lenses you need to run a calculation first or find the entrance pupil with that you are 90% on target use this test just to check you are ok and fine tune
If you wanted to use this method you would need a test thank with the dome only in water and a macro slider. Take photos in steps of 5mm and check alignment. Or use a test rig again super stable with a remote control and use a grid pattern to detect distortion assuming you have a perfect lens profile.
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The problem is that as the water line is always larger and the part in water goes off on both sides you will not know from the first shot if you are too long or short unless you have horizontal and vertical pattern to check for distortion and your rig is perfectly aligned.
I used this method 5 years ago and it was frustating I have since developed a set of formulas when I don't have the lens parameters and have a 90% hit rate normally I am off not more than 5mm
1 hour ago, Chris Ross said:I'm not saying that you imply anything I'm simply seeking an explanation of the apparent contradiction. They claim that distortion one way makes an object look bigger and the other way smaller and it seems you disagree with that premise.
I totally agree with the fact that distortion shows where to go however I disagree that the difference is as apparent as figure 3 makes it looks it is not
When you have distortion if you look at the absolute centre of the image you do not see it the centre is always straight so looking at the split line is not going to tell you much that is why I drew the lines
What you need to do is to use a measuring chart and check the part underwater in fact you don't even need to shoot a split you can just shoot a grid and measure it, obviously to do that you need to entirely submerge the grid but this is actually easier than taking a split
The challenge of this method is that you need a perfect lens to start with, so one where a lens profile is available and project totally straight lines (Like adobe one)
I therefore disagree that this is an easy method, it is not and I know because I have tried it years ago there are several challenges
1. You need to have a bunch of extention ring
2. You need a stable test rig
3. You need to be able to measure distortion
There is also another practical issue distorted optics are as the word says distorted so there is no lens profile and is a bit difficult to judge what is going on when you no longer have straight lines
In practical terms it is more effective and efficient to model the lens and then do this test at the very very end as a proof of concept, it is not effective not simple, not quick and not easy to start without any idea of what is going on because it is much more complicated and difficult to measure than figure 3 implies
One more thing nonetheless…
Why bother?
That’s arguably the most important question. If the port position is off the ideal location, then the port will cause diffraction as seen by the lens (because the light that is captured does not traverse the port at a 90° angle). Unless you are doing photogrammetry, you might not even notice the resulting distortion - and if you do, it may be straightforward to correct in Lightroom. However, the diffraction does not affect all colors to the same extent, hence it leads to color fringes (=chromatic aberration). Seeking the best possible port position i.e. extension ring size will thus reduce fringing.
[Of note: Light that hits the port „straight on“ will always traverse at a right angle, hence the center will always look good (and not be distorted). Therefore fringing increases off-center and is worst towards the corners of the image.]
Personally, I don’t care about the distortion (except maybe for splits in very calm water) but fringing can be distracting when it’s really bad. The good thing is that we anyways use a small f-stop to deal with the curved virtual image and that also helps a bit with the fringes. But a well-placed port may get you the same result perhaps one stop wider? I don’t think that it is necessary to achieve „ideal“ positioning and the limited set of extensions provided by the manufacturers only allows for a coarse-grained adjustment. For example, I am using an MFT Pen port and AOI only makes two extension rings for that (14 and 24 mm), so only about 3 reasonable combinations (14, 24 and 38 mm) are available. In addition, the position of the lens‘ entrance pupil may move when focusing. Thus, don’t let „perfect“ be the enemy of „good enough“! Whenever possible, the port charts or your own calculations will tell you most of what you need to know given the limited options you have.
Then why did I bother?
Because I could not find any information on the Samyang/Rokinon 7.5 mm fisheye lens with the Athena port under water. I like to buy second hand gear and could snap up the Athena port for a very reasonable price, plus the (manual focus) Samyang 7.5 mm fisheye sells for really cheap – less than 1/3 of the Panny 8 mm even second hand. AFAIK, the Athena is not as „mainstream“ as the Zen or Precision ports, and the Samyang lens is cherished by astro-photographers but nobody seems to use it below surface. Thus, no possibility to do any calculations beforehand. Once everything had arrived, I wanted to know whether this is totally off or worth a try under water.
[I will post some images elsewhere here once I get a chance to go diving with it.]
If you also want to try some funky lens that is off the beaten path, or adapt a well-known lens via Metabones to a different system, you may also be looking for a relatively easy way to check which combination you‘ll take below the surface for your first trial. The method I found in the paper is an easy way to compare the 2-3 combinations you have available and select the best one.
A couple of misconceptions need to be corrected:
1)Just like the port itself, any distortion caused by it is radially symmetric. It therfore absolutely does not matter whether you use vertical or horizontal objects for a test. But to reveal this distortion you need to cross the air-water line. Due to gravity on earth, I find this significantly easier with a vertical object. Just don’t place it dead-center, and try to shoot as much as possible the same perspective with the combinations you want to compare. If you don’t believe this, just turn the camera by 90° and take another shot – it will look exactly the same. (Except if you rigged it up to the cold shoe as I sugested, because then your object will also be rotated and it no longer crosses the water-line.)
2) It is a no-brainer that a fisheye will remain a fisheye when housed behind a dome port. I can't imagine anyone here would try to "de-fish" by deliberately mis-positioning the port. This would not work.
3)I picked this up from a publication that dealt with photogrammetry for deep-sea rovers. They indeed used checkered test charts, but that is because they also inserted them slanted. The goal was to develop an algorithm that corrects for distortion even after the best possible port positioning (e.g. due to the port not being of ideal spherical shape) because that allows for even higher precision. Calibrating distances, especially for computer vision, is of course much easier with checkered patterns than simple lines. But unless you care for highest positional accuracy in your image processing, you’ll be perfectly OK with a simple vertical thing (straw, ruler, thermometer, strobe arm…) that’s placed at about 2/3rd from the center of your image. Interceptor121 provided a really good proof-of-concept image above.
Of course, checkered test charts also work, but they are not easy to find when a workshop member invites you to try a new lens while you're on a liveaboard. If you’re lucky, however, Interceptor121 is also there. After you narrowed down the port position for your system experimentally, he can use his math to back-calculate the entrance pupil position (-range) of that particular lens and afterwards forward-calculate the best available scenario for the systems of all other workshop participants. That’s where the combination of theory and experiment really shines!
Outex Domes Summary
Outex makes 2 Domes: 120 mm, and 180mm. These are diameter measurements in millimeters. Both domes work with all/any lens, past present or future. There's additional information about the benefits, advantages, and comparisons elsewhere in our FAQ Questions pages.
All of the Outex flat lenses screw onto your lens' filter thread (just like a UV filter or polarizer) and can work with any lens. But some lenses, such as unthreaded, fisheye, or hooded lenses do not accommodate a filter thread and often have curved lenses in the front. Those require one of the Outex dome ports. Both Outex domes work with any lens in the market, past, present, and future, because the domes are designed to be universal, and anchor to specific lenses thru adaptors or clamps.
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Using Dome 120mm vs. the Dome 180mm is not a definitive difference maker, just as using different lenses. As explained above, the larger dome increases the variety, frequency, and compatibility of your use with your camera, lens, and set of conditions. In other words, the larger dome make it easier to "get the shot" more often, regardless of what equipment you're using, and the conditions. The smaller, more travel friendly, more affordable dome can yield the same desired outcomes. But it may force you to work "harder at it" - meaning a "more controlled" set of variables like camera, lens, focal length, setting, proximity to subject, lighting, lighting type, framing, composition, etc. And that difference is exacerbated for split level (over/under) work.
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