Klaus

I unfortunately don‘t know what you are referring to with „ambient light“ filter, but it certainly looks nice. And shallow, too - so there is at least some red left in the ambient light coming from the sun.

I am certainly dry good at producing backscatter - I‘ll spare you of the image proofs. Whether one calls it edge lighting or simply moving the strobes out is semantics, not physics. I really prefer the wording in Alex‘ book: reduce the illumination of the VOLUME between the lens and the subject. The light of the (two) strobes should reach the subject, but not the water in front of it. I think that is only possible when you get only the outer parts of the cone (to avoid „edge“) on the subject. And if you use two strobes, then you make two edges converge on the subject. The theory is easy, putting it to practice is difficult for a beginner like me. But when it does work, it makes a HUGE difference. At least in the quarries I dive when not traveling. Would it be possible to get back to „pleasing lighting“ and leave it with that?

Ahem, red or not so red - I‘ve seen many really nice pictures of fish schools in this thread. The best way to bring back color to a Rouget is to fry it in a pan, IMHO (that‘s the French name, no idea what fish that is in English). Delicious! I am intrigued by the concept of a blue filter on the flash. I assume this works by allowing you to adjust the white balance with more red overall, hence recovering more of the remaining red from ambient light in the „non strobed“ parts of the frame. If you used a warm color strobe, you would cool the rest of the frame, which gives the nice negative space blue. If you used a cooler color strobe, you could push the red further overall, hence deeper penetration of color. Thus, do I understand this right: It‘s like the red filter, just inverted logic? Then, do I also understand it right that this will work best in shallow waters (say, up to about 15 m) and that in theory you need to get the right blue filter for the depth you are shooting? Maybe we‘ll see a new line of magic filters, this time for the strobes?

Thanks for you efforts to spread the word that a scientific approach is key to approaching the challenges we are facing! For prevention of diabetes or lung cancer this is somehow easier, but getting public awareness for ecology at a planetary scale remains a tough nut to crack. Keep going!

Chip, I really, really like that manatee shot - because the manatee is crisp and the trees are blurred. That is a unique look and it immediately focused my eyes on the manatee. With the trees in focus, it would not be as strong because the manatee does not have too much contrast with its surrounding. The focused trees might have been a distraction. So, perhaps the WACAP is what made this shot so nice?

with that airline, you might have had more luck trying to shoot a sirène rather than a mermaid? 😉 I have not used my fisheye a lot for splits yet, but wouldn‘t in silico de-fishing be a reasonable option? I understand that this will not replace the effect of a larger dome on the water line, but at least getting the other lines straight might be possible?

If you know the dimensions, you can also try a simple eBay search. There are different materials (silicon, NBU etc.) - a lot of technical O-rings sold there will be NBU but the housing & port O-Rings are likely silicon. I‘d stick with that if possible. Even if you do find the exact same size, a third party O-Ring may not be quite as supple as the original. This probably does not matter in absolute terms, but a less supple seal means perhaps that sand, hairs or jolts will more easily lead to water seeping in. Less forgiving so to speak. If you can still find an original it‘s worth the extra money, but if not the third party ones are at least really affordable.

Come to think of it: my first split! The edges are a bit blurry, water-contact does not live up to its promise…

I did… This was my first stint beyond a cheap GoPro knock-off. A compact camera (Samsung wb-2000) in a second-Hand, „universal“ clear polycarbonate housing. Switched it off at some point once I realized and continued the dive; at the end the display was an aquarium - AFTER I took it out of the housing. Fortunately this was a freshwater quarry, I dried the camera and it still works today except for the built-in flash. I‘m taking it skiing sometimes now, it‘s one of the few compacts that saves a raw. Lembeh, however, is not freshwater as I‘ve read…

This time I‘d guess fisheye based on how much the anthias get smaller in the corners. But I only bother because of the challenge here. When the distortion is a „feature“ then it‘s usually not particularly obvious- otherwise it becomes a „bug“. whatever, I guess in the end it‘s all about releasing the shutter when you have a great subject, fisheye or not…

No - but great job hiding the sunball behind the soft corals to protect it from getting blown out! i can only guess - rectilinear?

Image quality aside (and I know this is now off-topic), the shots I admire most ist where the fisheye distortion is used as an element of the composition - a feature, not a bug. I can mostly remember this from CFWA, not so much from reefscapes. But I guess these occasion are not that frequent, hence zoomable options are more flexible. Just a thought on aspects beyond the pixel details.

