Browse By

Flat vs. Dome port test on wide focal length lens for 3D applications.

Last year I did some tests with a Focus Optics Ruby 14-24mm lens. I tested them on a 3D rig and underwater with a flat and dome port. So this is Part 2.

More info on the 3D lens testing here on my previous article where I tested a matched pair on a 3D Rig. (Part 1)

Using one of those lenses a RED One and a Element Technica REDONE underwater housing. Using a Dome port and a Flat port.

Here is the Element Techinca Housing with the Flat port.

The Dome port I used….a bit on the small side, a larger one would be even better and better suited to this amazing lens. Still for the purposes of this test it will do just fine.

Test Goals

The purpose of this test is too determine how bad is a wider focal length underwater on a flat port.

When shooting underwater with a beam splitter you are forced to use a flat port. This is because of the (virtual) overlapping of the lenses. A dome port is simply impossible when using a beamsplitter.

At the same time we all know that wide lenses optically perform extremely poor with flat ports.

Flat ports exhibit a few main issues underwater, the flat port is unable to correct for the distortion produced by the differences between the indexes of light refraction in air and water. Using a flat port introduces a number of aberrations when used underwater. They are:

This is the bending of light waves as they pass through different mediums of optical density (the air inside the camera housing and the water outside the lens port). Light is refracted 25 percent, causing the lens to undergo the same magnification you would see through a facemask. The focal length of your lens also increases by approximately 25 percent.

Demonstrated here in this split level shot of my assistant. His body looks huge compared to his head. This is a clear demonstration of refraction and the magnification it produces.

If the subjects moves things get even worse! Needless to say split level shots with a flat port are not advised. Unless you want the effect!

Radial distortion
Because flat ports do not distort light rays equally, they have a progressive radial distortion that becomes more obvious as wider lenses are used. The effect is a progressive blur, that increases with large apertures on wide lenses. Light rays passing through the center of the port are not affected because their direction of travel is at right angles to the water-air interface of the port.

Here is an example, flat port vs. dome on 14mm

Chromatic Aberration
White light, when refracted, is separated into the color spectrum. The component colors of white light do not travel at the same speed, and light rays passing from water to glass to air will be unequally bent. When light separates into its component colors, the different colors slightly overlap, causing a loss of sharpness and color saturation, which is more noticeable with wider lenses.

Dome Ports
The dome port is a concentric lens that acts as an additional optical element to the camera lens. The dome port significantly reduces the problems of refraction, radial distortion and axial and chromatic aberrations when the curvature of the dome’s inside radius center is placed as close as possible to the nodal point of the lens. When a dome port is used, all the rays of light pass through un-refracted, which allows the “in-air” lens to retain its angle of view. Optically a “virtual image” is created inches in front of the lens. To photograph a subject underwater with a dome port you must focus the lens on the virtual image”, not the subject itself. The dome port makes the footage marks on the lens totally inaccurate for underwater focus. Therefore lenses should be calibrated underwater. The dome port offers no special optics above water and functions as a clear window.

It’s really too bad were are stuck with the Flat port for 3D but there is little we can do. Sure you can use a “side by side” configuration with dual dome ports but this would result in a large inter-axial which is really not ideal for feature film 3D production.

So the purpose of this test is just to get an idea how poor and what the loss is and if that will be acceptable to potential cinematographer for use on feature productions.

First I assembled the camera, adjusted my settings, formatted the hard drive etc…

Here you see the lens in the housing before I put the final port on.

As you will see below I first shot the camera on the surface with no port, dome and flat. Then Underwater dome and flat at various focal lengths.

I setup a c-stand with my homemade geometrical chart i use for 3D aligment but I added 2 putora sharpness charts at each edge.

First as a baseline/control, I shot the chart on the surface clean, with the Dome and Flat ports at various focal lengths.
Click here to download the frame grabs.
full res 4k Tiff

Second, I shot the chart underwater with the dome port at various focal lengths.
Click here to download the frame grabs.
full res 4k Tiff

Lastly, I shot the chart underwater with the Flat port at various focal lengths.
Click here to download the frame grabs.
full res 4k Tiff

Now, naturally the results are predictable. The dome port performed well (not great, a bigger dome would of done even better) and the flat port performed poorly and got worse as we got wider. So we expected this. The questions really was how bad was it and is it acceptable?

My point is, the level of acceptable image quality is very subjective and is one that needs to be evaluated by the FX supervisor(post house), DP and Director. Ultimately in 3D feature productions we just don’t have a choice….we HAVE to use a flat port and in certain situations we might HAVE to use a 16mm lens.

I think that what might make it easier for Directors and Dp’s to “swallow” and image which is less than perfect so to speak is the fact that underwater and in 3D you tend not to put anything on the edges of frame to avoid “window violations” of the 3D space and in the center of the frame things are sharp. and the edges well they can tend to be the very “homogenous” blue ocean…. even when things are filling the frame if you also add motion blur into the mix a lot of shots can then get a “passing grade”. Just take the image of the anchor or the fish in my ocean shots below, you eyes are drawn towards the center and you tend to ignore the edges. I can guarantee that if those images were 3D that would be even more true.

To illustrate my point….

