Chris Ross

A 6"dome is really a little small for a rectilinear lens that wide even on APS-C. The 6"dome from Fantasea is not a full 180° dome so the geometry likely means the lens entrance pupil is positioned forward of the dome centre of curvature to avoid vignetting so is not optimal. A bigger dome helps of course but the largest offered by Nauticam is the 180mm dome. You could use the 8.5"acrylic dome and it seems to use 10-15mm more extension than the 180mm dome. Focusing closer has also been mentioned you don't need Manual focus to do that, just use back button focus to focus on something closer in the scene and stopping down to the f11-13 range will also help your corners. As far as distortion goes, all the wet lenses have some level of distortion probably best handled by where you place straight lines in the composition, they bend more as you go towards the corners of course, so for example the image DSC01971 you posted above would have obvious distortion with those straight lines top and bottom of frame.

To be fair though the centre softness goes away when stopped down which you would do anyway behind a dome, the lens is obviously designed with astrophotography and infinity focus in mind so the minimum focus distance is quite long and correction at f1.4 when at MFD not a priority. A rough calculation shows that it could focus at its closest approx 1m from a 140mm dome but in a 230mm dome could focus on an object about 0.2m away from dome surface, so it could be used for reef scenics with a bigger dome. However there is probably little reason to do that when an adapted Canon 8-15 fisheye works as well as it does.

I have used the Zen 100mm dome with 8mm Panasonic fisheye. That lens is very sharp but corners suffer purple fringing (fixable in post). I've just bought a Canon 8-15 and the N85-N120 adapter and looking for the 140mm dome port. The 170mm dome is actually a little small for the 7-14mm lenses (all varieties) and it is has been reported a number if times that they do better in a 100mm dome, both Panasonic and Olympus versions are placed forward of the centre of curvature to avoid vignetting in the Zen 170mm dome. They are really not CFWA candidates. The 12-40 can shoot thing very close in a 170mm dome but to get decent subject size you are zoomed right in so it is CF as as opposed to CFWA. I use the 12-40 a bit around Sydney.

yes, correct, it requires a custom port solution, probably effectively a custom port and housing, though the big opening on the gates housing seems to allow just using a custom port cover with two mini domes.

Apparently Marelux have announced a housing timing not known though: https://www.bluewaterphotostore.com/marelux-sony-a6700-underwater-housing/

We found them easily at Diver's Lodge at Lembeh house reef a few years back, swam down to the spot and they very quickly started spawning.

An APS-C should do fine printing A3. An A3 print at 300 dpi is just over 17 MP while you have 26 MP so you are over sampling by 50%. You'll have more depth of field out an APS_C at same framing as you'll be further back with lower magnification than full frame. Probably a bit optimistic to expect to find a used a6700 housing, so far only a couple of vendors have announced housings for that camera.

I know, but it's still a possibility that something could go wrong and totally agree the leak sentinel is a better option.

This loses at least 50% of the benefit of the vacuum system. Pre loading the o-rings with the vacuum helps them seal at the surface - that's why leaks are so common in the rinse tank. It also assume that you install the plug properly and don't damage its seal when you replace it.

This would be the post, looks like it needs the big port opening of the GAtes housing to fit it in: https://www.facebook.com/thesextonco/posts/pfbid02L4JMoMUUndAhqgVtqoUYWTF6oZ5PMFvfa2VvBce3Ed3Rp1Fd9gB3veDQ4UhmPkCkl And this appears to be the lens with a custom housing for an R5C? https://www.facebook.com/thesextonco/posts/817991040115612?ref=embed_post

Looking at my Nauticam ports the O-ring groove seems to be 1.5mm deep and the o-ring 2.0 mm OD. Getting the gap and groove depth right is the key to compressing the o-ring enough to ensure it seals and also so it won't extrude if the support below the o-ring is insufficient. I would measure and copy what Sea Frogs does with their o-rings. If you have a set of calipers you can measure your existing ports to replicate this on your extension.

If you need to remove the Sea Frogs vacuum system, it defeats much of the benefits of a vacuum system. A vacuum system does two things, it allows you to check for leaks and it pre-loads the o-rings. However, if you need to remove the vacuum valve what happens if the plug you replace it with is not installed properly? Pre-loading the o-rings is important, leaks are most common at the surface when the o-rings are not loaded, it's easy to bump the housing and lose the seal. Far better to have a system that keeps the vacuum on the housing the whole dive. The Leak sentinel is a much better option and is quite popular with people here.

If you want to use a WWL you get the nauticam port and lens combination and change the lug ring over. Regarding your shots, what are you focusing on? Depth of field is 1/3 in front and 2/3 behind your focus point. If you focus on a closer point you may get a little more in focus. Also try shooting at f13/14/16 and compare corner performance. A further option is using a fisheye, you would need the Nikon 8-15 as the the most straight forward option and you could use your existing dome and only buy a different extension ring.

Right, i see how that would help - do you need extra care placing the o-ring compared to the smooth surface of something like an aluminium port? Seems like it might drag a little on the grooves? or does it work out OK in practice?

The bottom ledge looks a little narrow, I would think having it sit flush on top of the ring would be best if the design allows. What is your o-ring cross section? The force pushing the dome and adapter combination into a 90mm ID port is equal to the differential pressure multiplied by the cross sectional area. I calculate the inward force at 40m to be about 250kg. This places the adapter material in compression pushing it inwards so you want to spread this over a reasonable cross sectional area.

