Chris Ross
Contributor
Whether you agree or not, you still need a larger dome to get the same corner quality with a bigger sensor, the dome size has nothing to do with long focal lengths, it's all to do with the optics of a dome port and is most critical for ultra wide lenses. Underwater the camera focuses on a virtual image which for an object at infinity is located 3 dome radii out from the dome surface and concentric to the dome. That's very close and the corners of the field are much closer and even stopped down there's not enough depth of field to make the corners sharp. A 6"dome is quite limiting for a wide rectilinear lens on a large sensor, but will probably work OK with a fisheye lens.I'd still rather have a A7RIII than an A6600, but used a6500's go for under $1000 and the seafrogs housing is much cheaper. I am not sure I agree the dome is less limiting for APS-C, I won't use the dome on land and am not taking a telephoto under water. The standard housing does a E 18-55mm and the 6 inch dome (I don't see an 8 for any Sony) goes to 70mm. If I understand correctly, 70mm is 22 degrees on APS-C and 35 degrees on full frame.
The other factor is the extension used with the dome - to get the best results you have to place the lens entrance pupil at the centre of curvature of your dome if you don't the image is degraded and distortion is introduced. You'll still produce an image but all the money you spent on the best sensor and lenses is somewhat wasted unless you get this right. The amount the image is degraded varies from lens to lens and will vary from pixel peeping issues with a 35 -50mm equivalent lens to obvious blurred corners with a 16mm equivalent lens in a 6"dome even at web sizes. Here is an extreme example full frame 17mm lens at f8, 170mm dome: Sea&Sea Internal Correction lens: the best solution… to date! | Jordi Chias: Underwater photography
Sea Frogs housings are very affordable but they are cheap for a reason, they don't optimize the dome port location and have a one size fits all approach and only have different domes to accommodate physically longer lenses. Don't get me wrong, they'll allow you to get the camera UW and take shots but they won't necessarily be the best optical quality some people accept this compromise others not. Manufacturer's like Nauticam test each lens and recommend how much extension to use with that lens to get the best results and have multiple different dome options to choose from - that is part of the reason for the price difference. It seems to me taking a top line line camera UW with a budget housing is not the best option, particularly when the results from a 1"compact are likely to be just as good and the expense no greater. People still do it and produce images from these systems that are somewhat happy with.
This is a complex subject and explaining every last aspect of it over a series of forum posts is difficult to say the least, just look at the length of this thread.