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I've produced IMAX, HD and 35mm stereo 3D projects and there are basically two ways you can set up the cameras. You can either put two cameras side by side or, if you want to get the lenses closer together than the lenses or bodies allow, you have to use a mirror rig. They’re called mirror rigs because inside the rig there is a half-silvered mirror angled at 45 degrees. One camera takes half the light through the mirror and the other camera is pointed down onto the mirror and receives half of the light reflected from the mirror.

I produced a film called Bugs!, which is an IMAX 3D movie, and we are currently repositioning it from IMAX 3D to digital – basically, the left eye needs to be moved across to the left so we can show it on digital cinema. I've been doing that and other 3D projects here at Pablo Post, Pinewood, with owner Ralston Humble. A few months ago Ralston and I went on a 3D production course in France with a stereographer/cinematographer called Alain Derobe, who has been building bespoke mirror rigs for people for the past 10 years. He’d got to the point where he didn’t want to build rigs for people any more: he wanted to back to being a DP. So he approached P+S Technik in Munich, gave them his design and said: “you build this rig, because it sounds like there are enough people out there who want to own there own.”

Up until this point you could only rent stereo rigs, often with the people who came with them, from one of three and a half companies around the world who provide them. There's 3eality, the people who did U23D and the Katzenberg live interview from LA at IBC, and then there's Vince Pace, who works with Jim Cameron and they first did a couple of IMAX movies, Ghosts of the Abyss and Aliens of the Deep; Pace also did the highly successful Hannah Montana and his cameras are being used on Avatar and all sorts of features and concert films. And then there's Paradise FX, who have done Dark Country with Geoff Boyle and then My Bloody Valentine. The half is NHK Technical Services whose cameras were used on indie horror, Scar. All of these companies have excellent rigs, but you can't buy them; you have to rent them and usually they come with people.

We discovered when we went on this course that P+S Technik was about to launch a mirror rig for sale and we decided that we wanted to play with one. With a mirror rig you mount one camera to look forward through the mirror and one camera has to face down to get light reflected from the mirror. The basic deal of using a mirror rig is that even the most engineered and geared cameras can only get the centers of the lenses about 7cm apart. For a whole range of 3D cinematography, that's still too wide. There is a current spate of people putting Sony 750s at 12, 13, 14cm apart and shooting things close, and that's a complete no-no in stereo 3D. Even with 7cm apart, anything closer than 3m and you start to set up problems in a deep background if you maintain this interaxial of 7cm.

With the mirror rig, you have one lens reflected in the mirror, so you can move one camera so that you can actually overlay the image of one lens over the other. When the two images are overlaid, effectively you have a 2D image, because both cameras are looking at exactly the same thing. You can smoothly move it out during a shot from this position, '0', to whatever you want, up to 12cm. That's something you need to be able to do for a significant amount of 3D filming; for instance if I'm filming a vista I could have my cameras 10 or 12cm apart and get wonderful 3D of the distant trees. But if someone comes into view closer to the camera, and you were cutting them off at the waist, they'd need to be very close to where the screen plane is. For this to work, the images need to be reconverged so that the subject is on the screen plane. This is achieved either by converging the cameras during shooting or shooting with the cameras parallel and shifting the mages relative to each other in post. As I shift in post, I get the subject closer and closer to looking like a single image, but the background separates.

Let’s back track, as this is both important, but a little tricky. There are three kinds of parallax – the horizontal differences between the left and right eye images on the screen. There's positive parallax, which is in the background, behind the screen plane, where the left eye image is to the left of the right; then there's zero parallax, the two images are overlaid and the subject would be 2D and at the screen plane; then there's negative parallax, where the subject could put their hand out into the audience say, and on screen the left eye image would be to the right of the right image and the eyes would fuse those together to make it look like the hand was coming out of the screen, in front of the screen plane.

The problem with shooting too close to subjects with the lenses too far apart is that the level of convergence required, either in shooting or in post, to put the subject at the screen plane means that the background parallax ends up too wide, and it enters into the potential discomfort zone. Our eyes see parallel when relaxed, and 6.5cm is as far apart as they normally go to look at infinity, in the distance. So, for comfortable (albeit a little cautious) 3D you would want the backgrounds to be a round 6.5cms of positive parallax maximum 13cms really. But, our eyes are very good, and they can theoretically bend out, one degree per eye, in order to fuse the images ad allow the brain to perceive single object. But if you do a bit of math, that 1 degree would end up being a few centimeters extra either side of the 6.5cm on a big screen, up to that 13cms. And so what you need to do in order to make a shots with subjects close to the camera work is to bring the lenses closer together. That way, the effective angle you're needing to converge the cameras to make the subject end up at the screen plane is much less, and the resulting positive parallax in the background is much smaller, and the 3D is comfortable to view. All the early good 3D in theme parks, such as Terminator 2 3D, Honey I Shrunk the Audience and Captain Eo, all used mirror rigs. In fact, all the people who started using side-by-side rigs in 2003 are now using mirror rigs as well, because they realized the side by side rigs only took them so far.

The P+S rig has been designed to be modular. The box we have is the medium box, and it will take a range of HDV and small HD cameras such as the EX1 and EX3. They make a larger one for the Genesis, Viper, Sony HDCAMs, for instance. The bracketry is the same, but you have a different camera plate and a different sized box. We specified the EX3 when we ordered it, and the plate was specifically measured to fit the camera. The system is designed so that once you've set it up there is minimal fine-tuning to get it ready to shoot.

The key thing in 3D is that your cameras need to be aligned in terms of roll, pitch and yaw. The image size needs to be the same, all sorts of line-ups have to be performed – one camera can't be higher than the other – because the whole point in 3D images is that the only difference between the two images when you come to the edit is horizontal parallax.

Our rig is a basic unit and is currently not motorized (the current rigs from 3ality, Pace and Paradise FX are already motorized); on the P+S there is a manual control to shift the interaxial – the distance between the centres of the lenses; but P+S are working on the motorization of this interaxial. The other crucial control is angulation (that's going to be motorized by P+S too), where you change the angle of one of the cameras, which is where the magic comes in. The magic comes in with balancing the angulation of the cameras with the distance between the centers of the lenses..

One of the standard ways of recording 3D currently is to an HDCAM SRW-1 deck, which has dual HD SDI inputs, so you can use one input for camera A and one for camera B and lay down two synced stereo HD images, albeit at 4:2:2 not 4:4:4, and play these back directly through a projector or take each eye off and ingest into something like a Quantel. It's a single deck – OK, it costs six times more than the EX3 camera – but it means that although these are relatively cheap cameras, and would generally be recording at a pretty compressed 35Mbit/s, we can take the 10bit uncompressed out onto HDCAM SR, or on to S.two or the Codex portable, and it would be good enough for HDTV, good enough for feature films, basically good enough for anything, even though you're using what is basically a small cheap camera with three 1/2in chips. Also, for 3D you want to be frame accurate, you can't afford for the cameras to be half a frame out, so having genlock on the EX3 is important.

The rig we used for Bugs! was 115kg, and it took four people to move it. The P+S Technik rig can be lifted by a single person, which is going to make shooting 3D a lot easier. We’re looking forward to it!

Mirror, mirror

Phil Streather