That should work, do you need a lip to limit how far it will slide into the extension ring?

Yes sucking the air out is probably a better representation, however as I understand it one of the issues is stability when materials are subject to compression, so for example if you press in lightly on the sides of the can it collapses even more easily. I have never done any 3D printing but it seems to me that the properties of the material are quite dependent upon the way the individual beads of plastic are laid down - does that vary with material and with the type of printer used? I've handled quite a few 3D printed items and they never struck me as something that would readily make an smooth surface to get an o-ring to seal upon. I assume different materials and print setting get around that? On the o-ring groove issue I would think you could make the walls thicker around the groove if needed just by reducing the ID of the port. The Tokina is 70mm OD and your port opening is 90mm - so the you could make the port 100mm OD at the base which would give a 5mm ledge where you insert port into the housing. My o-rings are 2mm OD with a groove ~1.5mm deep, so if you have 5mm wall thickness behind that, the ID would 100-2x(5+2+5) = 76mm ID and the top would be 80mm ID to take your port. You could taper the port out to that ID up at the top potentially. That ID is about equal to the N85 port of the Nauticam ports. Whether 76 mm ID is too small would depend on whether the zoom gear needs to fit through that hole or not. If you don't use mounting lugs and rely upon vacuum to hold the port in you can make the bottom of the o-ring groove a little thicker as well.

Designing things for external pressure is more problematic as the stresses are different. Issac's thin port withstood the pressure however a thin walled cylinder like that is not inherently strong under external pressure forces and may have collapsed if you grabbed the cylinder for example. Cylinders are not inherently strong under external pressure, only spheres are. Similar to the trick where you can stand on an aluminium drink can and it supports your weight until you flick the side walls and it collapses or the titan submersible where it was the cylinder that failed. I'm sure it can be done but I think one of the issues is probably going to be getting reproducible strength and water tightness from your print and couple that with the need to get smooth o-ring sealing surfaces. Properties of injection moulded plastics and metals are predictable, but I would suggest that similar to composites, 3D printed parts are not. If you want something custom I would think that machining it out of aluminium would be a better proposition. You can also machine acrylic or polycarbonate, if you could find tubing with the right dimensions to allow you to machine the part you need. regarding the entrance pupil, the 10-17mm is listed in this table of entrance pupil locations: https://wiki.panotools.org/Entrance_Pupil_Database#Third_party_lenses

I think the filter size is perhaps a proxy for front element and entrance pupil size. What has to happen is that the light bundle that corresponds to a 130° field has to fit through the entrance pupil. The entrance pupil is generally much smaller than the front element, so the front elemnt needs to capture the light bundle and as it passes through the elements in front of the entrance pupil it needs end up at a size that fits through without vignetting. The 14-35 is mentioned and that is on the FCP port chart and it was mentioned it works with the WACP-1 - presumably this is at 28mm focal length. The combination of front element and entrance pupil size and location would be allowing the light bundle through. The point of all this is that the lens in question 24-50 would potentially work with the FCP but the WACP-1 is probably less likely as it seems to favour 28mm f3.5 or f4 optics. We won't really know until someone tests it.

I've seen people do stuff with big heavy lenses that would make you wince, but throwing a mounted 500mm f4 on a Canon pro body into a backpack and driving thousands of km of corrugated roads I've done many a time and it's all still tight - in that you have two big masses joined by the lens mount and the lever arm of the long tele lens is much greater than what you have with the Sigma fisheye. The Sigma lens is 160mm long and weighs 1.3kg, a lot of that weight in the front element. The Nikon 14-24mm f2.8 lens is 130mm long and weighs 1kg - not too different to the Sigma fisheye and people wouldn't think twice about using that lens cantilevered off the lens mount. Similarly the Sigma 85mm f1.4 is 150 long in Sony mount and 1.2kg and 105mm f1.4 is 155mm long and 1.7 kg and both must be cantilevered to tripod mount. Regardless of the weight issues, it's all moot as the min focus and probable need for a large dome make it less attractive.

It's not ideal, but I have big tele lenses and the mounts are actually surprisingly robust, the bigger concern is the min focus distance and you probably needing the 230mm dome.

I think the only solution for the Sea Frogs port is to use the m67 version, the rim of the port is too wide to take the Nauticam bayonet adapter. This means the lens needs to be screwed in to the port everytime which is not as convenient. The other question if how well the Sea Frogs port fits the 28-60 lens, it needs to have the lens quite close to the port glass to allow the combination to achieve the maximum field of view.

Assume it's full frame but yes looks quite sharp enough to me - of course at 1200 px or so you aren't pixel peeping but looks every bit as sharp as any other pic I've seen at this resolution.

Yes you can upload pictures - just click where it says choose files or you drop them in that area. It will show as a thumbnal and you can click insert to place it inline.

Well the tripod collar looks to be removable, but it is a big lump for sure. The killer for UW use is the size: 104mm diameter won't fit through an N100 mount, presumably it would get through the ID on an N120 extension/dome. but the minimum focus is certainly a problem, close focus is 207mm from the front of the lens. The Sigma lens adapted is definitely more interesting as it is reported to AF quite well.