Paul M Clements

When designing and developing a steroscopic camera rig using a beam splitter, the orientation of the mirrored camera to the camera that sees through the mirror is very important, especially when using digital cameras. The main issue concerns electronic shutters and the effects caused when the mirrored image is flipped or flopped (flipped vertically). Skew is one of the biggest problems with electronic shutters such as that in the Red One. Skew causes objects to look as though they have tilted to one side like the ‘Leaning tower of Piza’. It occurs when a camera is panned or more commonly when an object moves across the frame quickly. An example of which would be a bus or train moving in front of a static background. When shooting in 2D it is often easy to ignore or relatively simple to correct with the right tools. Get it wrong when shooting stereoscopically and you’ll be spending a very long time in post fixing the issue and quite probably coming to the conclusion that the shot is next to useless.

In the following diagrams are four different setups. Each setup has two images beneath, which illustrate what the cameras see, as well as a third image which shows the mirrored camera (Camera 2) after the image has been flipped or flopped. If you were to combine what Camera 1 sees and what Camera 2 (once flipped or flopped) sees this would give us our stereoscopic image. There are eight figures which demonstrate how each mirror rig setup would see an object (In this case a letter ‘R’). The first figure of each setup shows a static shot and the second shows how the object would look if the camera was panned left to right or the object in shot moved from camera right to camera left rapidly.

Read to the bottom to see the conclusion.

Beam splitter 3D Stereoscopic camera rigs and the importance of correct camera orientation highlighting the issue of skew using digital cameras

As we can see Setup 1 and Setup 4 when flipped have the object ‘R’ skewing in the same directions. As such these two methods represent the best solution. There is the added benefit that both camera’s sensors scan top to bottom in the same direction. We can see this because the top of the ‘R’ is at the top of each image before being flipped. Because of this, other unwanted effects that electronic shutters create will also match. An example would be a photographer standing in frame pointing a stills camera and using a flash. When the flash goes off an electronic shutter might see that flash over two frames. The consequence is that half the flash is visible on one frame overlapping the shot and half is visible in another frame. But since both cameras scan the same vertically they will both match (Perhaps not ideal but significantly better than Setup 3 and 4 would be).

So what’s wrong with these electronic shutters then, why do they have this weakness? Well essentially it’s due to the “Read/Write rate” of the sensor. Like a television or computer monitor the sensor starts in the top left and reads the first line of horizontal pixels across to the right. Once it reaches the far right edge it goes back to the left hand side and reads the next line of pixels beneath this, repeating over and over again until it reaches the very bottom of the sensor. This makes up a single frame of the footage. It then goes back up to the top left where it starts the process again to produce the next frame. A fast moving object, or the frame being panned, is moving faster than the Read/Write rate which means that each line as it is recorded from top to bottom is seeing the moving object or moving frame slightly further into shot than the above line of pixels. As such they do not line up vertically. The result is skew.

Will a faster Read/Write rate negate this issue? It will improve it considerably but it is likely that such artifacts will still be obvious to some degree in any electronic shutter.

Why not just use film then which doesn’t have this issue? Well it does, although not to the extent of most electronic shutters. But the real reason is because film has other problems which makes stereoscopy tricky. The size of the rig is one thing but tiny artifacts that appear on film as well as differing position of grain would mean that the left and right eye see slightly different shots. Having clean, noise free, digital footage is ideal and is in fact one of the reasons that stereoscopic cinematography is making a comeback since digital cinema cameras are improving rapidly.

Which is the better setup, 1 or 4? It’s really a personal choice as they both have their pro’s and con’s. With setup 1 in position on a tripod the rig looks very similar to an operator as a normal camera might. It doesn’t appear at first sight to be so cumbersome. But if the operator wants to tilt down or do long pans it will likely interfere with the tripod. Furthermore positioning the camera at floor level is difficult. Distributing the weight of the rig over a tripod is also problematic because it is always going to be front heavy. Putting weights at the rear of the rig to counter balance is common practice. The plus side of the design is that it uses less material to make. Setup 4 requires a lot of fabrication to hold the camera securely in such a position. However it can be put at floor level and does not interfere with the tripod in any way. When on a tripod the rig can be manouvered so that the balance of the head is perfect.

In conclusion, if you are hiring, buying or even producing a stereoscopic rig and intend to use a beam splitter either Setup 1 or 4 is the ideal choice.