I was the stereographer on a $40 million feature called shark night 3D…/sharknight3d/

This was our underwater camera rig…..a now aging but capable PACE|FUSION side by side rig/housing. Also used in James Cameron’s underwater documentaries “Aliens of the Deep” and “Ghost of the Abyss” and others blockbusters like “Sanctum” and Resident evil 3D.

Sony F950 cameras and poor quality Fujinon zooms (I forget the focal range but they were very wide on the wide end maybe 16mm equivalent) on a FLAT GLASS PORT.

Open rear view of the housing

Coming out of the water on set.

It has a massive 2.75″ IO poor quality Fujinon lenses(when compared to cinema lenses) and flat port.

Most of the shots were done on the wide end.

We shot many charts underwater for VFX/post to correct the images. Post then takes the chart and corrects the distortion and applies the corrections to all the underwater footage. This way it gets rid of one of the flat port side effects. Unfortunately there is nothing that can be done to correct the soft edges….so not everything can be fixed in post!

The Fujinon lenses even above water displayed CA and significant barrel distortion on the wide end. Underwater it just got worse.

********PACE now has a new underwater rig…..they used it on “Life of Pi”. I have first hand confirmation(from the camera crew) that they did have to use 16mm on a flat port at times.*********

Point is…..all these features saw theatrical release. All these features were able to correct the distortion and other issues with the images this rig created. To what degree? Well pop in the DVD and watch for yourself.

Furthermore… is the “Panavision 3D splash box” we used on some above water scenes………..the “box” had just come back from shooting “Pirates of the Carribean 4″ 3D on RED one cameras.

The “box/fishtank” features a large FLAT glass port”. Lens most used in this setup was… guess it a 16mm. I have friends that worked with the box on pirates and can verify that.

We used the Box/fishtank with a 18mm cooke zoom (18-40)

Here it is in all it’s glory.

Did you watch Pirates of Caribbean 4? Then you saw images created using this box/fishtank and a flat port with a wide lens.

Here is a side shot.

I’m not doubting the laws of physics that we are fighting. My job is test stuff and present it to people above my pay grade regardless how ridiculous it might seam.

It’s up to them (mostly VFX guys/post) to evaluate it work their million dollar post tools on it and tell us if it’s an image they can work with and make better.

That’s is the main purpose behind this test…..

Based of my past experiences (posted above) coupled with this test, I think that with a 5k epic (more pixels and data for post) great lenses (ruby) instead of Fujinons that 16mm will can be usable in a flat port.

I would generally advise going below 20mm but it’s up to them to decide.

Filmmakers “might” be willing to live with “good enough”. Certainly has been the case in the recent past as I pointed out.

Why this is relevant to me and others,

Because for feature film production we are forced to used “Beamsplitter” rig because this allows us the smaller IA needed for large screen 3D production. The only way to get parallax into reasonable parameters for large screen production is to use a beamsplitter and therefore we are also forced to live with Flat ports.

Here is the GATES “DEEP EPIC” underwater housing for the 3ality Technica “ATOM” rig.

from the other side, you can see the lens controls and the 12″ transvideo 3D monitor housing.

The other feature quality (Red Epic 5k) underwater beamsplitter housing I will be using and that is currently available is the “Mocean Armor Magicine GV4″ Red Epic beamplitter housing. (full review article coming soon)
Used by underwater cinematographer Ken Corben recently in the Artic.

Here is a look at it ….. you can just see the top camera on the beamsplitter and the large front flat port.

In the end I just point my camera at what the DP tell me to!

So I leave the final evaluation and decision to you the reader, DP, director…..etc….

Here some “real world” image grabs from video shot with a Red One with a Flat port in Catalina Island, CA (Casino Point) on a very sunny day…..

Red One…Ruby Lens..salt water…flat port.
here is a link to all these JPEG and Full Res 4k Tiff


Some Fish….

Bouey Anchor 30ft depth…

another chart shot…you can see the chromatic aberrations towards the edges and the distortion and the soft edges…..hehehe it’s all in this picture! Naturally notice the center is tack sharp.

You can see the glass is sharp(pun intended) but the rocks along the edges get soft.

similar issue on this shot of a sea cucumber

In the end ….. form your own opinion.

Last image from my dive…I saw a shadow on the sea floor then looked up to see this….I was around 60ft unfortunately so most of my light was gone….it did give this a eerie feel to it.

1stAC E.Gunnar Mortensen for his help on this day.
Element Technica for use of their housing.

I’ll leave you with a gallery of the test,


2 thoughts on “Flat vs. Dome port test on wide focal length lens for 3D applications.”

  1. Pingback: Life Behind the Lens: Flat vs Dome Port Test on Wide Focal Length Lens For 3D | Kelly On A Tangent
  2. Trackback: Life Behind the Lens: Flat vs Dome Port Test on Wide Focal Length Lens For 3D | Kelly On A Tangent
  3. Pingback: Life Behind the Lens: Flat vs Dome Port Test on Wide Focal Length Lens For 3D | Kelly On A Tangent
  4. Trackback: Life Behind the Lens: Flat vs Dome Port Test on Wide Focal Length Lens For 3D | Kelly On A Tangent

Leave a Reply