Dome Ports for Video

[djensen]

Hi gang,

I have a general question re: dome ports. I gather that the reason to use a dome rather than flat port in u/w imaging is to widen your angle and cut your physical distance to the subject. Is that about right?
If my cam/housing with a flat port currently has no vignetting, how can I find out if a 60* or 90* dome port will cause vignetting?

I don't really understand how the dome changes the focal length or angle. Can somebody explain it?

Thanks.

 

[ronscuba]

Objects underwater look approximately 25% larger because of light refraction. This is true for cameras and the human eye.

A dome port attempts to correct this. Tricky part is the dome port and camera lens need to work together correctly or you will get vignetting, blurring, distortion and usually you cannot use zoom with the dome.

If you know the dimensions of the dome port and how it will attach to your housing, you can predict if it will vignette by shooting footage of a ruler and comparing the dimensions.

Not sure if there is a way to predict blurring and distortion. What camera and housing do you have ?

 

[djensen]

I have a Sony HDR-PJ710V in a Mangrove MVHS-L. I currently have a flat port and I find I have to put too much water between the subject and myself in order to get a wide enough shot. I want to get closer to my subject while getting a wider shot at the same time. Add-on W/A lenses are not an option with my camera.

 

[ronscuba]

I thought Mangrove had wide angle port/lens options for their housings ?

 

[djensen]

They provide a flat port (what I have currently), a 60* dome and a 90* dome. No other options on the website.

 

[jcclink]

When using a flat port the effective focal length of the lens is increased by 4/3 due to index of refraction thru air/water interface. With a properly configured dome port the focal length remains the same. However with a dome port the lens must be able to focus on the virtual image which is close to the port. In general infinity would be 15-18 inches. Will your lens focus at this distance & closer. If not you will need to add a diopter to your lens to essentially pull the focal range in a lot closer to the lens, letting it focus on the virtual image. Using a 60 or 90 deg dome depends on the angle of view of your lens. If you're shooting macro use a flat port. Otherwise a dome would be a better choice.

 

[djensen]

This is good. I'm learning stuff. The Sony has a focal range of 26mm - 260mm (16:9), slightly less in 4:3. By far I shoot more wide angle than macro. I'm not always in clear water, so cutting my distance to subject is the desired goal of the dome port purchase. My Sony does focus close, and it has a TeleMacro feature that allows it to focus very close for full-frame macro. So... Is there a way to know if the dome/lens combination will provide sharp focus and no edge to edge distortion? Can I zoom with a dome port? Auto focus? Thanks.

 

[ronscuba]

I believe Mangrove makes universal housings designed to work with several different models of camcorders. This is great for flat ports, but can make it difficult to ensure sharp focus and no distortion or vignetting for dome ports with all the camcorders with their different focal ranges.

Can the camera mount shift forward/backwards inside the housing ? If so, you can probably adjust the camera position into the ideal spot for the dome. The only for sure way to know is to find someone who has the dome and your camera or if Mangrove has actually tested the combination underwater and can show you footage.

Zooming with a dome port is usually limited because you don't want to zoom past the virtual image.

 

[jcclink]

In theory the center of curvature of the dome should be located at the nodal point of the lens for optimum performance. With a zoom lens the nodal point location varies with the focal length of the lens, so as you zoom the NP shifts slightly in/out. Since you said the housing manufacturer offers dome ports they should be able to tell you how to position it properly. If you're lucky they designed the mount so it locates the center of the dome in the right place for your camera/lens. If you have soft focus on the edges add a diopter to the front of the lens but keep in mind that this also affects the focal range. You can test setups in a pool or in a plastic tub (like a big rubbermaid tub) that is 12-18 inches deep. Only the dome has to be underwater to test - not the whole housing. Put something with fine print on the bottom of the tub as a test subject (like newspaper but waterproof).

 

[ronscuba]

.....Since you said the housing manufacturer offers dome ports they should be able to tell you how to position it properly. If you're lucky they designed the mount so it locates the center of the dome in the right place for your camera/lens. .....

As you stated, the position of the lens in a dome port is critical. Since the Mangrove housing is universal for over 40 different Sony camcorders, hopefully the mount allows some shifting inside the housing. Ideal lens location for the dome might require shifting the camcorder both horizontal and vertical.