26 minutes ago, ashic said:
The Rx100 (vii) does have the 24-200mm equivalent zoom on it. But with that range, would a flat or a dome work better?
RX100 VII works best with flat ports and wet lenses, but its long lens introduces an additional challenge. Wet lenses need to be very close to the actual lens front element in order to work properly, but the RX100 VII lens extends and retracts a lot as it zooms, and nobody has figured out how to make a telescoping water-tight port so far. Therefore, the housings for RX100 VI and VII usually feature interchangeable ports - you have a short flat port that limits your zoom range but allows the use of wet wide lenses, and you have a long flat port that lets the lens extend to its full extent, but vignettes at wider FoVs - this port is meant to be used with close-up lenses for shooting macro. You can also have a long port with a wide opening that will let you use the entire zoom range without vignetting, but that front opening is too large for the typical 67mm threading. In case of Nauticam, they have a special flip holder that attaches to the port, so that you can shoot the lens at full range without wet lenses, and flip down a close-up lens when you want to shoot macro. The SeaFrogs port does not have that.
36 minutes ago, ashic said:
I'm sort of getting an idea when it comes to wide lenses - wide + flat = no!
It all has to do with refraction. Light travels at different speeds in water, glass and air, and when light rays strike a boundary between different propagation mediums, they get bent (refracted). The sharper the angle, the more they get refracted. Therefore, when you have a wide-angle lens that's 'looking' at something like 55 degrees off-nominal (110 degree FoV) the refraction encountered is quite significant. This is what domes solve - when you place the entrance pupil of your lens at the exact geometric center of the dome's curvature, then anywhere that the lens is 'looking', the light passes through the dome glass at exactly right angle, so no refraction occurs. Of course, in the real world, such precise placement is impossible, not least because the entrance pupil is not a discrete point, but we try to get close enough. This is why proper extension lengths on dome ports are so important.
43 minutes ago, ashic said:
Form a bit of searching, that appears to be a "wet" lens. Do wet lenses perform much better than flat ports and domes?
A wet wide lens has multiple lens elements inside that correct for various aberrations introduced by shooting through a simple dome. A dome, in effect, is also a lens element, as the water/glass/air boundary refracts light just like any other lens (this is, in fact, how all lenses work). Both domes and wet lenses project a sort of 'virtual image' into the lens, but whereas domes are a single element that fails to correct for things like field curvature (the virtual image is curved, and has trouble fitting into the camera lenses depth of field, this is what causes soft corners), the WWLs (and their cousins, the Nauticam WACPs and EMWL system) are designed to present a camera lens with the best image that it can relay to the sensor.
Because the window where the image is relayed between the wet lens and the actual lens is limited in size (usually 67mm, or 52mm for the smaller compact housings), as are the actual lens elements in the wet lens (they are, in fact, already huge, rivaling the elements in long telephotos), the wet lens designers have to operate with certain assumptions about the lens that's going to be behind their device. Most wet lenses are designed for 28mm focal length lenses, some (Nauticam WWL-C, Weefine/Kraken WFL-01/KRL-01) target the more modern 24mm. A special case is Nauticam MWL-1, and Weefine/Kraken WFL-09S/KRL-09S - these project a ~150 degree field of view into a 50-60mm macro lens. At any rate, what's important with all these wet lenses is that the entrance pupil of the camera lens be as close as possible to the wet lens rear element. This is typically signified by small front elements - if you see a large front element on a lens, this is usually indicative of the entrance pupil sitting deep inside, thus requiring this large element to avoid vignetting. Another important criteria is internal zooming - if the lens extends and retracts a lot, then it will either withdraw into the port and vignette, or bump into the port glass, or both. Sony 16-50mm f/3.5-5.6 PZ for APS-C and 28-60mm f/4-5.6 for full frame are notable examples of lenses that work very well with WWLs, as they tick both of the above criteria.
1 hour ago, ashic said:
That particular Nauticum is over £1K - so I would expect it to blow any domes out of the water.
Read @Phil Rudin's articles in UWPMag; he has tested just about everything under the sun. In short, for overall image quality shooting wide-angle underwater, the best solution appears to be Nauticam WACP-2 ($8442), followed by WACP-1 ($5159), followed by WWL-1/WWL-C, with traditional 230mm domes (which aren't cheap either - the Nauticam #18812 dome recommended for for Sony 12-24mm and 16-35mm lenses retails for $2439) trailing far behind.
Remember, however, that your system overall performs only as well as its weakest link. Put together the best housing, best port, best glass, best everything, but omit strobes, and your results will be disappointing. Use a port that doesn't fit your lens well, and the images will be ruined no matter how good your glass and lighting are. Use a housing that doesn't let you access important camera functions, and you will simply miss short opportunities as you struggle to set up.
Seriously - insurance companies are known for avoiding payouts, some are better than others of course. I would strongly suggest using the vacuum system, I assume your housing has it?
