Wetpixel Live: Dome Port Choices

@Alex_Mustard and @adamhanlon set out to answer some of the questions that are commonly asked about dome port choices on the Wetpixel forums. They include in their discussion: Port size, extensions, materials, and lens choices. There is no physics or optical theory!

Thanks Adam and Alex, the question comes up often how close you need to be to the correct extension in the forums - you touched on this a bit, but do you have a feel for how close?  Seems like being with 5-10mm which is the a common gap between extension tubes would be mostly OK?

Another point to note in general terms is that some ports are 180° dome ports - termed fisheye ports generally and others are segments of a larger diameter dome and often they cover around 105-110° of arc or about the field of view of a 15-16mm lens.  To achieve this without vignetting the lens entrance pupil needs to line up the base of the dome which may or not be the optically best position to place it, as the centre of curvature of the dome is generally 10-50mm down inside the extension.  I've done calculations for the 170/180mm domes from Zen and Nauticam  for the 7-14mm lenses in m43 systems and the centre of curvature is around 30mm behind the entrance pupil. 

The short version is that this type of dome if often compromised on optical centre placement so they can get the full angle of view without vignetting for lenses in 14-16mm full frame equivalent range, depending on where entrance pupil of the lens is located - most wide angle zooms seem to have the entrance pupil towards the front of the lens.   Lenses narrower than 16mm seem to be progressively less impacted as there is more freedom on placement to avoid vignetting.

Chris Ross, I am aware of this too. I think there are two issues, big hemispherical acrylic domes are crazy buoyant. And fisheye's seem to manage fairly well behind section domes like the Nauticam 8 1/2 acrylic in spite of the no parallax point of the lens being well in front of the centre of curvature of the dome. With wide rectilinear I think it's much more important to get the no parallax point and centre of curvature in the same place for best results. My 230 Nauticam glass dome is about 70% of a hemisphere and seems to work ok with a fisheye but needs at least 70mm extension with the 16-35. Water contact optics seems attractive for this role.