I had never heard of the myth that fish-eyes have good corner performance. Top-side tests usually conclude that the field of view is too large for the test charts available (I.e. the patterns don’t extend into the very corners because of distortion and AOV), „real world“ shots have a chromatic aberrations which of course can be better or worse depending on the lens. Most of the times, the tests conclude with something like „ … fisheyes are extreme lenses with inherent compromises.“ Now underwater, it‘s a different story. The distance argument stated above is what I have read pretty much exclusively as the argument for using a fish-eye and it pairs well with the fact that the distortion is not as obvious for most UW subjects. There is of course nothing wrong with discussing very special aspects of optical setups - but I had never read about the supposed myth that started this thread.

I think he posted a picture in the thread on experimentally testing the dome port position. Looked good to me, it‘s the one with the nice pool thermometer.

Sorry, posted it out before inserting the link : https://www.on1.com/products/photo-raw/download/

I don‘t know about Sony, but with m43 raw files the adobe editors get a lot of info from the cameras and some of these you cannot even revert unless you use other editors (at least for my epl5 raws, newer stuff may be different). So there is definitely a lot more in the raw files than just the pixel intensities. I currently use on1 as an editor, certainly not comparable with LR but once I got used to it, it can do a really good job for my needs. They have a free trial version that has all features enabled - check it out if you‘re interested.

I suggest using the personal message feature for this kind of exchange.

I’d be careful with the contact cleaner /silicon oil spray. Underneath the buttons there probably are O-rings, so I would not use very light silicon oil. It will diffuse into the O-ring and dilate it, eventually this can provoke leakage. Essentially the same story as with the O-ring for the housing. But a little lubrication is sometimes needed on those hard to reach surfaces. This is what I do: I have a small bottle of medium-viscosity silicon oil (around 250 cSt I believe) and place 2-3 DROPS into a bucket of water. Then I stir this vigorously to create a suspension (does not dissolve but the droplets get smaller). I then bathe the equipment in this shortly and work the buttons while submerged. You really need only very little of the silicon oil, because it will find its way to the hydrophobic surfaces by diffusion (@Chris: Ostwald’s principle for colloids…). The medium viscosity silicon oil is still not really the best for the O-rings, but this approach deposits only a really small amount. The rest of the plastic case will also feel a bit sticky when you take it out, because this also got coated. I rinse with clean water+ wipe dry, after a few days the plastic will feel nice and smooth again. The silicon oil diffuses into the plastic. This can be good for older equipment, as the silicon replaces the softeners that the plastic looses over time and hence this restores a certain elasticity. But as always - too much of this and you will wreck the case. I try to do this procedure only once every two years at max.

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!

Very interesting, thanks for sharing it here!

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

That is exactly what I meant, and if the information is correct that the image on the LEFT is with a longer spacer then that fits the interpretation. The shorter extension leads to barrel distortion and that causes a displacement to the outside (because that distortion does not happen in air). I‘m not trying to give a lecture on optics but to point out a simple test that will indicate in which direction one should make a change (displacement to the outside > longer, displacement to the inside > shorter). If that is true, then it is quite likely that already the second configuration tested will be the best one available to a given person. Let‘s just leave it with this and see whether anyone finds it useful. That is the main point here. For me it already is, because I couldn‘t find the information for either the Samyang lens or the Athena port (lack of perseverance). But I now know that this will have some limitations that I can only solve at the expense of more vignetting, so I won‘t bother. GAS cured, thanks to your help!

Thanks for the clarification. So let’s sum this up: If the lines in the test shot continue with an offset to the outside of the image (i.e. the uw part is slightly enlarged), you need to increase the distance between lens and port. That‘s the opposite of my initial interpretation of the publication text. I suppose that if you go beyond the optimum, then the lines will be off-set to the inside. And even a 5mm difference from the optimal dome position is visible in this test. Whether 5 mm matter in practice is probably a different question and practical considerations, such as vignetting or plain availability of parts are also limiting the options. For example going back to my own shot I wold thus need to add an extension to avoid the offset, but there is already some vignetting so I won‘t do this. It‘s reassuring that theory and test come to the same conclusion (as they should). With the calculations you can get an answer BEFORE you have the parts in hand, provided all the required information is available. Cool!

Dome port physics aside, I have to say it is a nice thermometer… I like it.

Nice picture, thanks a lot! Which one has the longer extension- the one on the left or the one on the right? Sorry if I don’t get it when I should…