Dome port size /positioning

Hi all, i'm searching some informations on how to correctly choose the right combination of dome size and extension port. 

Especially for Sony system there are few informations around. I'm using an Easydive houseing with Seacam dome (special adapter made by me).

 Here my questions:

- Is it correct to position the flat surface of the dome in line with the lens nodal point? 

- Best dome size for a 14mm 1.8G wideangle?

- Big vs small dome? (Fisheye small, wideangle big?)

thanks all

E

1 minute ago, enricomarzico said:

- Is it correct to position the flat surface of the dome in line with the lens nodal point? 

Not quite. The lens nodal point needs to align with the geometric center of the dome. If the dome is a full hemisphere, then yes, the flat surface is your alignment point. However, if the dome is a smaller cut of the sphere, then the nodal point needs to sit behind the flat surface.

3 minutes ago, enricomarzico said:

- Best dome size for a 14mm 1.8G wideangle? 

Nauticam recommends 250mm for Sigma 14-24mm; it stands to reason that it will work with 14mm Sony as well. The bigger the better.

4 minutes ago, enricomarzico said:

- Big vs small dome? (Fisheye small, wideangle big?)

Corner quality benefits from larger domes regardless of fisheye vs rectilinear, but fisheye lenses suffer less from smaller domes. However, bigger domes don't let you place the lens very close to the subject. With rectilinear, bigger dome is typically the way to go, but with fisheye, it's a trade-off - smaller dome will give you better CFWA framing, at the cost of corner sharpness. If you are shooting over/under splits, then you want as large a dome as possible, regardless of lens.

Rectilinear wide angle lens performance behind a dome is dependent on nuances of lens design. Some just don't work well (Nikon 14-24 f/2.8 for example.) My working theory is that close focus is not a design parameter that is used when designing wide angle lenses, so those that do focus closely enough is a happy circumstance, rather than a function of focal length.

As @Barmaglot has mentioned any rectilinear wide angle lens on full frame will need a (very) big dome. I would have a really good search here on Wetpixel, there is a bunch of information on the subject here. 

Unless someone on here has used your specific combination, the only way to find the correct lens position and hence extension, along with whether this specific lens will work behind a dome port, is to try it out in the water! Please let us know what you find.

 

14 hours ago, enricomarzico said:

Hi all, i'm searching some informations on how to correctly choose the right combination of dome size and extension port. 

Especially for Sony system there are few informations around. I'm using an Easydive houseing with Seacam dome (special adapter made by me).

 Here my questions:

- Is it correct to position the flat surface of the dome in line with the lens nodal point? 

- Best dome size for a 14mm 1.8G wideangle?

- Big vs small dome? (Fisheye small, wideangle big?)

 

thanks all

E

The easiest to to work out extensions choice is probably to use existing port charts.  To do that you need to work out how your housing compares to the housing you are using the port charts for with respect to distance from the camera lens flange to the housing.  You need to adjust the amount of extension specified in the Seacam port charts by the difference between the two different housings  and also allow for any extension added by your adapter (if any) .

Basically what you want is the measurement from the camera lens flange to the end of the extension to be the same with your housing as the Seacam housing and this may change the recommended extension in the Seacam port charts a little  You may need to find someone who has a Sony housing to make this measurement. 

While you can calculate where to place the lens entrance pupil based on dome radius of curvature - the recommended port charts as I understand don't always follow this rule and most housing manufacturers test the recommended lenses behind their domes and sometimes a compromise works better.

For a 14mm rectilnear lens at least a 230mm dome is recommended and even then you may need to stop down to f11-16 and the far corners may still not quite perfect.

Yess, what i noticed is that also with the seacam superdome the corners are never perfect, and i tried differend port extensions. But probably i'll never solve it.